Don't use a $5 range outlet for EV charging

Many level 2 home chargers plug into a NEMA 14-50R outlet, which is the same type a kitchen range would plug into.  A 40A EVSE is considered a continuous load under the electrical code, and therefore needs a circuit rated for 25% more current.  Even though the 14-50R outlet is technically a 50A outlet, when used as a kitchen range plug, they are often wired on a 40A circuit.  This is permitted by the Canadian Electrical Code rule 26-744 5).  Cheap 14-50R outlets intended for a kitchen range are not designed for continuous 40A use, as a range is not a continuous load.  A 40A EVSE needs an outlet designed for high-current continuous loads to avoid overheating and melting the outlet.

I purchased two different heavy-duty 14-50R to evaluate them for EVSE use.  The Leviton 1450R is designed specifically for EVSE use.  The legrand 3894 is a heavy-duty outlet for ranges and EV chargers.  I paid $60 for the Leviton 1450R at Home Depot, and $23 for the 3894 from Wesco.  Given the price difference, and the fact that the Leviton 1450R is designed specifically for EVSE use, I was expecting the Leviton to be a better quality outlet.  I was not disappointed.

The Leviton 1450R, at 207g, is heavier than the legrand 3894 at 153g.  The front steel plate is thicker at 2mm, while the plate on the legrand is 1.4mm thick.  The circular receptacle has a diameter of 2 7/16" on the Leviton and 2 1/8" on the levtion.  That will be important to note when choosing a face plate.  The most obvious electrical difference with the Leviton is the solid copper back electrical terminals.  The legrand appears to use common brass, which has a resistance about 3.5 times as high as copper.  The lower resistance copper terminals means lower temperatures while in use.

I was not able to measure the thickness of the metal used for the receptacle contacts where the plug fingers are inserted, as that would require a destructive teardown.  Given the overall robust construction of the Leviton 1450R, I think it likely uses heavier gauge metal to reduce resistance, and to maintain a stronger mechanical contact with the plug.

For installation, the Leviton specifies 50 lbs of torque on the terminal screws, while the legrand specifies 20 lbs.  Neither specifies the use of oxide inhibitor, but I'd suggest some DE-OX grease for extra protection.

Despite the high quality of the Leviton 1450R, it's not my preferred solution for an EV charger.  A hard-wired charger is cheaper since it avoids the cost of the outlet, and will usually make better electrical contact than any plug-in solution.  If you want a 14-50R outlet in your garage to support future plans for a home level 2 charger, then the Leviton 1450R would be a good choice.

2026 Update

I no longer recommend a hard-wiring an EV charger.  I think the best choice is a good quality 6-50R or 14-50R outlet, with the charging limited to 30A for daily use.  Charging at 30A generates 44% less heat than charging at 40A, reducing the chances of overheating.  If your charger dies, plugging in a new charger is a easier than disconnecting and reconnecting a a hardwired charger.  And if you move, it's easier to take your charger with you.

Hyundai Level 2 EV Charging Efficiency about 88%

 

Unlike PV inverters, EV on-board chargers usually don't have efficiency specifications published by manufacturers.  Studies done on charger efficiency are limited in the number of vehicles that can be tested.  I decided to test the charging efficiency of a Canadian 2023 model Kia Sportage PHEV.

I performed the test with the Kyungshin EVSE that was provided with the vehicle, set to 12 amps.  The EVSE was plugged into a 240V outlet via a 18m, 10 AWG extension cord.  The L14-30R outlet is wired to the electrical panel with 6/3 aluminum cable.  Measurements were made at the electrical panel using a Peacefair PZEM-016, logged with a python program I wrote.  I initially took measurements with a clamp-style meter, but the readings were too variable, and could not account for power factor.

Considering the connectors and wire resistance, I estimate the losses between the electrical panel and the EVSE to be about 1%.  For a typical home level 2 30-amp charger installation, the losses would be much higher, likely over 2%.

The Sportage PHEV battery capacity is 13.8 kWh.  To charge from 20% to 100%, the PZEM-016 recorded 12.55 kWh of energy.  The energy stored in the battery was 80% of 13.8, or 11.04 kWh.  The efficiency is therefore 11.04/12.55 = 0.8797, or about 88%.

Level 2 EV Charging Deep Dive

 

Electric vehicles have an on-board charger, which converts an AC input voltage to DC to charge the batteries.  These chargers are designed to accept 120 volt input for level 1 charging, and 208/240 volt input for level 2 charging.  The specs for the Hyundai OBC shown above indicate it can accept a wide range of 70 to 285 Vac, allowing it to work with almost any power grid in the world.  Note that it is the OBC, not the EVSE, that rectifies and boosts the voltage to charge the battery.  That's why an EVSE with a 120V 5-15R plug can be connected to a 240V source.

Although the Hyundai OBC is rated for 7.2 kW of output power, getting much more than 6 kW of input power has been difficult.  The first reason is that power to commercial buildings is usually 3-phase 120/208V, so charging stations usually provide 208V.  At the maximum input of 32 amps, that's 6656 watts.  Having used both ChargePoint and Flo charging stations, I've noticed the majority of them are limited to 30 amps for level 2 charging.  Those stations rated for 30 amps use 10 AWG flexible cord, which is limited to 30 amps according to table 12 of the Canadian Electrical Code.  30 amps at 208 volts is 6240 watts.  Charging at more than 30 amps requires a more expensive larger cable.

I've also found the OBC doesn't seem to pull the full amperage advertized by the EVSE.  The signalling used for J1772 charging doesn't communicate a precise amperage available to the OBC.  It transmits a sequence of pulses, and the duty cycle timing of the pulses indicates the available amperage.  I think the OBC reduces the current by a safety margin to allow for imprecise timing of the control signal pulses.  When connected to a Kyungshin IC-CPD set to 12 amps, the on-board charger draws about 11 amps.

Lastly, some charging stations don't always provide the power that they advertize.  Several of the ChargePoint chargers I've encountered are Leviton 4000 units.  They support 16 amps per head, or charging from a single head at 30 amps.  These stations are usually listed on the ChargePoint network as 6.6 kW, but with a 208V supply, you'll never see more than 6.2 kW.  When both heads are being used, your vehicle will charge at no more than 3.3 kW.

I think home charging makes a lot of sense, but I see limited value in public level 2 chargers.  I am not aware of any public chargers in Nova Scotia that accept payment by credit card.  They require users to first set up an account and install an app in order to active chargers.  When you do get a charger working, at a charging rate of 6 kW, you can't get much of a charge while you shop at a store or eat at a restaurant.  I've seen a few businesses that offer free charging for customers, but after the novelty factor of free charging wears off, I wonder how much use they will get.  Since charging at home costs 18.5 c/kWh including GST, getting free charging while you shop for a half hour only saves you 50c.

240V EV Charging for $5

 

We recently purchased a PHEV which came with a portable home charger/EVSE.  It plugs into a NEMA 5-15R outlet, and supports a maximum charging rate of 12 amps.  At 120 volts, the maximum charge rate is 1440 watts, and the charge rate reported by the vehicle is usually 1.3 kW.  The vehicle supports level 2 charging at up to 7.2 kW, but I didn't want to spend $400 to $500 for a good quality 30 amp level 2 EVSE.

The label on the portable EVSE listed an input of 12 amps and 120 volts, however I suspected 240 volts would be fine.  The EVSE just passes through the AC power, generating a PWM signal on a control wire to indicate the amount of current the vehicle's on-board charger can draw.  Of course, it's possible some home EVSEs for the North American market are built as cheaply as possible, and may not handle 240V.  I am confident our Kyungshin IC-CPD is built to accept 240V.  On the vehicle side, I checked the on-board charger label and saw that it has a wide input voltage, with a rating of 70-285Vac.  I have a 14-30R 240 volt outlet in my garage, which is the same type of outlet an electric dryer uses, giving me an available source for 240V power.

To make an adapter for the portable EVSE, I used the cord I cut off a broken dryer, and a 5-15R connector.  I used an Eaton 4887, which costs about $5 at local electrical suppliers.  The Leviton 515CV is another option.  The specs for the 4887 lists an input wire size of 12 to 18 AWG, however the 10 AWG stranded copper wires on the dryer cord were just able to fit.

With my portable EVSE adapter hack, the vehicle now charges twice as fast.  It's probably more efficient too.  The output of the on-board charger is 240-430 Vdc, and boost converter efficiency increases with a smaller difference between the input and output voltages.

Calculating Copper Wire Characteristics

For the early of my life I've relied on tables or similar references to look up things like copper ampacities and resistance.  Now I just remember a few constants, and can calculate what I need to know.

The photo above is a 12 AWG copper wire, commonly used in building wiring in the US and Canada.  It has a diameter of .0808 inches or 2.052 mm.  When used for building wire, it is typically limited to carrying 20 amps of current by breakers or fuses.  For circuits carrying more current, 10 AWG wire with a diameter of 2.587 mm can be used.  For a change of 2 AWG in wire size, the change in diameter is always 1.26, and therefore the change in cross-sectional area is 1.26^2.

The cube of 1.26 is 2.0, so an increase in size of 3 AWG will double the cross-sectional area of the wire, and reduce the linear resistance by half.  The resistance of 10 AWG wire is 1 ohm per thousand feet (304.8 m), so the resistance of 16 AWG wire, often used in extension cords, is 4 ohms per thousand feet at room temperature.  The amount of heat generated in a wire is calculated with the formula P=I^2xR.  If 10 amps is flowing through 1000 feet of 16 AWG wire, the power dissipated will be 10^2 x 4 or 400 watts.  If the wire is 10 AWG, and the current is 20 amps, the power dissipated will be the same 400 watts.  However voltage drop will be lower, since V = I x R.  The resistance of 10 AWG wire is a quarter of 16 AWG, so the voltage drop in the 10 AWG wire with 20 amps will be half of the voltage drop of the 16 AWG wire with 10 amps.

The resistance of copper increases with temperature, by 0.393% per degree C, so increasing the temperature by 25 C will increase the resistance by almost 10%.  Calculating temperature increase due to power dissipation is quite complicated, so it is common in electrical codes to consider an ambient temperature of 30 C, and a temperature rise in the wire of no more than 30 C.  Because of that, most wire sold in Canada that is CSA certified will use insulation rated for at least 60 C.  The most common category of wire used in residential construction, NMD90, has an insulation temperature rating of 90 C.

On a final note, a wire labeled 16 AWG might not really be 16 AWG.  I generally trust the electrical distributors like Rexel and Wesco, however before using some battery wire from a discount hardware store I'd inspect it carefully first.

MODBUS communication with Solis 4G-US inverters

Solis single-phase inverters have a circular RS485 connector supporting MODBUS communication.  The connectors can be difficult to find for sale outside China, so using a wifi data logger stick is a more straightforward way of communicating with the inverters.  IGEN Tech is the OEM for the Solis wifi data loggers, which IGEN also sells under the SOLARMAN brand.  While the same circular connector is used by many other inverter manufacturers such as Solax, RENAC, and KSTAR, the logger firmware is customized to read and report the MODBUS registers for a particular manufacturer.

The LSW-3 series of wifi loggers allow external programs to perform MODBUS queries via a TCP connection port 8899.  I believe this is a variant of the MODBUS/TCP protocol that is assigned TCP port 502.

To perform MODBUS queries, I used pysolarmanv5.  To connect to the logger pysolarmanv5 requires the logger serial number and IP address.  Initially I read the serial number off the label of the logger, and looked up the IP address from the admin page of my router.  Later I noticed solarman_scan.py, which sends a broadcast UDP packet which the data logger replies to.  I sometimes had to run it more than once before the logger responded to the scan packet.

The Solis 4G-US series inverters are Sunspec MODBUS certified, and have the well-known 32-bit ‘SunS’ identifier (0x53756e53) at address 40001.  This means it should be possible to read the registers by decoding the SunSpec information models and inverter device IDs.  SunSpec shares some example code, however I haven't been able to figure it all out.

I couldn't figure out the Solis registers using SunSpec, but I was able to find the register documentation from Ginlong.  The AC output power and DC input power are 32-bit registers at address 3005 using MODBUS function code 4 (input registers).  I wrote a python program to read the output and input power and calculate the efficiency.  It also reads the inverter temperature, and outputs the data every 5 minutes.  I also wrote a small AWK program to calculate the weighted average of multiple samples.  The code can be found in my github repo.

Over multiple days of output in the spring of 2023 including sunny and cloudy days, I observed an overall efficiency of 94% for a Solis 1P4K-4G-US.  For a Solis 1P6K-4G-US I observed an average efficiency of 95%.  This compares to respective advertised CEC weighted efficiencies of 97.5% and 97.0%.

KSTAR Single Phase String Inverters

 

KSTAR New Energy makes single phase grid-tied inverters ranging from 1 kW to 10 kW.  I tested a 3000S, a 5000D, and a 6000D that were produced in KSTAR's factory outside of Shenzhen.  Their single phase inverters are marketed for locations with a 230 V line to neutral (L-N) grid.  They also work with the split phase 240 V line to line grid that is typical in the US and Canada.  They do not have UL 1741 certification, so they would require special engineering approval to be used for permanent installations with most US and Canada power utilities.

Residential inverters used in the US and Canada usually have an attached junction box with terminal connections for DC and AC wiring.  In the rest of the world, inverters usually have MC4 connectors for the DC string input, and a watertight three-pin plug connection for the AC output.  It is much more convenient having the plug connections when testing inverters and PV panels.  It also avoids potential electrical code concerns when DC wiring up to 600 V and 240 Vac are in the same junction box.

 

The KSTAR inverters all included MC4 crimp connectors for terminating the DC strings.  The AC connector will accept SOOW or SJOW cable with a outside diameter of up to 16 mm.  I used 3-wire 12 AWG SOOW cable that is rated for up to 25 Amps.

The 3000S has a single string input, and a "nominal" output power of 3 kW.  It is a light inverter, with a stated weight of 8 kg.  Out of the box, the measured weight was 7.3 kg.  The light weight makes it very easy for a single person to install.  When hooked up to a test string of 10 72-cell panels, the efficiency was 85-86%. This is much lower than the spec efficiency of 97% or the 96% efficiency at nominal 380 V listed on the inspection and test sheet that was included with the inverter.  With input power of 3070 W and input voltage of 367.7 V, the output power was 2620 W, for an efficiency of 85.3%.  KSTAR sales and engineering were unable to explain the low efficiency.

The 5000D and 6000D have the same external dimensions and connections on the bottom.  The weight of the 5000D is 11.74 kg, while the 6000D weighs 12.48 kg.   This suggests the 6000D has different internal circuitry, likely larger inductors and capacitors, to support the higher power rating.

The efficiency of the 5000D and 6000D inverters ranged between 89 and 91%.  The screenshot of monitor data below shows a total input power of 6240 W with AC output power of 5570 W, for an efficiency of 89.3%.  This test was done with a large difference between the PV1 and PV2 voltages to represent typical residential PV installations which are not optimized for the inverter's 380 nominal string voltage.

The KStar inverters are reasonably priced and easy to install, but the low efficiency makes them unattractive compared to Growatt and Ginlong Solis inverters.

DC Wiring Losses in String and Microinverter Solar PV Arrays

There are two common ways of wiring solar PV arrays.  Each panel can be connected to a microinverter, with each microinverter connected in parallel to an AC bus.  Alternatively, panels can be connected in series, with one or more DC strings connected to an inverter.  Although there is debate over which design is best, at Solar Si, we prefer string inverters.  This is an analysis of DC wiring losses with an array of 8 72-cell LONGi PV modules of about 450 Watts each.

There are two sources of wiring resistance in the array.  The first is from the wire itself, and the second is from the connectors.  The 12 AWG wire used for the panel output cables has a resistance of 5.2 mOhm/m.  The MC4 connectors are specified to have a contact resistance of less than 0.5 mOhm.  While this may be the resistance when tested in a clean and dry factory, test results in warm and humid conditions show a much higher resistance.  Reliability Model Development for Photovoltaic Connector Lifetime Prediction Capabilities indicate resistance in the field is likely to be around 2.5 mOhm.

For the string array, the panels are arranged in the portrait configuration, with the inverter situated 1m from the array.  The panels are about 1.06 m wide, making the length of the array 8.5 m.  Each panel has a 20cm and a 40cm negative and positive output cable.  Unlike the 12 AWG wire used for the PV panel output cables, in Canada, field wiring for PV strings is almost always done with 10 AWG RPVU wire.  This has a resistance of 3.28 mOhm/m, and a total of 10.5 m are used for the array.

With 8 panels, there are 7 connections between panels, plus two connections at the ends mating with the RPVU wire.  The DC connections on the inverter are usually not MC4, but for simplicity the resistance is assumed to be the same.  Adding the positive and negative connections connections to the inverter, the total comes to 11.  Here's the calculations for the total resistance:

10.5 m * 3.28 mOhm/m = 34.4 mOhm
12 AWG 0.6 m panel cables * 8 = 4.8m, * 5.2 = 25 mOhm
11 contacts/string * 2.5 mOhm = 27.5 mOhm
total: 86.9 mOhm

For the microinverter array, the optional 1.4 m PV panel output cables will be needed in order for the cables to reach the corresponding microinverter.  This increased the total length of 12 AWG wire to 22.4 m.  Here's the calculations for the total resistance:

12 AWG 2.8 m panel cables * 8 = 22.4 m, * 5.2 = 116 mOhm
16 contacts * 2.5 mOhm = 40 mOhm
total: 156 mOhm

Although the microinverter configuration higher resistance losses, they are not significant.  During peak power output, DC current is about 10 Amps.  Using P = I^2 * R, power losses are around 0.5%.  Most of the time the array output current is much less than 10 Amps, so the average power loss is much lower.  There are additional losses from the AC bus connectors, which are also not significant.

In conclusion, power losses are higher with microinverters than string inverters, but they are not significant.  The justification for choosing string inverters lies more with the cost savings in material and labor.  For an array with 16 panels, the cost of a 6 kW inverter with 2 string inputs is less than half the cost of 16 Enphase IQ7A microinverters.

Pi ethernet gadget with reverse SSH proxy

 

I love my Pi Zeros.  I think every hacker should have one in their toolbox.  When I got my firs Pi Zero several years ago, I used a USB-TTL serial adapter to connect to the console UART on pins 8 and 10 of the Pi header.  Once I learned how to setup the Zero as an ethernet gadget, things were a bit easier.  However updating software was still a cumbersome process of downloading files to the host computer and then using scp to transfer them to the Pi.  This blog post documents how to setup the Pi to use a SSH reverse proxy so utilities like git and apt work.

When I got my first Pi Zero, I chose the Pi OS Lite image.  I decided to update to the March 4, 2021 release, and this time I used the Pi OS with desktop because it includes development tools like git.  I followed the ethernet gadget setup instructions, modifying config.txt, cmdline.txt, and creating an empty file called "ssh".  The next step is to configure the multicast DNS component of Zeroconf.  As mentioned in the Adafruit instructions, if you are using Windows, the easiest way to do this is installing Apple's Bonjour service.

To use a reverse proxy over ssh, Windows users can't use putty as that feature is not supported.  OpenSSH supports reverse socks5 proxies as of version 7.6.  For connecting from Windows, I installed MSYS2, including OpenSSH 8.4.  On Windows 10, WSL is probably the easiest option.  To connect to the Pi and enable a reverse socks5 proxy on port 1080, enter, "ssh -R 1080 pi@raspberrypi.local".

Once connected to the Pi, set "http_proxy" to "socks5h://localhost:1080".  The "h" at the end is important as it means the client will do hostname (DNS) resolution through the proxy.  I added the following line to .profile to set it every time I login:

export http_proxy="socks5h://localhost:1080"

Programs such as git and curl will automatically use the socks proxy when the http_proxy environment variable is set.  Note that github defaults to showing https URLs for repositories, which need to be changed to "http://" for the proxy to work.

The last configuration I recommend is setting the current date, since the Pi does not have a battery-backed RTC.  I normally use ntpdate from the ntp project for manually setting the date and time on Linux, but it does not work with a socks proxy.  After some searching I found a suggestion of using the HTTP Date: field from a reliable internet server.  The command I use is:

date -s "`curl -sI google.com | grep "^Date:" | cut -d' ' -f3-7`"

Once the Pi Zero is configured and has the proper date and time set, I recommend running "apt update".  If everything is working properly, it will use the socks5 reverse proxy to connect to the raspbian servers and update the local apt repository cache.

Honey, I shrunk the Arduino core!

 

One of my gripes about the Arduino AVR core is that it is not an example of efficient embedded programming.  One of the foundations of C++ (PDF) is zero-overhead abstractions, yet the Arduino core has a very significant overhead.  The Arduino basic blink example compiles to almost 1kB, with most of that space taken up by code that is never used.  Rewriting the AVR core is a task I'm not ready to tackle, but after writing picoCore, I realized I could use many of the same optimization techniques in an Arduino library.  The result is ArduinoShrink, a library that can dramatically reduce the compiled size of Arduino projects.  In this post I'll explain some of the techniques I used to achieve the coding trifecta of faster, better, and smaller.

The Arduino core is actually a static library that is linked with the project code.  As Eli explains in this post on static linking, libraries like libc usually have only one function per .o in order to avoid linking in unnecessary code.  The Arduino doesn't use that kind of modular approach, however by making use of gcc's "-ffunction-sections" option, it does mitigate the amount of code bloat due to the non-modular approach.

With ArduinoShrink, I wrote more modular, self-contained code.  For example, the Arduino delay() function calls micros(), which relies on the 32-bit timer0 interrupt overflow counter.  I simplified the delay function so that it only needs the 8-bit timer value.  If the user code never calls micros() or millis(), the timer0 ISR code never gets linked in.  By using a more efficient algorithm and writing the code in AVR assembler, I reduced the size of the delay function to 12 instructions taking 24 bytes of flash.

In order to minimize code size and maximize speed, almost half of the code is in AVR assembler.  Despite improvements in compiler optimization techniques over the past decades, on architectures like the AVR I can almost always write better assembler code than what the compiler generates.  That's especially true for interrupt service routines, such as the timer0 interrupt used to maintain the counters for millis() and micros().  My assembler version of the interrupt uses only 56 bytes of flash, and is faster than the Arduino ISR written in C.

One part that is still written in C is the digitalWrite() function.  The Arduino core uses a set of tables in flash to map a given pin number to an IO port and bit, making for a lot of code to have digitalWrite(13, LOW) clear PORTB5.  Making use of Bill's discovery that these flash memory table lookups can be resolved at compile time, digitalWrite(13, LOW) compiles to a single instruction: "cbi PORTB, 5".

ArduinoShrink is also designed to significantly reduce interrupt latency.  The original timer0 interrupt takes around 5us to run, during which time any other interrupts are delayed.  The first instruction in my ISR is 'sei', which allows other interrupts to run, reducing the latency impact to a few cycles more than the hardware minimum.  The official Arduino core disables interrupts in several places, such as when reading the millis counter.  My solution is to detect if the millis counter has been updated and re-read it, thereby avoiding any interrupt latency impact.

The only limitation compared to the official AVR core is that the compiler must be able to resolve the pin number for the digital IO functions at compile time.  Although the pin may hard-coded, even with LTO enabled, avr-gcc is not always able to recognize the pin is a compile-time constant.  Since AVR is not a priority target for GCC optimizations, I can't rely on compiler improvements to resolve this limitation.  Therefore I plan to write a version of digitalWrite that is much smaller and faster, even when avr-gcc can't figure out the pin at compile time.

Although ArduinoShrink should be compatible with any Arduino sketch, given some of the compiler tricks I've used it's not unlikely I've missed a potential error.  If you do find what you think is a bug, open an issue in the github repository.

Writing USB firmware on the CH55x MCUs

Over the last several months, I've been familiarizing myself with the CH552 and CH551 MCUs.  Most recently, I've been learning how to program the USB serial interface engine on these devices.  The USB interface is powerful and flexible enough to implement many different kinds of USB devices, from HID to CDC serial.  The highlights are:

• support for endpoints 0 through 4, both IN and OUT
• 64-byte maximum packet size
• DMA to/from xram only
• multiple USB interrupt triggers

One of the first requirements for writing USB firmware is writing the descriptors.  The examples from WCH are difficult to use as a template due to the descriptors being uint8_t arrays instead of structures.  There are USB structure and constant definitions in ch554_usb.h, which I recommend using instead of arrays.  For instance, I changed the CDC serial example from :

__code uint8_t DevDesc[] = {0x12,0x01,0x10,0x01,0x02,0x00,0x00,DEFAULT_ENDP0_SIZE,

0x86,0x1a,0x22,0x57,0x00,0x01,0x01,0x02,

0x03,0x01

};

to:

__code USB_DEV_DESCR DevDesc = {

.bLength = 18,

.bDescriptorType = USB_DESCR_TYP_DEVICE,

.bcdUSBH = 0x01, .bcdUSBL = 0x10,

.bDeviceClass = USB_DEV_CLASS_COMMUNIC,

.bDeviceSubClass = 0,

.bDeviceProtocol = 0,

.bMaxPacketSize0 = DEFAULT_ENDP0_SIZE,

.idVendorH = 0x1a, .idVendorL = 0x86,

.idProductH = 0x57, .idProductL = 0x22,

.bcdDeviceH = 0x01, .bcdDeviceL = 0x00,

.iManufacturer = 1, // string descriptors

.iProduct = 2,

.iSerialNumber = 0,

.bNumConfigurations = 1

};

Once the descriptors are written, the code to handle device enumeration is mostly boilerplate and can be copied from one of the examples.  During the firmware development stage, I recommend adding a call to disconnectUSB() near the start of main().  It's a function I added to debug.h which forces the host to re-enumerate the device.  This way I don't have to unplug and re-connect the USB module after flashing new firmware.

Setting up the DMA buffer pointers requires special attention when multiple IN and OUT endpoints are used.  Even though five endpoints are supported, there are only four DMA buffer pointer registers: UEP[0-3]_DMA.  When the bits bUEP4_RX_EN and bUEP4_TX_EN are set in the UEP4_1_MOD SFR, the EP4 OUT buffer is UEP0_DMA + 64, and the EP4 IN buffer is UEP0_DMA + 128.  Endpoints 1-3 have even more complex buffer configurations, with optional double-buffering for IN and OUT using 256 bytes for four buffers starting from the UEPn_DMA pointer.

When I first started writing USB firmware for the CH551 and CH552, I was concerned that it may be difficult to meet the tight timing requirements, particularly for control and bulk packets that can have multiple in a single 1ms frame.  For example, with small data packets, the time between the end of one OUT transfer and the end of the next OUT transfer can be less than 20uS.  If the USB interrupt handler is too slow, the 2nd OUT transfer could overwrite the DMA buffer before processing of the first has completed.  This situation is avoided by setting bUC_INT_BUSY in the USB_CTRL SFR.  When this bit is set, the SIE will NAK any packets while the UIF_TRANSFER flag is set.  Therefore I recommend setting bUC_INT_BUSY, and clear UIF_TRANSFER at the end of the interrupt handler.

I am currently working on the CMSIS_DAP example.  It implements the DAPv1 (HID) protocol supporting SWD transfers, and works well with OpenOCD and pyOCD.  I'm working on adding CDC/ACM for serial UART communication.  The first step is creating the descriptors for the composite CDC + HID device.  The second step will be integrating the usb_device_cdc code.  The final step, although not absolutely necessary, will be optimizing the CDC code for baud rates up to 1mbps.  The current code uses transmit and receive ring buffers with data copied to and from the IN and OUT DMA buffers.  With double-buffering, the transmit and receive ring buffers can be omitted.  The UART interrupt will copy directly between SBUF and the appropriate USB DMA buffer.

Quirks of the CH55x MCUs

Over the past several months, I've been been learning to use the CH551 and CH552 MCUs.  Learning generic 8051 programming was the easy part, as there is lots of old documentation available, with Philips having written some of the best.  The learning curve for WCH's additions to the MCS-51 architecture has been steeper, requiring careful reading of the datasheets, and reading the SDK headers and examples.  I've found that the CH55x chips have some quirks that I've never encountered on any other MCUs.

The GPIO modes are controlled by two registers: MOD_OC and DIR_PU.  The register values are explained in the datasheet and in ch554.h in the SDK.  Figure 10.2.1 in the datasheet shows a schematic diagram for the GPIO.  Modes 0, 1, and 2 are for high-Z input, push-pull, and open-drain respectively.  Mode 3, "standard 8051 mode" is the most complicated.  It's an open drain mode with internal pullup, but with the output driven high for two cycles when the GPIO changes from a 0 to a 1.  This ensures a fast signal rise time.  The part that took me the longest to figure out was the operation of the pullup.  The GPIO diagram shows 70k and 10k, but section 10 of the datasheet does not explain their operation.  Therefore I've highlighted a part of the schematic in green.  When the pin input schmitt trigger output is 1, the inverter in the top right of the diagram will output a low signal to turn on the pFET activating the 10k pullup.  When port input value is 0, only the weak 70k pullup is active.

The pullups aren't actually implemented as resistors on the IC.  They are specially-designed FETs with a high drain-source resistance (RDS).  Since RDS varies with gate-source voltage (Vgs), the pullup resistance will vary inversely with Vcc.  Using a 5V supply, the pullup resistance will be close to the 70k shown in the schematic.  Using a 3.3V supply, the pullup resistance is close to 125k.  Although it is not obvious, this information can be found in section 18 of the datasheet, with the specifications for IUP5 and IUP3.  These numbers are the amount of current a grounded pin will source when the pullup is enabled.

The reset pin has an internal pulldown, which seems to be weak like the GPIO pullups.  At times when working with a CH552 running at 3V3, the chip reset when I inadvertently touched the RST pin with my finger.  This was easily solved by keeping the RST pin shorted to ground.

The last issue I encountered is more of a documentation issue than a quirk.  The maximum reliable clock speed of an IC is depended on the supply voltage.  All of the AVR MCUs I've worked with have a graph in the datasheet showing the voltage required to ensure safe operation at a given speed.  For the CH55x MCUs, there is a subtle difference in the electrical specs at section 18 of the datasheet.  At 5V, total supply current at 24MHz is specified, whereas the specs for 3.3V specify total operating current at 16Mhz.  When I tried running a CH552T at 24MHz with a 3.3V supply, it never worked.  The same part worked perfectly at 16MHz.

Despite the quirks, I think the CH55x MCUs are still a good value.  Current quantity 10 pricing at LCSC is 36c for the CH552T, and 26c for the CH551G.  I recently purchased a small tube of the CH552T, and have plans to test the touch, ADC, PWM, and SPI peripherals.

GD32E230: a better STM32F0?

 

On my last LCSC order, I bought a few GD32E230 chips, specifically the GD32E230K8T6.  I chose the LQFP parts since I have lots of QFP32 breakout boards that I've used for other QFP32 parts.  Gigadevice is much better than many other Chinese MCU manufacturers when it comes to providing English documents.  After my past endeavors trying to understand datasheets from WCH and CHK, going through the Gigadevice documentation was rather pleasant.

Although Gigadevice makes no mention of any STM32 compatibility, but the first clue is the matching pinouts of the STM32F030 and GD32E230.  To prepare for testing, I tinned the pads on a couple of breakout boards, applied some flux, and laid the chips on the pads.  I laid the modules on a cast-iron skillet, and heated it up to about 240C.  The solder reflowed well, however I noticed some browning of the white silkscreen.  Next time I'll limit the temperature to 220C.  After testing for continuity and fixing a solder bridge, I was ready to try SWD.  I connected 3.3V power and the SWD lines, and ran "pyocd cmd -v":

0000710:INFO:board:Target type is cortex_m
0000734:INFO:dap:DP IDR = 0x0bf11477 (v1 MINDP rev0)
0000759:INFO:ap:AHB5-AP#0 IDR = 0x04770025 (AHB5-AP var2 rev0)
0000799:INFO:rom_table:AHB5-AP#0 Class 0x1 ROM table #0 @ 0xe00ff000 (designer=4 3b part=4cb)
0000812:INFO:rom_table:[0]<e000e000:SCS-M23 class=9 designer=43b part=d20 devtyp e=00 archid=2a04 devid=0:0:0>
0000823:INFO:rom_table:[1]<e0001000:DWT class=9 designer=43b part=d20 devtype=00 archid=1a02 devid=0:0:0>
0000841:INFO:rom_table:[2]<e0002000:BPU class=9 designer=43b part=d20 devtype=00 archid=1a03 devid=0:0:0>
0000848:INFO:cortex_m_v8m:CPU core #0 is Cortex-M23 r1p0
0000859:INFO:dwt:2 hardware watchpoints
0000866:INFO:fpb:4 hardware breakpoints, 0 literal comparators

I did little probing around the chip memory.  The GD32E23x user manual shows SRAM at 0x20000000, like STM32 parts.  The contents looked like random values, which I could overwrite using the pyocd "ww' command.  Writing to 0x20002000 resulted in a memory fault, indicating the part does not have any "bonus" RAM beyond 8kB.

Next, I tried using the built-in serial bootloader.  After connecting BOOT0 to VDD and connecting power, PA9 and PA10 were pulled high, indicative of the UART being activated.  However my first attempt at using stm32flash was not successful:

After attaching my oscilloscope, and writing a small bootloader protocol test program, I was able to determine that the responses did seem to conform to the STM32 bootloader protocol.  I did notice that the baud rate from the GD32E230 was only 110kbps, so it wasn't perfectly matching the 115.2kbps speed of the 0x7F byte sent for baud rate detection.  To avoid the potential for data corruption, I switched to 57.6kbps.  Before resorting to debugging the source for stm32flash, my test of stm32loader gave better results:

$ stm32loader -V -p com39

Open port com39, baud 115200

Activating bootloader (select UART)

*** Command: Get

    Bootloader version: 0x10

    Available commands: 0x0, 0x2, 0x11, 0x21, 0x31, 0x43, 0x63, 0x73, 0x82, 0x92, 0x6

Bootloader version: 0x10

*** Command: Get ID

Chip id: 0x440 (STM32F030x8)

Supply -f [family] to see flash size and device UID, e.g: -f F1

Next, I was ready to try flashing a basic program.  I first checked for GD32E support in libopencm3.  No luck.  Then as I read through the user manual, I noticed GPIOA starts at 0x4800 0000 on AHB2, the same as STM32F0 devices.  The register names didn't match the STM32, but the function and offsets were the same.  For example on the GD32E, the register to clear individual GPIOA bits is called GPIOA_BC, rather than GPIOA_BRR as it is called on the STM32.  The clock control registers, called RCU on the GD32E, also matched the STM32 RCC registers.  Since it was looking STM32F0 compatible, I tried flashing my blink example with stm32loader, and it worked!

The LED was flashing faster than it did with the STM32F030.  A little searching revealed that the ARM Cortex-M23, like the M0+, has a 2-stage pipeline.  The STM32F030 with it's M0 core has a 3-stage pipeline.  My delay busy loop needs to be four cycles per iteration, and on the M23, the bne instruction only takes two cycles.  My solution is adding a nop instruction based on an optional compile flag.

One problem I have yet to resolve with the GD32E is support for the bootloader Go/0x21 command.  With the STM32F0, I left BOOT0 high, and used DTR to toggle nRST before uploading new code.  The stm32flash "-g 0" option made the target run the uploaded code after flashing was complete.  I went back to debugging stm32flash, and discovered that it is hard-coded to use the "Get Version"/0x01 command, and silently fails if the bootloader responds with a NAK.  After a few mods to the source, I was able to build a version that works with the GD32E230, however the Go command still doesn't work.  Perhaps a task for a later date will be to hook up a debug probe to see what the E230 is doing when it gets the Go command.

Overall, I'm quite happy with the GD32E230K8T6.  They cost less than half the equivalent STM32 parts, and are even cheaper than other Chinese STM32 clones I've seen.  They are lower power and their maximum clock speed is 50% faster than the STM32F0.  In addition to the shorter 2-stage pipeline, the GD32E devices support single-cycle IO, making them faster for bit-banged communications than the STM32F0 which takes 2 cycles to write to a GPIO pin.  The GD32E230 also has some new features, which might be worth discussing in a future blog post.

Trying to test a "ten cent" tiny ARM-M0 MCU part 2

After my first look at the HK32F030MF4P6, I wondered if the HK part, unlike the STM32F030 it is modeled after, does not have 5V tolerant IO.  I changed the solder jumpers to 3V3 on the CH552 module I'm using as a CMSIS-DAP adapter, which caused it to stop working.  This was because the CH552 requires a 5V supply in order to run reliably at 24Mhz.  After re-flashing the CMSIS-DAP firmware set to run at 16MHz, the module worked, and I was finally able to talk to the HK MCU via SWD.

In the screen shot above, I chose the stm32f051 target because pyocd does not have the HK MCU nor the STM32F030 among it's builtin targets.  For basic SWD communications, the target option is not even necessary.  With the target specified, it's possible to specify peripheral registers by name, rather than having to specify a memory address to read or write.

In the screen shot above, I'm using the "connect_mode" option to bring the nRST line low on the target device when entering debug mode.  Usually this is not necessary for SWD, however some of the probing I did would cause the MCU to crash.  This required a power cycle or reset to restore communications via SWD.

The first tests I did with the HK MCU were to probe the flash and RAM.  The HK datasheet shows the flash at address 0.  In the STM32F0, the flash is at address 0x8000000, and is mapped to address 0 when the boot0 pin is low.  Although the HK MCU doesn't have a boot0 pin, data at address 0x8000000 is mirrored at address 0 as well.  What was most unusal about the HK MCU is that the flash was not erased to all 0xFF as is typical with other flash-based MCUs.  Most of the flash contents was zeros, except for some data at address 0x400, which was the same on the 2 MCUs I checked:

By writing to memory starting at 0x20000000 using the 'ww' command, I discovered that the MCUs I received have 4kB or RAM, rather than the 2kB specified in the datasheet.  Writing to 0x20001000 (beyond 4kB) results in a crash.

For writing and erasing the flash, I initially tried using the pyOCD 'erase' and 'flash' commands.  Since the MCU flash interface is not part of Cortex-M specification, the flash interface peripheral will vary from one MCU vendor to the next.  The flash interface on the STM32F051 is almost identical to the flash interface on the STM32F030, however the 'erase' and 'flash' commands caused the HK MCU to crash when I ran them.  Testing on a genuine STM32F030 crashed as well, and after some debugging and reading through the pyOCD code, I realized the STM32F051 flash routines need 8kB of RAM.  Even after downloading and installing the STM32F0 device pack, I could not erase or flash the HK MCU.

Next I reviewed the STM32F030 programming manual, and tried to access the flash peripheral registers directly.  This was when I found a pyOCD bug with the wreg command.  I was able to unlock the flash by writing the magic sequence of 0x45670123 followed by 0xCDEF89AB to flash.keyr.  I tried erasing the first page at address 0, and although flash.sr and flash.cr updated as expected, the memory contents did not change.  What did work was erasing the page at address 0x8000000, which cleared the contents at address 0 as well.  I still find it strange that the erase operation sets all bits to 0 instead of 1.  The HK datasheet says a flash page is 128 bytes, and erasing a page resulted in 128 bytes set to all zero.

I was only partially successful in writing data to the flash.  Writing to 0x8000000 did not work, however writing a 16-bits to address 0 using the 'wh' command was successful.  Trying to write 16-bits to address 2 updated the flash.ar and flash.sr as expected, but did not change the data.  Writing to any 4-byte aligned address in the erased page worked, but writing to addresses that were only 2-byte aligned left all 16 bits at zero.  I tried writing bytes with 'wb' and full words with 'ww', both of which crashed the MCU, likely from a hard fault interrrupt.  I even made sure there isn't a bug with the 'wh' command by writing 16-bits at a time to RAM.

While searching the CHK website for more documentation, I found a page with IAR device packs.  Although pyOCD uses Kiel device packs, I downloaded the HK32F0 pack, which is a self-extracting RAR file, which saves the uncompressed files in AppData\Local\Temp\RarSFX0.

Since .pack files are just zip files with a different extension, I zipped the files back up as a .pack file.  However pyOCD couldn't read it: "0000731:CRITICAL:__main__:CMSIS-Pack './HK32F0.pack' is missing a .pdsc file".  Manually examining the files confirmed some of my earlier discoveries, such as flash at address 0x8000000, remapped to address zero.  I found a file named HK32F030M.svd, which contains XML definitions of the peripheral registers.  pyOCD's builtin devices appear to use svd files, so it may be possible to add HKD32F0 support to pyOCD.

Copies of the IAR support pack, datasheet, and pyocd page erase sequence can be found in my github repository.

Trying to test a "ten cent" tiny ARM-M0 MCU

 

A few months ago, while browsing LCSC, I found a surprisingly cheap ARM M0 MCU.  At the time it was 16.6c in single-unit quantities, with no higher-volume pricing listed.  From the datasheet LCSC has posted, there was enough information in English to tell that it has 2kB RAM, 16kB flash, and runs up to 32MHz with a 1.8V to 3.6V power supply.  Although the part number suggests it may be a clone or is compatible with the STM32F030, it's not.  The part number for the STM32F030 clone is HK32F030F4P6.

Some additional searching brought me to some Chinese web sites that advertised the chip as a 32-bit replacement for the STM8S003.  The pinout matches the STM8S003F3P6, so in theory it is a drop-in replacement for the 8S003.  Unlike the STM32F0, it has no serial bootloader, so programming has to be done via SWD.  And with no bootloader support, there's no need to be able to remap the flash from 0x0800000 to 0x0000000 like the STM32.  A small change to the linker script should be all it takes to handle that difference.  Even though I wasn't sure how or if I'd be able to program the chips, I went ahead and ordered a few of them.  I already had some TSSOP20 breakout boards, so the challenge would be in the software, and the programming hardware.

Since I'm cheap, I didn't want to buy a dedicated DAPlink programmer.  I have a STM32F103 "blue pill", so I considered converting it to a black magic probe.  But since I've been playing with the CH554 series of chips, I decided to try running CMSIS-DAP firmware on a CH552.  If you're not familiar with CMSIS-DAP and SWD, I recommend Chris Coleman's blog post.  Before I tried it with with the HK32F030MF4P6, I needed to try it with a known good target.  Since I had recently been working with a STM32F030, that's what I chose to try first.

The two main alternatives for open-source CMSIS-DAP software for downloading, running, and debugging target firmware are OpenOCD and pyOCD.  pyOCD is much simpler to use than OpenOCD; after installing it with pip, 'pyocd list' found my CH552 CMSIS-DAP:

However that's as far as I could get with pyOCD.  There seems to be a bug in the CMSIS-DAP firmware or pyOCD around the handling of the DAP_INFO message.  Fixing the bug may be a project for another day, but for the time being I decided to figure out how to use OpenOCD.

To use OpenOCD, you need to create a configuration file with information about your debug adapter and target.  It's all documented, however it's very complicated given that OpenOCD does a whole lot more than pyOCD.  It's also complicated by the fact that since the release of v0.10.0, there have been updates that have made material changes to the configuration file syntax.  I had a working configuration file on Windows that wouldn't work on Linux.  On Linux I was running OpenOCD v0.10.0-4, but on windows I was running v0.10.0-15.  After installing the xPack project OpenOCD build on Linux, the same config file worked on both Linux and Windows, which I named "cmsis-dap.cfg":

adapter driver cmsis-dap

transport select swd
adapter speed 100

swd newdap chip cpu -enable
dap create chip.dap -chain-position chip.cpu
target create chip.cpu cortex_m -dap chip.dap

init
dap info

With dupont jumpers connecting SWCLK, SWDIO, VDD, and VSS on my STM32F030 breakout board, here's the output from openocd.

After making the same connections (factoring the different pinout) to the HK32F030MF4P6, I was getting no response from the MCU.  Before connecting, I had done the usual checks for shorts and continuity, making sure all my solder connections were good.  Next I tried just connecting VDD and VSS, while I probed each pin.  Pin 2, SWDIO, was pulled high to 3V3, as was nRST.  All other pins were low, close to 0V.  The STM32F030 pulls SWDIO and nRST high too.  I tried reconnecting SWDIO and SWCLK, and connecting a line to control nRST.  I added "reset_config trst_and_srst" to my config file, and still didn't get a response.  Looking at the debug output from openocd (-d flag) shows the target isn't responding to SWD commands:

Debug: 179 99 cmsis_dap_usb.c:728 cmsis_dap_swd_read_process(): SWD ack not OK @ 0 JUNK Debug: 180 99 command.c:626 run_command(): Command 'dap init' failed with error code -4

Since the datasheet says that after reset, pin 2 functions as SWDIO, and pin 11 functions as SWCLK, I'm at a bit of an impasse.  I'll try hooking up my oscilloscope to the SWDIO and SWCLK lines to make sure the signals are clean.  I've read that in some ARM MCUs, DAP works while the device is in reset, so I'll peruse the openocd docs to figure out how to hold nRST low while communicating with the target.  And of course, suggestions are welcome.

Before I finish this post, I wanted to explain the reference to a "ten cent" MCU.  LCSC does not list volume pricing for the part, but when I searched for the manufacturer's name, "Shenzhen Hangshun Chip Technology Development", I found an article about the company.  In the article, the company president, Liu Jiping, refers to the 10c ($0.1) price.  I suspect that pricing is for quantities over 1000.  Assuming these chips can actually be programmed with a basic SWD adapter, then even paying 20c for a 20-pin, 32MHz M0 MCU looks like a good deal to me.

Read part 2 to find out how I got SWD working.

STM32 Starting Small

 

For software development, I often prefer to work close to the hardware.  Libraries that abstract away the hardware not only use up limited flash memory, they add to the potential sources of bugs in your code.  For a basic test of STM32 library bloat, I compiled the buttons example from my TM1638NR library in the Arduino 1.8.13 IDE using stm32duino for a STM32F030 target.  The flash required was just over 8kB, or slightly more than half of the 16kB of flash specification on the STM32F030F4P6 MCU.  While I wasn't ready to write my own tiny Arduino core for the STM32F, I was determined to find a more efficient way of programming small ARM Cortex-M devices.

After a bit of searching, looking at Bill Westfield's Miimalist ARM project, libopencm3, and other projects, I found most of what I was looking for in a series of STM32 bare metal programming posts by William Ransohoff.  However instead of using an ST-Link programmer, I decided to use a standard USB-TTL serial dongle to communicate with the ROM bootloader on the STM32.

To enable the bootloader, the STM32 boot0 pin must be pulled high during power-up. then the bootloader will wait for communication over the USART Tx and Rx lines.  On the STM32F030F4P6, the Tx line is PA9, and the Rx line is PA10.  In order reset the chip before flashing, I also connected the DTR line from my serial module to NRST (pin 4) on the MCU as shown in the following wiring diagram:

For flashing the MCU, I decided on stm32flash.  While installation on Debian Linux is as simple as, "apt install stm32flash", I had some difficulty finding a recent Windows build.  So I ended up building it myself.  Although my build defaults to 115.2kbps, I found 230.4kbps completely reliable.  At 460.8kbps and 500kbps, I encountered intermittent errors, so I stuck with 230.4kbps.  After making the necessary connections, and before flashing any code to the MCU, do a test to confirm the MCU is detected.

One thing to note about stm32flash is that it does not detect the amount of flash and RAM on the target MCU.  The numbers come from a hard-coded table based on the device ID reported.  The official flash size in kB is stored in the system ROM at address 0x1FFFF7CC.  On my STM32F030F4P6, the value read from that address is 0x0010, reflecting the spec of 16kB flash for the chip.  My testing revealed that it actually has 32kB of usable flash.

I used William's STM32F0 GPIO example as a template to create a tiny blinky example that uses less than 300 bytes of flash.  Most of that is for the vector table, which on the Cortex-M0 has 48 entries of 4 bytes each.  To save space, I embedded the reset handler in an unused part of the vector table.  Since the blinky example doesn't use any interrupts, all but the initial stack pointer at vector 0 and the reset handler at vector 1 could technically be omitted.  I plan to re-use the vector table code for other projects, so I did not prune it down to the minimum.

The blinky example will toggle PA9 at a frequency of 1Hz.  That is the UART Tx pin on the MCU, which is connected to the Rx pin on the USB-TTL dongle.  This means when the example runs, the Rx LED on the USB-TTL dongle will flash on and off.

I think my next step in Cortex-M development will be to experiment with libopencm3.  It appears to have a reasonably lightweight abstraction of GPIO and some peripherals, so it should be easier to write code that is portable across multiple different ARM MCUs.

LGT8F328P EDMINI board

Earlier this year I purchased a EDMINI board from Electrodragon.  It uses a LGT8F328P chip, which supports the AVR instruction set.  The instruction set timings and peripheral registers vary slightly from the ATmega328P, so it is not 99% compatible as claimed by Electrodragon.  I bought one to see just how compatible it is, and possibly to port some of my AVR libraries to the LGT MCU.

The module arrived in an anti-static bag, inside a padded envelope.  After connecting 5V power to the board, the D13 LED blinked on and off every second, suggesting that it comes with the Arduino blink sketch pre-loaded.  I then hooked up a USB-TTL adapter, installed the LGT board file in the Arduino IDE, and tried flashing a modified blink sketch to the board.  The upload failed, and after some debugging I found that the reset was not working on the MCU.  Neither pressing and holding the reset button nor grounding RST would reset the board.  After contacting Electrodragon, Chao agreed replace the board, with two new boards.  He told me that they see a higher than average failure rate with the LGT8F328P chips.

In addition to Chao's frank comment about reliability, another concern I had about the LGT parts was the lack of markings on the chip.  I suspect LGT sells the parts without markings so vendors can label them with their own brand.  This also makes it easier for more nefarious manufacturers to label them as an ATmega328p.  

When the new boards arrived, the first thing I did was make sure the reset button worked.  After pressing reset the LED flashes quickly three times for the bootloader, and then flashes on and off every second.  However when I tried uploading sketch using the Arduino IDE, the upload still failed.  After some more debugging, I found I could upload if I pressed the reset button just before uploading.  This meant the bootloader was working, but auto-reset (toggling the DTR line) was not.  These boards use the same auto-reset circuit as an Arduino Pro Mini:

A negative pulse on DTR will cause a voltage drop on RST, which is supposed to reset the target.  When the target power is 5V and 3V3 TTL signals are used, toggling DTR will cause RST to drop from 5V to about 1.7V (5 - 3.3).  With the ATmega328P and most other AVR MCUs, 2V is low enough to reset the chip.  The LGT8F328P, however requires a lower voltage to reset.  In some situations this can be a good thing, as it means the LGT MCU is less likely to reset due to electromagnetic interference.

The EDMINI board has a 3V3 regulator which can be selected by a solder jumper.  This is mentioned on the Electrodragon site, but it is not clearly documented which pads need to be shorted to switch from 5V to 3V3.  After a bit of debugging I was able to run the board at 3V3, and was able to use the auto-reset feature.

I do most of my AVR development using command line tools, not the Arduino IDE.  I compiled a small program that toggles every pin on PORTB using avr-gcc 5.4.0, and flashed it to the EDMINI board using avrdude.  Nothing happened.  Since the Arduino blink sketch worked, I know that the LED on PB5 was working.  My conclusion is that the LGT Arduino core must do some setup to enable PORTB.  This is common on modern MCUs such as the ARM Cortex, but on AVRs like the ATmega328p, writing 255 to the PORTB and DDRB registers is all it takes to drive every pin on port B high.

I won't be doing any development work with the LGT MCUs.  Although they are cheaper and can run a bit faster than authentic AVR parts, their compatibility is rather limited.  Any code that relies on the standard AVR instruction set timing, such as my picoUART library, will not work.  The 8F328P cannot be programed with a USBasp, as the native programming interface is SWD, not Atmel's SPI-based protocol.  For a cheap and powerful MCU, the CH551 looks much more interesting.