Overthinking UART Pinout

Quite a few of my boards have a 4-pin UART output. And it’s always the same: [TXD GND RXD VCC]. But this pinout seems strange to quite a few people. I promise, there’s a method to the madness.

First of all, what is the most common UART pinout and why don’t use it? Well, the most common output I’ve seen on other people boards is [VCC RXD TXD GND] and I hate it because it’s really unforgiving if you accidentally plug it the wrong way. When I started playing with electronics, I used to use this pinout and, after frying a couple of boards, I’ve learned to triple-check that I don’t plug it rotated 180°. Even so, it still makes me nervous.

The other common pinout I’ve seen around is [VCC GND RXD TXD] and this one is slightly better as you can safely rotate it 180°. When it comes to common accidents (180° or off-by-one), it’s really hard to fry the board using this one. But this pinout also has its twin brother [VCC GND TXD RXD] thus ensuring you need 1x1 Dupont connector for each wire.

For my “standard” pinout I wanted to be able to easily switch TXD and RXD lines. Since UART really needs only three pins, it was clear that the first three pins had to be [TXD GND RXD]. This way rotating 180° allows you to connect two cables together allowing for point-to-point connection.

In order to provide power, I just added the extra pin for the final [TXD GND RXD VCC] pinout. If you accidently rotate it 180°, nothing happens. If you plug it off-by-one, nothing happens. And, if you don’t need power, you can plug only the first 3 pins and everything still works.

It’s as full-proof as it gets for me.

Calculating Mean for Microchip PIC

Averaging ADC readings on microcontroller is often needed but also often quite problematic. The easiest way to do it is remembering the last N values, summing them together and simply dividing by count. Simple, reliable, and uses a BUNCH of memory. If you want to store 50 16-bit values, you will need 100 bytes. When dealing with Microchip PIC, that’s a fortune.

However, if you’re ok with a mean rather with an average, there’s an easier way (and I would argue that mean is a better measure anyways in 90% of cases). You can use Welford’s online algorithm to get mean value using only 3 bytes of static storage and a few bytes more when executing. Of course, use of such algorithm in microcontroller environment will require a bit of a trade off (the least of them is me using average and mean interchangeably further down in post).

For my case, I assumed I need it for unsigned 10-bit ADC values. In the context of Walford algorithm, this means my intermediate values can be up to 15-bits (16-bit signed integer is needed internally) and this leaves 5 bits to use as a fixed decimal point. My case also called just “averaging” a handful of values, so I was ok with a 8-bit counter.

This means my variables are as follows:

uint8_t walfordCount = 0;
int16_t walfordMeanEx = 0;

As compared to the standard Walford’s algorithm, a few things are different. First of all, if we have more than N entries, it will reduce count to half over and over again. This trickery is allowing for “sorta” running average. And yes, you shouldn’t really use Walford’s for running average but it’s close enough. Secondly, when adding the value into the internal variable, we convert it to the signed integer and shift it 5-bits to simulate a decimal point. The only expensive (in CPU terms) operation here is dividing by count as everything else is quite cheap (shift, subtractions, and comparisons).

void walford_add(uint16_t value) {
    walfordCount += 1;
    if (walfordCount > WALFORD_CUTOFF) { walfordCount >>= 1; }
    int16_t delta = (int16_t)(value << FIXED_DECIMAL) - walfordMeanEx;
    walfordMeanEx += delta / walfordCount;
}

To get a value out, we just shift variable 5-bits back to remove the “decimal point”.

uint16_t walford_getMean(void) {
    int16_t value = (walfordMeanEx >> FIXED_DECIMAL);
    return (value > 0) ? (uint16_t)value : 0;
}

And that’s it. It’s a cheap (memory-wise) way to get an average on a 8-bit microcontrollers. As compromises go, I had worse.

In any case, you can download code here.

Why My Serial Port Device Is Detected as a Mouse

When designing electronics, probably the easiest way to make it communicate with a computer is to connect it as a serial port. However, occasionally you might see your device detected as a serial mouse instead of a plain serial port. When this happens the device will obviously not work and you might even see your mouse cursor jump around.

This behavior is an artefact of how Windows detect a serial mouse since that was written way before plug’n’play times. Widnows will first raise DTR and RTS signals and then monitor if there’s an M or B character coming at 1,200 baud. If either character appears, congratulations - your device is now a mouse. And anything it sends will be processed as if it was a mouse action.

There’s a brute force solution of disabling serial mouse in registry and that would honestly be the easiest way around. However, what if you cannot modify the registry? What if you need to make your device work in the face of an unknown machine? Well, there are certain tactics you can employ to either make this behavior unlikely or avoid it altogether.

The simplest solution of not sending M or B will unfortunately not work as easily. Yes, if your device works at the 1,200 baud rate, avoiding those characters will be fine. But if your device works at faster baud rate (and it probably does) the other end receiving at 1,200 will misunderstand its output. Depending when the actual bit sample is taken and what your device is actually sending, you might see an offending character appear either every time or every 10 times. Even worse, the frequency of the issue might change depending on the computer or even the room temperature. So, not really a solution.

The second obvious choice is detecting DTR and RTS combination and pausing whatever output your device gives until that state passes. If you can afford this, it’s a great and surefire way to sort the issue out as DTR and RTS are not baud rate sensitive. The only problem happens when you need those signals for flow control or for something completely custom. Even if you’re not using them they’ll still “cost” you two pins.

The most practical way I found of stopping this issue is by having device not speak until spoken to. If you design your serial protocol so that a device must receive a valid command before it will send any data, it will pass serial mouse detection unharmed. And, if you really need a streaming output (a perfectly valid requirement), just use a command to turn it on. If you can additionally somehow get CRC check into your protocol, the world is your oyster.

Framework Expansion Board

One of most exciting recent developments in laptop world for me is definitely the framework laptop. A major component of that concept are its expansion cards. And, of course, you can build your own.

This repository is quite encompassing if you’re using KiCAD. However, for those who love nicer tools (ehm, DipTrace), it’s annoying to find that there is no board size specification in human readable format (and no, KiCAD XML is not). So I decided to figure it out.

To cut the long story short, here are the board outline points for the expansion card PCB:

• (0.0, 0.0)
• (26.0, 0.0)
• (26.0, 26.5)
• (25.0, 26.5)
• (25.0, 30.0)
• (17.7, 30.0)
• (17.7, 28.0)
• (16.0, 28.0)
• (16.0, 29.0)
• (10.0, 29.0)
• (10.0, 28.0)
• (8.3, 28.0)
• (8.3, 30.0)
• (1.0, 30.0)
• (1.0, 26.5)
• (0.0, 26.5)

In order to make it slightly nicer to handle, each corner is additionally rounded with a 0.3 mm radius.

And let’s not forget two holes at (1.7, 10.5) and (24.3, 10.5), both with a 2.2 mm diameter and 4.9 mm keepout region.

With that information in hand, one can create PCB board in any program they might prefer. Of course, I already did so for DipTrace and you download the files here.

And yes, PCB is just a first step in a development process. What I found the hardest is actually getting appropriate connectors for the enclosure as there’s not too much height to work with.

PS: No, I do not own framework laptop at this time. I am waiting for 15.6" model as 13.5" is simply too small for me when not used external monitor.

Battery Pack Load

Powering device via USB has its advantages even if device doesn’t need connectivity. Most notable of those advantages is a wide availability of USB battery packs. That makes powering device really easy. However, what if your device uses so little power that batter pack keeps turning off?

All three battery packs I had available had the same issue. Put something that pulls just a trickle of current and they would turn off. One of them did have a trickle-charge functionality and that sorta worked but required manual intervention. And yes, I forgot to turn it on more times than I care to remember.

So I went looking for solution and found Dorkbot PDX article by Paul. The way how he solved this issue was quite a nice one. Just put intermediate load on the bus and you’re golden. Instead of going with his solution, I decided to use my favorite hammer - Microchip PIC microcontroller - to make my own board using the same concept.

Realistically, there’s no reason to use microcontroller for this. Having a programmable part just introduces assembly complexity and you cannot use it out of box, without programming it first. But there are advantages too. The major one being adjustability and that came in really handy when figuring out the timings needed. Instead of dealing with large value capacitances, one can use inherit time keeping mechanisms of microcontrollers. Furthermore, it also allowed for easy placing of blinky light. And who doesn’t love those?

The whole solution is based on a small 6-pin PIC10F200 microcontroller. When you discount programming pins, that leaves you one pin to work with. And yes, you can pretty much use programming pins too (3 of them) if you use a few extra passives but I generally leave them alone if I can. Fortunately, application I had in mind was simple enough to solve with a single pin.

The device logic is as simple as it gets. To keep the battery pack turned on, it uses MOSFET to pull 200 mA over two resistors for about 20 milliseconds. Once pulse is sent, wait for 10 seconds. Rinse-repeat. On all packs I’ve tried this combination worked flawlessly. Of course, if there’s a pack where this doesn’t work, adjustment is easy enough (yes, I will update code with some constants later :)).

The heavy lifting is done by MOSFET driven directly by microcontroller. Once turned on, it will allow two resistors to dissipate a bit over 200 mA of current. While this seems like a significant amount of power to pass over puny 0805 resistors, short duration means they will never get hot enough to present a problem.

One could also take an offense to how MOSFET is driven too. Due to the nature of the beast a lot of current will flow out of the gate pin and by common knowledge one has to have a resistor to limit it. However, this MOSFET is small enough that current limiters built-in to Microchip PIC microcontrollers are fine. I had even bigger MOSFETs work in circuit for years without gate resistor so I am not too worried. That said, if I ever change to different microcontroller, some reevaluation will be needed.

And yes, there is no bleed-off resistor on MOSFET gate either. If circuit is powered on, PIC will be driving it either low or high so pulling the MOSFET gate down is not really needed. If circuit is not powered on, who cares - there’s no power to go around anyhow. That leaves only the short amount of time when circuit is powered on and PIC is not yet driving the gate. And that is the interval we can safely ignore as it’s shorter than 20 milliseconds and even erroneously active MOSFET will cause no damage to the circuit in that time.

Thanks to Microchip’s semi-standard pinout, this device can use any number of PIC microcontrollers. As long as footprint fits (SOT23-6), you’re golden. I have tried it with already mentioned PIC10F200 and PIC10F320 but any variant should work too. Yes, the firmware did need a bit of adjusting between methuselah PIC10F200 and newish PIC10F320 but those were simple enough. In these times of chip shortages, having multiple parts fit is always a nice thing.

How much battery this uses, you might ask. It would be a bit annoying to have your battery drained by a device whose only purpose is to keep it awake. Well, to start with, we need to account for all power usage. The main one is of course our 220 mA pulse (5V/(47Ω÷2)) lasting 20 milliseconds. Since it repeats every 10 seconds, that leads us to a duty cycle of 0.2%. Multiply one by another and we can see that average power usage is about 0.5 mA.

Second power user is our LED that blinks with the pulse. Due to large value resistor, it will require slightly less than 1 mA for each pulse. Using our duty cycle, that us average consumption of 2 µA (0.002 mA). Pretty much a noise compared with our load.

And lastly there’s the PIC itself. Good news is that this is well under 1 mA. How much under? I have no idea as I didn’t have anything capable of measuring it. I will definitely need to create a board for that too. However, I did use USB power meter to get a long term reading of the usage and it was slightly under 0.5 mA (averaged over an hour) if using PIC10F320. For older PIC10F200 usage is a smidgen above it.

Those reading more carefully might wonder, if a power pulse needs about 0.5 mA, and total consumption is under 0.5 mA, we have the free energy as microcontroller is actually using the negative power from a far dimension to run itself. Sadly, it’s not so. Due to how MOSFET is driven, it won’t turn on nor it will turn off instantly. So our 200 mA pulse average will actually be lower. And PIC consumption is low enough to “hide” in those numbers.

Code actually runs constantly in the background due to a quirk of PIC10F200. If you put it in sleep, it resets itself (by design) making it annoying to keep track of time longer than 2.3 seconds. I was toying with idea of just using the newer PIC10F320 but power usage of constantly running PIC was low enough that I decided not to care.

Either way, if you have 10000 mAh battery, this device could theoretically run for 20000 hours. Suffice it to say that it shouldn’t reduce battery life too much. If you really want to squeeze the last possible mAh out of it, you could adjust timings. For example, my Anker power bank was quite happy with more than a minute between pulses (0.033% duty cycle). However the default 0.2% duty cycle is probably a good starting point when it comes to compatibility.

Lastly, there are actually two versions of the device. The “normal” one just plugs in USB port while pass-through has a female USB connector on it allowing it to be used on battery packs with a single type A output. You can find gerbers for both on GitHub alongside with the source files.

Dealing with part shortage - Microchip edition

For a small electronics project of mine, I needed literally only one output pin. My go-to part for these situations is PIC10F200. It’s a 6-pin SOT-23 device offering internal oscillator and not much more. Microcontrollers don’t really get smaller/simpler than this.

Due to the hamster habits I have, I actually had enough enough parts to finish up the prototype so the only thing left was to order a few more parts of DigiKey. Well, as many components lately, my favorite PIC was out of stock.

However, when one door closes, another one opens. Microchip is really good at keeping pinout similar over multiple microcontrollers. And DigiKey had PIC10F202, PIC10F206, PIC10F220, PIC10F222, and PIC10LF322 available in stock. While all these PICs are slightly different, they share the same basic pinout. And for my project any of them would do. Even if I used some less common feature, Microchip often has multiple products differing only in memory amount.

While hardware might be similar enough, firmware does have significant differences - especially between older PIC10F206 and newer PIC10LF322 setup. Even turning LED on/off uses different registers between them. Instead of having different firmware for each, one can make use of compiler directives and check which PIC is actually being used. Something like this:

#if defined(_10F200) || defined(_10F202) || defined(_10F204) || defined(_10F206)
    GP2 = 0;                // turn off GP2
    TRISGPIO = 0b11111011;  // GP2 is output
#else
    LATAbits.LATA2 = 0;    // turn off RA2
    TRISAbits.TRISA2 = 0;  // RA2 is output
#endif

While out-of-stock syndrome has hit Microchip too, with a bit of care, they do make transition feasible if not always trivial.

Adding USB to A-BFastiron SS-305MP

I needed a cheap power supply for a project and it was easy to find a nice one in A-BFastiron SS-305MP. It was small enough, looked good, and had shiny display. What could man want more?

Well, when I got it and saw cutout for USB, I know what more I wanted. An USB port.

And strangely enough once you open the power supply, you’ll find connector providing about 8.2 V already there without anything to plug into. It’s almost as if somebody placed it there to be an input for 5V linear voltage converter and then later figured electronics and heatsinking would cost too much and covered the hole. And yes, it’s a proper hole cover that you can remove - no drilling necessary.

If you open power supply you will even find standoffs already in place. It’s simply begging to have PCB mounted in.

First thing to figure out was which USB connector will fit. Searching on DigiKey found quite a few of them roughly matching the dimensions. So I just selected the cheapest one that matched standard footprint. And yes, looking on side you might find it protruding a bit too much but not criminally so. It might be original designers were fine with this or the had a custom length connector in mind. For me this was as good as it gets.

With connector found, it was time to figure PCB. And I decided to keep it really simple. The whole setup would revolve around VXO7805-1000. It’s a nice DC-DC switching regulator that will take any input higher than 8 V and drop it down to 5 V with some efficiency. In its pinout it emulates beloved LM7805 but at 90% efficiency and without all the heat.

Regulator itself requires just two capacitors and I decided to go just with them. I was tempted to add a smaller 1 µF capacitor to output and maybe even a 100 nF one for decoupling purposes but decided against it. Due to wide variety of cables and outputs USB device might face, all of them already have more than sufficient decoupling and adding more wouldn’t really do anything. So why waste a component.

The only really unneeded components would be an LED and its accompanying resistor. While they serve no function, I really love to have an indicator of output. If there was ever an issue, looking at LED would at least tell me if power is going out. And quite often that’s quite a big help.

Speaking of power going out, I don’t consider a fuse optional. It’s a minimum you need in this setup. Another thing I would consider bare minimum for power supply would be a short-circuit detection but that’s fortunately already a part of voltage regulator. And yes, I could have gone further, especially by adding reverse polarity protection to the input and I was tempted but in reality you’ll just connect this thing once and leave it connected. As long as you connect it correctly the first time, you’re good.

Connecting all this to the power is entrusted to any JST-XH 2-pin cable - 10 cm in length. Just make sure that the negative wire is going next to COM marking on the power supply motherboard. If in doubt, just double-check with voltmeter.

And that’s it. For a few bucks more and some extra soldering, we have a nice 500 mA USB port at the front of the power supply. Just in case we need it.

On GitHub you’ll find source files and releases with gerbers and part list.

Changing A-BFastiron SS-305MP Binding Posts

I needed a cheap power supply for a project and it was easy to find a nice one in A-BFastiron SS-305MP. It was small enough, looked good, and had shiny display. What could man want more?

Well, how about proper binding posts?

And no, I am not only talking about quality albeit one coming with it are quite flimsy and it already arrived with one cracked. I am talking about spacing. I simply hate when binding posts don’t observe standard ¾" distance between them.

And this power supply almost had it right. I measured spacing to be a smidgen over 20 mm while standard would call for 19.05 mm. With such a small difference, there was literally no reason to go non-standard. But non-standard they went.

If you open the power supply, you’ll see that binding posts are held by the PCB in the back. Thick wires are soldered onto it and nuts are used to connect to posts themselves. So the whole operation can be done with a simple PCB update with correct spacing. Only thing needed extra is a bit of filing action and you can reassemble it all.

However, since my binding post was already cracked, I decided to swap them for Pomona 3760 (black and red) set. But that brought another issue - panel cutout for them is completely different. And yes, a patient man might shape it enough, but for those with 3D printer there’s an easier solution.

To mount it all, I used some nice red MH Build PLA to print really tight mounting base and spacer for posts.

After filing plastic a bit to expand holes toward each other, I placed binding posts into the printed base, pushed it through the hole, used another 3D printed spacer on inside and added some more height to set using spacers that came with binding posts themselves. Then in goes the custom PCB and finally all can be fastened using lock washer and nut that cane with posts.

Result are nice binding posts at proper spacing. :)

On GitHub you’ll find source files and releases with gerbers and part list. 3D model can be found on TinkerCAD.

PS: The only downside of Pomona is that it uses ¼" imperial nuts while the power supply originally had 7 mm nuts. So, in addition to metric socket set you already have, you’ll need a witchcraft-sized set too.

Well Grounded

Playing with electronics as a hobby has its advantages. Most notably, I don’t need to deal with high-speed signals or EMC most of the time. However, in the days of faster and faster I/O, high-frequency content “sneaks in” whether you want it or not. Just because your microcontroller works at 48 MHz, that doesn’t mean your I/O edge is not (much) faster. And sorting out those issues is hard.

Fortunately, there are many “rule-of-the-thumb” guides out there, but I found none better than Rick Hartley’s. Well worth the watch.

DKRed

While I use OSH Park most of the time, I always like to look at different services, especially if they’re US-based. So, of course I took a note of DigiKey’s DKRed. Unfortunately, review will be really short as I didn’t end up using it.

On surface it looks good. Board requirements are reasonable. If you want to use DKRed service you need to have your design fit 5/5 mil minimum - more than sufficient for all I do. While more detailed specifications are available for other PCB vendors on their platform (albeit Royal Circuits has a bad link), there’s nothing about DKRed itself. Yes, I know DKRed might use any of manufacturer’s behind the scene but I would think collating minimum requirements acros all of them should be done by DigiKey and not left to a customer.

And not all limitations are even listed on that page - for example, fact that internal cutouts are not supported is visible only in FAQ and whether slots are supported is left as a complete secret. Compare this to OSH Park’s specification and you’ll see what I mean.

But ok, I went to upload one small board just to test it. And I was greeted by error that length is shorter than 1 inch. As I love making mini boards smaller than 1", I guess I’m out of luck. But specification page did correctly state that fact so I cannot be (too) angry. Never mind - I’ll try a slightly bigger board. Nope - it has to be more than 4 square inches in area. Something that I didn’t find listed anywhere.

Well, I had need for one board 65x72 mm in size - that one would work. And yes, DKRed finds that board OK. But cost is $43.52. OSH Park charges $36.25 for the same board. And yes, DKRed gives 4 boards ($10.88 per board) while OSH Park only provides 3 ($12.08 per board) so it’s slightly cheaper if you really need all 4 boards. If you’re hobbyist requiring only 1 board like me, you’re gonna pay more.

And this is where I stopped my attempts. Breaking deal for me was the minimum size as this makes it a no-go for most of my boards. And cost for just a prototyping is just too high. Mind you, it might be a good deal for people regularly working with bigger boards at a small quantity. But it’s not for me.