Pancakes for ... Lunch?

2017-07-08 11:55 - General

Mmmm, pancakes.

When you wake up late on the weekend and your first meal is at noon, is it breakfast or lunch? Who cares when it's pancakes!

Cable Hangers

2017-04-09 18:53 - Making

The first cable hanger, holding mostly micro- and mini-USB cables.

The second cable hanger, holding mostly portable game system charging adapters, and a few miscellaneous things.

A while ago I came up with an idea which I finally brought to fruition this weekend. I've got a lot of cables, mostly USB cables. They generally sit in messy piles and are difficult to find. This is a system to hang the cables on the wall so they remain out of the way, but also easily found.

There are two locations with plenty of wall space in my apartment which are also very out of the way, and they now both hold one of these three foot long organizers, for a total of just over sixty spaces to hold cables. Most of which are full!

It started with some very cheap mini clothespin style clips. Then a one-by-two board with a small groove routed into it. The clips are glued into that groove, then the board is screwed to the wall with a couple drywall anchors. That's it!

One (pictured left) holds mostly micro USB and mini USB cables, with a couple USB extensions and full size USB cables. The other (pictured right) holds mostly charging adapters for various portable game systems, but also some wall adapters, earbud headphones and a couple USB C cables as well.

Now I just need to come up with a way to straighten all those cables, so that it looks neater!

PCB Fab Review: EasyEDA

2017-03-30 22:39 - Making

Following my two previous such posts, EasyEDA reached out to me requesting a review of their service. Disclaimers up front: First, they offered a $50 coupon so I paid only the shipping for these boards, but this article remains my own unvarnished opinion. Second: I did not use their design tools, I already know KiCad and can't spend the time to learn a new one. This is review is about only the PCB fab service, so point one is that they support both modes (use their design tools, or upload your own produced separately).

Three revisions of my wi_ther board.  Rev A by Elecrow, rev B by Seeed, Rev C by EasyEDA.

To the right is a comparison of three revisions of the same board. The green board on the right is the back and front of the board made by EasyEDA, with a PCB ruler showing centimeters besides that. Click to embiggen, open in a new tab to go even bigger.

The Good

The quality of the boards I received was very consistent. And quite good overall. There was the occasional light scratch, and a semi-consistent sort of wrinkle in the solder mask, but they are only evident under close inspection, never deep enough to cut through the solder mask. The silkscreen is bold and legible, even under a microscope it's consistent and solid. Compare especially the line around U4 to the rev B board in the middle, whose silkscreen is spotty and hard to read. Routing of the internal cutouts is also quite good (although I intentionally rounded as many corners as I could in this version, after experiencing this in rev B). The square corner cutouts around U3 show the worst, still minor, issue here: one corner has a bit of over-cutting.

The Bad

There's a fair deal of dust, especially visible along the right side of M1 on the front-up board at far right. Easy enough to wipe off before assembly. Then of course those darn added part numbers! I specifically put instructions to place the number inside the outline of M1. That will be covered by a part once assembled, becoming invisible. Instead they put it right smack along the edge of the front layer. There's hardly a more visible spot.


Cost was very good. I also got a stencil, which due to size/weight forced express shipping. So I'll do a separate cost comparison, for this 99x33mm board, cheapest options including shipping:

QuantityEasyEDASeeed Fusion 1Elecrow 2Dirty PCBs

1 Seeed had a (temporary?) $5 off shipping special, in these prices.

2 Always the "special two layer, 10qty" service. Edit to multiple quantity of that in the cart for 20/50 total quantity.

When I first put this chart together, EasyEDA had the best price after shipping at anything but quantity 10, for this board. I then had the idea to use multiples of Elecrow's "special" service, which brought them down to the same price range. It's worth noting that isn't clever enough to try this. If you're cost sensitive, invest a little time into exploring all the options offered from several fabs.

I've got a different larger board that I'm working on (149x68mm) and I only need one of it. In this case five from EasyEDA is easily the least expensive among several fabs (coming in about 25% cheaper than the next lowest option). Once again it's worth checking several sources, and even designing to fit into a particular envelope which is efficiently priced if you can. For this larger board, it has to be that size.


These boards are perfectly good, and also very inexpensive. They match or beat the quality of the previous revisions from other fabs, for similar or lower costs. I'm very happy with this service. That said, when I'm not getting the service for free I'll probably pay the premium to not get extra part numbers added to hardly-controllable usually-too-visible parts of my boards.

Remapcro: A Hardware Keyboard Macro Recorder

2017-01-21 15:39 - Making

The assembled boards, with keys, top view. The assembled boards, with keys, bottom view.

I posted once recently about the prototype of this project, but as of today I've finished the first real one. The name Remapcro is a portmanteau of remap and macro, because that's what it does. It's an array of extra keys onto which macros can be stored, and then replayed at a touch. It works by sitting between your keyboard and computer. Normally key presses are passed straight through, but when recording a macro they are stored and then later replayed.

I made a handful of small errors along the way, but none were fatal. The worst is in the picture on the right, above. There are two PCBs to hold the electronics. On top one holds the key switches and the diodes which help make the keyboard matrix easy to read and error free. Then a second smaller one, holding the "smarts". There are a couple connections between the two of them, and the cross-wise one (top to bottom, at the edge of the smaller board) is off by one space. I ended up mounting the smaller board "upside down" and even though I thought I had the connector perfectly centered, it was off by a tenth of an inch. Thanks to the size of the key caps, there was already some extra room in the case I designed, and cutting a bit of the corner off the smaller board helped squeeze everything in anyway.

There's also one LED tucked into the top-left key (bottom left in the left picture above). It's actually three LEDs (red, green, and blue). And specially designed to fit exactly into these key caps. I could only find it in a lot of 50 from China, and I was too impatient to wait for it to arrive (and carefully check my design against it) before I ordered the PCBs. I ended up needing to put it in backwards, trim off a bit of the bulge that now sticks out rather than into the slot it's designed for, and re-wire the connections a bit.

Fully assembled Remapcro, front side view. Fully assembled Remapcro, rear view. Fully assembled Remapcro, rear side view.

The case the boards fit into was a challenge both to design and to produce with my 3D printer. But (perhaps aside from the screaming orange color, that's what was easily on hand for this first test run) I'm quite happy with it. A couple pieces slot together at the front, then a screw holds the back on, wedging everything together. The only problem was getting the back piece sized correctly, especially with all those holes. It's still not perfect, but it's functional. And if you were curious about the shape and arrangement of keys:

The first Remapcro, in-situ.

Of course, it's designed to look like it belongs, sitting next to a standard PC keyboard. I hope to re-design the case, ideally with more rounded corners and tweaking the alignment of a few things. Plus, supporting the insides better: right now it's only supported at the extreme front and back edge. This is mostly fine, but it can flex when pressing the keys towards the middle, which would be nice to avoid. I'd really like to fit everything onto a single board. That little space between the top and second rows could probably just fit all the electronics, but the problem is the connectors. With all the keys poking holes everywhere, not much room is left to mount the USB in and out ports. Who knows, maybe I'll come up with a brainstorm.

Geeetech G2S Pro Power Supply Cover

2017-01-16 17:12 - Making

The cover which I designed and printed for the power supply for my 3D printer.

Back in October I finally decided to get a 3D printer. I got a low cost kit, and one of the glaring issues is the power supply. As shipped, there were loose wires between the plug, the switch, and the supply. No bare conductors, but still not awesome. The switch would just flap in the breeze. I saw some similar designs already made online but none of them really satisfied me. I decided to design my own. It took some time before I could successfully print it, but it's finally done, much safer now not to mention easier to use!

I've shared the design files on Thingiverse. The set of hexagon holes were designed partially to save material, but mostly to try to reduce curling. They weren't sufficient, which is why I only now finally got it printed successfully. I had to get my bed leveled very well plus use a brim, to get this successful print in ABS. The PEI print surface was probably also required; it helped at least.

PCB Fab Review: Seeed Studio's Fusion

2017-01-08 23:20 - Making

In early November, Seeed Studio contacted in regards to my PCB fab comparison post from 2015, asking that I also review them. After a short discussion they agreed to give me a $50 service credit in exchange for a review. This post is that review, of the Fusion PCB Prototyping Service.

Board One

Several copies of a simple square PCB manufactured by Seeed Studio's Fusion service, with issues marked.

I had two boards fabbed, plus a metal solder paste stencil. Let's get into the first board. This is for an incomplete project which I won't describe yet here, instead just covering the quality of the PCB.

They're fully routed, which seems to be common from Chinese fabs. There's one tiny "nipple" of extra FR4 consistently in one spot. I assume this is the very end of the tool path which somehow isn't quite completed. This seems to be common for individually routed boards, as I've seen this sort of issue from three separate (low-cost) fabs.

The good: there's both plated slots and non-plated holes, and they came out as desired.

The bad: a few boards have extra blobs of silkscreen scattered about, see the left two. In consistent locations, and there's two more boards (not pictured) with slightly more faint spots but again in the same locations. Those two I might have missed if it weren't for these very obvious two pointing out the locations. Several of the boards have scratches, more visible in person than in the scanned image here. One of them has a bit of solder mask missing from a spot over one of the traces. There are several blemishes in the solder mask, most too subtle to notice in the scan but one (on the bottom right pictured board) is severe. Perhaps one or two of the ten boards arrived without some form of blemish or scratch.

The ugly: order numbers added to the silkscreen layer. On this board, it's at the top right of the front layer, near the power connector. It seems they put a tiny bit of effort into hiding it under a component, but the presence at all is disappointing. I'd normally check this sort of thing ahead of time, but I selected Seeed because they approached me, not because of my normal evaluation process. I only got to it now that I have the boards and I'm taking the time to write a review. I went looking, and I found as the last entry in their FAQ the Seeed Production Code will be printed on empty space of your PCB page. Certainly could have been more up front with it. I didn't mind when Dirty PCBs did this, they're transparent about it. A way to hint to them where the printing would be least obtrusive would be an improvement. (In this case the fat DIP pads in the middle will be fully covered by a part, and anywhere on the back would be better than almost anywhere on the front.)

Board Two

Another more complex board, manufactured by Seeed Studio's Fusion service.  Again, issues highlighted.

This second board is a revision of my environmental sensor project. It's mostly the same, but a USB/UART chip was added along with a few other small tweaks. I'm putting a front and back copy each of the first revision into the picture, for comparison. They're in the bottom right, clearly identifiable as the pair with a different color.

The good: Seeed's blue solder mask is actually blue! More of these boards came out okay: I put four of ten in my defect-free pile, as I was laying the rest on the scanner bed to show. All the issues I've got to bring up next are generally minor.

The bad: Still several defects, including scratches on lots of boards. One, pictured in the mid right, goes across several holes, deep enough to expose copper. Two of the boards, but one really in particular, had a surface finish issue, the whole thing being mottled. The silkscreen shows stippling everywhere on this board. Side-by-side with the rev A board (the bottom-right two) it's nearly unreadable by comparison. One of the boards has a stray speck of metal (solder, surely) sitting off in the middle of nowhere. Thank goodness it's not shorting something. Another has a foreign object in one of the through holes.

The ugly: this board cares about thermal properties, so it has several internal cutouts to help limit heat transfer. The Seeed boards are all routed sloppily. Most of the inner holes have at least one corner where the cut goes a bit too far, creating an extra divot. And still those order numbers in the silkscreen. Again sort of hidden under a part, at least they tried.

A curiosity: Silkscreen between fine pitch pads. Both rev A and B as well as my old prototype include the exact same TSSOP part (U3), with 0.65mm between pins (center to center, 0.40 wide). As best I can tell, the silkscreen gerber specifies one wide empty area across these pins in both revisions. But:

Comparison of the fabbing of the same footprint by three PCB fabs.  Left to right: OSH Park, Elecrow, and Seeed Studio's Fusion.

That's the same footprint manufactured thrice by three different fabs. On the left is the prototype by OSH Park, in the middle rev A by Elecrow, and on the right rev B by Seeed. It's the same "Housings_SSOP:TSSOP-8_3x3mm_Pitch0.65mm" default KiCad footprint in all three cases, with no custom clearance setting. The mask clearance ends up overlapping into one large rectangle covering each of the four pins on either side. That's just how OSH and Seeed fabbed it. But Elecrow actually filled in the mask between these pins anyway. I can't fault Seeed for fabbing the board as specified. Overall I'm not excited about the idea of the fab changing the design on my behalf. In this particular case, I like what Elecrow did, however. Perhaps I'd never have noticed if I wasn't scrutinizing the boards for this review, as I never had any trouble soldering them, even completely by hand.


A close-up view of the silkscreening on a PCB fabricated by Seed Studio's Fusion service.

I've laser cut my own stencils out of plastic before, but they've only worked okay. For roughly $20, and with a $50 coupon, I decided to order a metal one to see what that was like. Board two has several surface mount parts, and I added a QFN part in this revision.

There's not actually a lot to say here. The measurements were listed online in the order page (28 x 30 cm) but it was a case of ordering sight unseen over the internet and not truly understanding the size. I ended up cutting it down to make it practical to use by hand, and then it worked very well.

I have now used it once, and it was a fine experience. Especially including the narrow pitch QFN leads, which worked great the first time. But then, once I was done, I whipped the board under the microscope to check my joints. The soldering was fine, but up close, the silkscreen's quality issues were clearly revealed. Was the inkjet under-extruding? I can see that it's a bunch of separate dots, which do not connect to form any sort of line. The upper case letter "I" in "IO" is especially bad, sometimes one-and-a-half sometimes just one row of dots not quite making a line. (This is visible in the comparison shot of U3 above, as well.)


The order I made for everything reviewed above was two PCBs, 10 qty each, and one metal stencil. That's $9.90 x 2 plus $19.90 which seemed a reasonable $39.70. But that weight/size came to $84.20 total after shipping (!). If I try again without the stencil I get $34.89 total. The same no-stencil order comes out $32.84 at Elecrow.

For an order of just board one, Seeed quotes $9.90 for a basic board (1.6mm FR-4, 2 layer, qty 10, green mask, HASL, 1 oz copper). Great. But go through checkout and you'll discover that the cheapest shipping is $12.68, for 10-18 day service (3-5 day costs $19.43). Elecrow charges $11.50 for their 5x10cm service with otherwise identical specs, but they have a shipping option at only $6.60 (their 2-3 day costs $18.05). If you're patient that's $22.58 from Seeed, or $18.10 from Elecrow, and a wider difference if you're impatient. And if I were ordering there, I'd have squeezed into 5x5cm and saved another $2.

Which is another subtler issue. Elecrow puts their size-based pricing right up front. I've definitely designed to a 5x5 or 5x10 cm envelope before. But Seeed just has you type in your size and only then tells you the cost. It might be that this gives you more/better choice for what to do and what it will cost, but it's very hard to know what your board will cost ahead of time, or to know how to focus the design phase to optimize cost. Where is the line? Let's compare some known prices at Elecrow to what Seeed would charge:

Size (cm)ElecrowSeeed

A lot of data to say that they're both competitive, always within a few dollars of each other. When I use Elecrow's size increments. I've got another practical board in progress, it's 41 x 130 mm and external concerns force this particular size. At Elecrow I'd need to buy the 5x15 cm service at $23.48 for five copies. For five copies at this size Seeed charges $16.84, which is a significant difference (roughly 28%). But since Seeed will only tell me the final cost of a board after selecting all options, it's hard for me to take advantage of these possible savings.


For this review I've pulled out another set of two boards I ordered from Elecrow in late 2015. (A failed (suspended?) project which I won't go into...) They're of similar sizes to the Seeed boards reviewed here, and when looking at them with as critical an eye as I did when I started writing this detailed review, I did find two small scratches, out of eleven boards. But I did not find any other issues, most importantly no order numbers added to the silkscreen. The prices are very similar, and I can predict the price easily ahead of time.

If I don't need more than three and my boards are small, I'll definitely go for OSH Park. If I've got bigger boards or I want more than three copies though, Seeed probably won't be my go to in the future.

Remapcro Keyboard on the Breadboard

2017-01-02 17:38 - Making

The final (?) prototype of my Remapcro keyboard project, on the breadboard.

Here's the probably final prototype of my Remapcro project. I don't think I've mentioned it (here in writing) before, so: long ago I had a Gateway Anykey keyboard. It had an extra row of Function keys down the left, plus a "Record Macro" button in the top right. Tap record, any target key, then any series of keys (and finish by tapping record again) and the macro is recorded. Tap the target key and now it replays the whole series that you typed the first time. It's a small thing, but it can help to iron out tons of small tedious tasks. For example, while I was working on this schematic I had a need to rename a couple dozen items to add a "k" in front of the name. It was several keypresses to do so, but I used the prototype itself to record a macro, then it was just one keypress, and some pointing with the mouse, all done!

Anyway, I'm working on recreating that same functionality with a piece of hardware. This is easy with software, but the extra row of keys from the Anykey is what makes it really useful, somewhere to put the macros. All the keys on my keyboard already do something! In the picture above at the bottom left is an Arduino Pro Micro, which already has the ability to masquerade as a USB keyboard when hooked to the computer. I'm pairing it with a USB Host Shield, top right in the picture, to read USB keyboard input, and pass it to the Arduino. Normally it is passed through unchanged, but when the record macro button is pressed they're also stored, so that they can be replayed. The chip on the bottom right is the flash storage for that.

This is actually the second breadboard prototype. The first was on a full size Arduino Leonardo, with full size USB shield. It was easy to work with, and the hardware was specifically for that prototype. The array of (six) buttons was going to stand in for the extra keyboard keys, but I'd have to do something unusual to access the extra I/O ports on the host shield, which I'm using for the buttons, and I'd like to leave that bit of hardware free for the real thing, so I've skipped that. I did in fact discover several small things (or maybe, one thing and extra things I created in my confusion, trying to find and fix that one thing ...) but now everything else is working here, so I'm confident enough to spend the time and money to put together the final version with real keys and all.

One of the things I got wrong was setting my logic analyzer to too slow a sampling rate. That made so many things look to be completely wrong, and sent me on several wild goose chases. Never again! I hope ... I also broke out the 8-SOIC socket, to hold the flash chip, for the first time. Very nice, to put this surface mount part on a breadboard. I'll have to get more shapes and sizes of those.

Schematics for my Killer Queen Tapper Box

2016-12-11 16:56 - Making

A few months ago I made a "tapper box" to share with the local Killer Queen arcade game community. A side effect of the way the game is played means that your ability to tap the button both very quickly and very consistently goes a long way towards your ability to play well. It was great fun and enjoyed by many. This first box was a spur of the moment weekend project. I took a LCD shield and an arduino, grabbed a button from a video game store and threw it in a box from radio shack with a pile of batteries.

At some point after learning about the nationals tournament this year and decided it would be fun to make a few more to give away there. This involved a bit of redesign in order to be something that I could make several of (rather than just the one) and to be sure the parts would be cheaper (as I'd be buying them rather than mostly using extras just lying around).

The Design

Understanding the LCD shield. Hand drawn schematic for the Tapper 2.

First I looked at the LCD shield and understood how it worked. There's power on the left, and more power on the right, dedicated to the LED backlight. In between is signals: contrast adjustment, various control lines, and four data lines. With that understood I could build a simple unit based on the LCD only, rather than the larger and more expensive shield I used the first time.

Then I started laying out both the schematic and the parts. The battery is a holder for two AAs, which goes to a switch, which goes to a 5V boost module. This provides power for the Arduino (a Leonardo Pro Micro) and the 1602 LCD. At the bottom of this sketch I began figuring out how to connect everything: the sixteen pins of the LCD across the top (notice the four empty spots in the middle, this LCD can be driven with either four or eight data lines, and we're just using four here) connect to power and the Arduino. Pins 4 through 7 of the Arduino hook to data pins 4 through 7 of the LCD, Arduino 8 to the LCD's RS (register select) pin, and Arduino 9 to the E (enable) pin.

The Build

The first assembly test of a Tapper 2, checking the depth after assembly. Four of the five Tapper 2 boxes mostly assembled.

Here's the first one partially built, inserted into a failed (bad spacing) face plate I still had from the original as a test for size. I needed to verify my ideas about exactly how big they would be. Then the only other picture I took, when I had four of the five boxes made. There you can see the letters A, B, C, D, E hand written on the perf boards. That is the order they were made in, and you might notice that I got them neater and more compact by the third try! The cases were 3D printed, the insides were all hot glued in and the case front and back were super glued together.

The Source

For those who are interested I've published the source on GitHub.

Making One

If you're interested in making one of these for yourself, there's an easy way and a hard way.

First the (mostly) easy way: Start with almost any full size Arduino. You can get the Uno from (e.g.) Sparkfun or a cheap clone from China. Then get an LCD Keypad Shield, again available both from Sparkfun or China. These should plug right together, one atop the other. Hook the Arduino board to your computer and install the Arduino IDE to build and flash the tapper.ino program linked above. Hook any button's two terminals between the Arduino board's GND and 2 pins. Now it's up to you to figure out how to provide power (your computer can do this) and how to hold it all together. If you have access to a laser cutter, you can use face.svg from the source linked above; it's designed to fit into Radio Shack's 6x4x2 inch enclosure.

Next the hard way. Get an Arduino Leonardo Pro Micro and a 1602 LCD module. Hook them together like my crudely drawn schematic above. Power can be added in several ways, I used a 2xAA battery holder into a 5V boost module, with a switch between the battery and booster. Assemble it all onto something like the perf board pictured. The same button and case concerns apply as for the easy way.

Good luck!

A Quick RFID Hack

2016-12-11 16:01 - Making

I recently became interested in an RFID-based project. In actuality, revamping an existing project incorporating RFID reader(s). In truth, the application wants five separate cards in five separate (nearby) locations to be readable at roughly the same time. Today they're using what appears to be a RDM630 module, with some electronics to switch in five separate antennas, and this is slow. (Power down, switch in an antenna, power up, wait long enough to be sure the card would answer if it was there, power down, switch in the next antenna, repeat x5.) It would be very nice if it was more responsive.

The thing is that the RDM630 is a "no name" brand product, with documentation scattered and spotty at best. What they've got in common is a set of three headers. On the main five pin header: two of the pins are power, one is unused, and the other two are consistently labeled as the TX/RX pair for a UART. A two pin header goes to the antenna, and a third is both the power and signal line to drive an LED indicator.

These RDM630 modules all speak 9600 baud UART, and for my application the problem is that they all simply output the ID of the card exactly once, when it appears in range. I can't control that timing, so I'd need to be prepared to read from five separate readers all at once. It's not easy to have five UART inputs all working in parallel with independent timing. But there's an RX pin labeled in addition to the TX pin, might it be possible to put these things into a mode where they could be queried on demand, with timing I can control?

These modules appear to be an implementation of a common design for low frequency RFID readers floating around the internet: a microcontroller, an op amp, and some glue. There's no reason that the microcontroller couldn't be programmed with further abilities. But I can't find any word anywhere about what they are, if they exist.

An RDM6300 module.

There's also a related module with the same footprint called the RDM6300, with at least two variants. One of them has an extra four-pin header compared to the otherwise common pinout, in the top middle in the picture just above. I managed to convince myself, by pictures available online, that I knew this was the debugging port for the microcontroller. In these variants it was always a Silicon Labs F330, an 8051 variant. So I got one of these RDM6300 modules, and found a cheap SiLabs USB Debug Adapter from eBay. My goal was to dump the code and discover more about how it works, especially if there is an existing way to ask it to behave differently over UART.

Before I continue, for posterity the pins are (from left to right starting at the square one in the picture above): 1 GND, 2 Vdd, 3 C2CK, 4 C2D, which are the power and debug pins for the SiLabs C0851F330 microcontroller (see page 37). All of the pins besides GND lead to vias, but they're trivial to trace through the other side of the board to the vias that pop back to this side of the board where the microcontroller is. It's got pin one on bottom left, and the bottom right three are the three pins in question (besides GND). Note that it's a 3.3 volt part, the regulator is right next to it which drops the 5V "normal input"; this part is also 5V tolerant on all its pins.

I have dumped the flash, and disassembled it. The disassembly seems imperfect, especially all the addresses in the middle of instructions. But there is clearly:

X0db1:  mov     sbuf,r7         ; 0db1   8f 99      ..
X0db3:  jnb     ti,X0db3        ; 0db3   30 99 fd   0.}
        clr     ti              ; 0db6   c2 99      B.
        ret                     ; 0db8   22         "

Exactly one place that the sbuf register is referenced, and it seems pretty clear to be a "write the byte in R7 to the serial buffer (memory mapped) "register", loop until the transmit bit is set, clear it, return" routine, which makes total sense for a UART transmission. There is no corresponding read from sbuf, so the default firmware will not, for example, respond with the ID when queried over the UART. This write-to-UART routine appears to be called (lcall X0db1) in three places, all within 20 instructions from each other.

In theory perhaps I could patch the firmware to put such functionality in at this point, but there are challenges. First would be writing such code, in assembler. I could probably write it in C, compile that, and get the assembler that way. Then I'd have to insert it into the current firmware somehow which is even more difficult. Where is the main loop? How do I insert it without breaking existing functionality? And how do I fit it? The dump I've got is 7,681 bytes of a theoretical 7,678 maximum, and the clearly empty bit at the end starts at 7,603, leaving only a tiny bit of space.

In the mean time I've found the MFRC522. It operates on a different kind of RFID tag, but already supports the sort of slave mode "speak only when spoken to" protocol that makes it easy to interface with several at the same time, so I'm continuing by working in that direction.