Tuesday, September 29, 2015

Under $3 Chinese Arduino UNO compatible board

With UNO compatible boards selling for under $3 on AliExpress, I decided to order one.  Some people might think you're only going to get cheap crap for that price, but in some ways it is even better than the original Arduino UNO R3.

Some boards can be found with a microUSB connector, but I prefer the full-size connector since it is much more robust, and I have half a drawer full of the old cables with no more use for them.   One thing that could be better on the board is the choice of USB-ttl chip.  It uses the CH340G chip, which, depending on your OS, can be problematic when it comes to drivers.  Prolific and FTDI have both done some nasty things with their drivers to deal with clone chips, so I wouldn't recommend either of them.  The Silicon Labs CP2102 would probably be my first choice.

The board arrived with the female headers and ICSP header already soldered on.  The vendor could have done a better job with the packing, as the ICSP header pins were bent (though easily straightened out).  The red power LED, like many other boards I've seen, is a little bright for my liking.  It uses a 1K SMD current limiting resistor, which I may replace if it gets too annoying.

There's a part by the ICSP header that I haven't been able to identify, that looks like some kind of diode.  It is in parallel with the adjacent 10K reset pullup resistor.  If anyone knows what it is, drop a line in the comments.  The board uses a 16Mhz SMD resonator to clock the ATmega328p, and strangely also has a couple of (22pf?) loading capacitors.  While crystal oscillators usually require loading capacitors, ceramic resonators do not.  A quick frequency check by toggling a pin at 2Hz confirmed it is clocked slow.  With the frequency counter on my multimeter I measured 1.984Hz, or 0.8% slow.

To make a more exact frequency measurement, I used my Rigol 1054z to probe the oscillator pins, and measured 15.8537, or 0.9% slow.  I then decided to remove the caps.  I have a hot air gun, but was concerned the heat could damage the SMD oscillator.  Instead I used the 2 soldering-iron technique to heat both ends of the SMD capacitors and remove them.  With the caps removed, the frequency measured 15.9410Mhz, or 0.4% slow, within the +- 0.5% typical rating of ceramic resonators.

I think the board is well worth 268c price, for someone starting out with AVR MCUs I think one of the Nano clones is a better idea.  For less than $2 you get a board that is simpler to use with a breadboard than a full-size UNO.

Thursday, September 24, 2015

145c ATmega328 Pro Mini

Last month when I saw an Aliexpress listing for a 145c Pro Mini (no longer listed), and it was a version that I hadn't seen before, I quickly ordered it.  The board design has pads for a regulator, although I haven't seen a version of this board for sale with the regulator populated.  I power most of my Pro Mini projects from 5V or even 3V battery power, so the lack of an on-board regulator is not an issue for me.

While waiting for the board to arrive, I reviewed the photo posted by the seller and noticed there doesn't seem to be any decoupling capacitors on the board.  The two small (0603) caps appeared to be connected to the reset button (for the DTR-controlled auto-reset), and to AREF for reducing ADC noise.  So when the board arrived, the first thing I did was probe the connections, confirming that there is no on-board decoupling cap.  The pre-tinned pads for a regulator provided a good spot for a 0.1uF 0805 decoupling cap.  So in addition to soldering the side header pins, I added a decoupling capacitor.

When I powered up the module, the green LED connected to pin 13 (PB5) started flashing every two seconds or so.  The power LED is also green, and almost blindingly bright.  A quick check with my meter revealed the current-limiting resistors for both LEDs were 470 Ohm.  Most of my SMD parts are 0805, but I do have a reel of 15K 0603 resistors.  Changing the resistor for the power LED to 15K changed it to a modest but still clearly visible glow.  Having a 470 Ohm resistor with a low-Vf LED (I measured ~2V) risks attenuating the slew rate of the SPI SCK signal which is the secondary function for PB5.  While I was able to check the fuse settings with a USBasp without problems, I still decided to change the resistor from 470 Ohm to 7.5K Ohm.

Speaking of the fuse settings, here's the readings from avrdude:
avrdude: safemode: Fuses OK (E:00, H:DA, L:FF)

Those fuses correspond to a 2K bootloader, 4.3V brown-out detection, and low-power clock 8-16Mhz, slow rising power (65ms from reset).  The clock fuse settings are detailed in table 9-4 of the ATMega328 datasheet (pdf).  I flashed my picobootSTK500 bootloader to free up 1.5K of space for code, and I may also change the brownout fuse bits to 2.7V so I can power it at 3.3V.

Unlike the Baite Pro Mini's I've bought before that have a crystal oscillator, these modules have a SMD ceramic resonator.  While the resonators take up much less board space, they are not as accurate as a crystal oscillator.  The accuracy of crystal oscillators is typically +-20ppm, and ceramic resonators are typically +- 0.5% (5000ppm).  I probed PB6/PB7 of the ATmega with my scope, and measured a frequency of 15.9837Mhz, which is 0.1% slow.  The timing wouldn't be good enough for a clock (it would run about 1.5 minutes slow per day), but it is perfectly adequate for high-speed (115kbps) serial UART or even low-speed (1.5mbps) USB.

While I am satisfied with my purchase, I've noticed that the price for the Baite Pro Minis with the on-board L05 regulator and large MLCC capacitors are now selling for 149c.  Even if I don't use the regulator, I'd spend the extra 4c just for the on-board decoupling caps.  The latest Baite boards also use > 1K Ohm resistors for the LEDs.

Monday, September 21, 2015

Diodes, diodes everywhere

I recently was breadboarding a circuit and needed a couple diodes.  I couldn't find my 1n4148s, so I used a couple of 1n4006s.  With their heavy gauge leads for high current carrying, the 1n4006 diodes are hard to plug into a breadboard.  Then I remembered the bag of tl431s I have, and that just like the zener diodes they are meant to replace, they can be used forward biased.  The tl431 block diagram above shows the diode from the anode to cathode.

One other difference I noticed with the 1n4006 diodes is their lower voltage drop.  As I discussed in my 5c lithium ion battery charger post, diodes don't have a fixed voltage drop, but have a voltage drop that increases with increased current.  At 1mA, 1n4148 diodes have voltage drop of 0.6 - 0.65V, while 1n4006 diodes have a voltage drop around 0.5V.  None of the datasheets for the 1n4148 or 1n400x had detailed specs for low current.  The first couple of 1n400x datasheets I found didn't even show current below 100mA.  However Jack Smith has a great write-up on the forward voltage of the 1n400x diodes including curves going down to 10uA.   I measured the forward voltage of the 1n4006 diodes using my multimeter (which uses a current of about 500uA) in the .502V to .515V range.  Jack's tests don't include the 4006, though my measurements would suggest that it falls in between the curves of the 4005 and the 4007.

Next I measured the voltage drop across the tl431 anode-cathode diode:
.542, .546, .540, .543, .539, .546, .541V

I also measured reference-cathode diode.  It isn't a discrete diode like one between the anode and cathode, it is the diode formed by the base-collector of the reference input transistor:
The variability of the A-K diode was about +-0.7%, the R-K diode was more consistent at around +-0.2%:
.729, .730, .728, .728, .727, .728, .730V

I also measured the base-emitter voltage of some old 2N2222a NPN transistors:
.601, .608, .612, .608V

and I measured the base-collector voltage too:
.605, .613, .616, .614V

Another common but often forgotten diode is the clamping or protection diodes on MCU input pins.  Here's a diagram from an Atmel AVR datasheet:

Although the datasheet and schematic suggest there is no difference between the protection diodes, there is a noticeable difference in the voltage drop.  The measurements for the protection diodes going to Vcc on an ATtiny13a were as follows:
.631, .631, .633, .629, .628V
and from ground:
.560, .561, .538, .537, .538V

The measurements from an ATtiny85 going to Vcc and from ground were as follows:
.634, .612, .613, .633, .634V
.554, .555, .555, .551, .553V

The voltage drop across the ground protection diodes is close to the 1N4006, which would suggest a current handling ability in the hundreds of mA.  Although circuits that take advantage of the ground protection diodes are extremely rare, the Vcc protection diodes allow for simple level shifting.  An AVR running at 3.3V can receive 5V signal simply by using a current limiting series resistor of around 1K Ohm on the input.