Tuesday, July 28, 2015

Externally clocking (and overclocking) AVR MCUs

People familiar with AVR boards such as Arduinos likely know most AVR MCUs can be clocked from an external crystal connected to 2 of the pins.  When the AVR does not need to run at a precise clock frequency, it is also common to clock them from the internal 8Mhz oscillator.  Before CPUs were made with internal oscillators or inverting amplifiers for external crystals, they were clocked by an external circuit.  Although you won't see many AVR projects doing this, every AVR I have used supports an external clock option.  One (extreme) example of a project using an external clock is Brad's Quark85 video game platform.  Some AVRs such as the tiny13a and the tiny88 do not support an external crystal, so the internal oscillator or an external clock circuit are the only options.  The 4-pin metal can pictured above is a clock circuit hermetically sealed for precision and stability.  They can be bought from Asian sources for under $2.

A common reason for needing an external clock for an AVR MCU is from accidentally setting the fuses for an external clock.  Once the fuses are set to external clock, they cannot be reprogrammed without providing an external clock signal.  Wiring the oscillator is simple; connect power and ground, then connect the output to the CLKI pin of the MCU.   On the ATtiny13a, this is pin 2 (PB3).  On the ATmega328-AU, this is pin 7 (PB6).

The output of the oscillators is very stable and accurate, around a few ppm, as measured by my Rigol scope.  The output is almost rail-to-rail (0-5V), and quite clean:

Although the connection is simple, it's not foolproof.  During my experimentation, I accidentally plugged my oscillator backwards (connecting 5V to Gnd and Gnd to 5V), which quickly fried it.  Now I'll be extra careful with the M-Tron 40Mhz oscillator so I don't kill that too!

AVRs are known for being easy to overclock, but I was uncertain whether an ATtiny13a rated for 20Mhz would work when overclocked to more than double it's rated speed.  I experienced no problems flashing code with avrdude and running my bit-bang UART at either 40 or 44.3Mhz with a 5V supply.  At 3.3V it crashed most of the time, only running OK occasionally.

Another way to provide an external clock is to build a ring oscillator using a 7404 hex inverter or similar chip.  A 3-stage ring oscillator I build using a 7404 generated a clock close to 30Mhz:

Since the frequency is inversely proportional to the number of stages, a 5-stage oscillator using the same 7404 would generate a frequency of 18Mhz.  I tried to make a single-stage oscillator with the 7404 and also with a 74LS00, but was unsuccessful,  They are just not fast enough to generate a 90Mhz clock.  Considering the 7404 I used is a Fairchild part with a 1984 date stamp, I'm pleased with how well this 30-year-old part works.

The last way of getting a clock source I'll describe is to tap off the XTAL pin of an AVR (or other MCU) that is using an external crystal.  Most AVRs can drive the external crystal in low-power of full-swing mode.  For the ATmega8a, the CKOPT fuse enables full swing mode.  If the AVR is driving the crystal in low-power mode, the peak-to-peak voltage will not be enough to work as the external clock for another AVR.  By soldering a wire to one of the XTAL pins you can use it to clock another MCU.  I've labeled the XTAL pins in yellow on a chinese USBasp clone:

And here's a shot from my scope connected to the 12Mhz crystal on the USBasp:

Finally, if your external clock is slower than 8Mhz (like if you were to use a 555 timer to generate the clock) you'll probably need to use a slower SPI bit clock setting with avrdude.  I've found avrdude -B 4, specifying a 4 microsecond clock period will work with AVRs clocked as low as 1Mhz.

Saturday, July 18, 2015

$3 USB gamepad teardown

I recently received a USB gamepad I ordered off Aliexpress for a little more than $3.  I got it for a RetroPie box I'm planning to build, so I don't need anything fancy.  A USB controller chip alone can easily cost $1, so I was curious to see what went into making these.  The photo shows it is pretty simple.

The PCB is single-sided bakelite, since it is really cheap.  While double-sided FR4 PCBs cost around 5c/sq in, even in volume, a single-sided bakelite board is under 2c/sq in.  The USB controller chip is on the other side of the board covered in an epoxy blob, so I can't say what kind of controller chip it is.  besides the controller chip, the only electronic components are the 6Mhz resononator and the ceramic capacitor.  The wires connecting the L/R buttons to the PCB are cheap - similar to the wires twist ties are made from.  The controller looks like it has good strain relief, with the cord winding around a few plastic posts.

The controller was detected (under Windows 7) as a HID-compliant game controller.  I haven't finished setting up my RetroPie box yet, so I tried it out with Doom.  The button feel wasn't the greatest, but all 12 of the buttons worked.  Overall, I'm satisfied with the controller considering the low price.

Tuesday, July 14, 2015

Rigol DS1054Z frequency counter accuracy

I recently found out that in addition to a software frequency measurement (shown in the bottom right) the DS1000Z series has a hardware frequency counter (shown in the top right).  The hardware counter is enabled by pressing the "measure" button, select counter, CH1.  The display shows 6 digits, or 1 ppm resolution, but I was unable to find a specified accuracy for the counter.  My testing suggests the accuracy at ~25C ambient temperature is 1-2ppm.

The first measurements I took were with a couple old metal can 4-pin oscillators I had salvaged.  One is a Kyocera 44.2368MHz that I measured at 44.2369MHz.  The second was a M-tron 40.000000MHz that I measured at 39.9999MHz.  The next thing I measured was a generic 12.000MHz crystal on a USB device which measured 12.0001MHz.   Together those measurements suggested an accuracy of <10ppm.  I don't have a high-precision clock source such as an oven-controlled crystal oscillator or GPS receiver with a timing output, so I needed another way to precisely measure the accuracy of the frequency counter.

My solution was to accurately measure the 1Khz test signal output from the scope since the frequency counter measured it at an exact 1.00000kHz.  I don't have access to a calibrated frequency such as a 5381A, but I do have Kasper Pedersen's nft software.  I connected the test posts for the 1kHz output to the Rx line on a USB-TTL module, and started up nft.

From the mode options I selected pulse at 1kHz.  I could tell the pulses were being detected because the "Events" count was going up by about 1000 per second.

I did a few 300s runs that gave an average error of -0.98ppm.  I then let it run for two 1000s tests which resulted in an average error of -1.68ppm.  I don't recall Dave's teardown identifying the timing source, but given the amount of error, I'd rule out an OCXO.   The accuracy is a bit better than the +-10ppm for a typical crystal oscillator, so maybe it uses a temperature-controlled crystal oscillator (TCXO).  If anyone knows for sure, drop a line in the comments.

In addition to testing accuracy, I tested the frequency range.  I probed the antenna output from a 433.92Mhz ASK/OOK transmitter.  The software frequency counter identified it as 435Mhz, but the hardware counter showed 66.1680Mhz.  The signal level was low (around 300mV), so that may have caused problems for the hardware counter.  I suspect it is good up to 100Mhz, which is more than I expect to need in the foreseeable future.  The accuracy and frequency range is sufficient for the things I want to do like checking oscillators on MCUs.  I found one of my $2 Chinese Pro Minis was oscillating at 15.9973Mhz.  The -169ppm error would be acceptable for a ceramic resonator, but this was with a HC-49S package crystal oscillator.

2018 Spring Update
I did a few 1hr calibration runs with NFT, which averaged out to -2.68ppm.  That suggests an aging drift of -1ppm since the original test I ran almost 3 years ago.