Category: electronics

Recently my Kindle (Paperwhite 3, 2015 edition) had a mishap wherein it took a trip through the washing machine. After it dried out, I tried booting it up again only to get a “you should really go repair this” graphic on the display and it was toast. I took the opportunity to upgrade to the more recent, somewhat waterproof edition.

Meanwhile, I wondered if I could do something with the e-ink screen on this old one. After manhandling it open, I found I could get it back on after disconnecting and reconnecting the battery and shorting the traces where the power button used to be. So, naturally I rooted this thing.

Besides trying out KOReader and the like, one possible use for the device would be as a very low power black-and-white photo frame, so here’s a picture of one of the kiddos on the screen. Perhaps encasing this in a better frame and setting up a cron job to cycle images every few days would make a nice build.

I recently picked up one of these $10 CH341A USB flash programmers and a test clip. I didn’t really have any initial plans for that it but thought it might come in useful someday while messing with router bootloaders. This unexpectedly turned out to be a lot of fun so far and not a bad tool for the price.

Rummaging through my parts bin, I found a Winbond 4K SPI flash of unknown provenance, hooked it up, and read out the contents with flashrom. Turns out this was an AMI BIOS that I pulled from some motherboard way back when. Disassembling the BIOS might make a good rainy day exercise.

I tried using chavrprog to program an ATTiny13, which did not work at all. The chip did not appear to respond to any of the SPI commands. Oh well.

On a bricked router I had sitting around, I read the flash directly from the board using the test clip. I happen to remember the confluence of events that lead to me bricking this (writing bootloader to wrong partition), and could finally verify the damage on the flash dump. When I get a free moment, I’ll try bringing it back to life by reflashing a repaired image.

On a different model, unbricked router, reading out the flash on the board didn’t work. Applying power to the flash chip powered up other components on the board, so I’m guessing I’d have to desolder this flash chip in order to read/write it. Maybe worth retrying after spending a few weeks practicing SMT rework.

In the previous installment, I was messing with the serial library on Arduino to get it to work on the older board. The purpose was this:

Pictured is an RFID reader from SparkFun that I picked up years ago and sat in my parts box since then. This RFID reader is intended to read Mifare tags and as such predates the whole NFC thing by a year or three. The programming interface is to send commands over serial to the device such as “Authenticate sector N”, “Read sector N”, etc. I’ve connected D7 to D2 so that I could use D2 as the interrupt pin to suit my Arduino.

It appears to work, in that I can dump sectors from my yubikey and a tap-and-go credit card. The sector trailer parts look valid, and the first few bytes of sector 0 have the device ID (verified by the NFC reader on my phone). I can’t seem to read any NDEF information; whether that is due to limitations in the reader, bugs in my code, or my almost zero knowledge about how all this stuff works, I’m not sure. Sketch code is over here.

As to possible applications, for that I have no real idea either, which is probably why it sat in the parts bin all this time.

As mentioned before, I have a really old Arduino, the Arduino NG with an Atmega8 on board. No fancy embedded Linux wifi-enabled SoC on that thing.

Recently, I found myself doing a bit of yak shaving when I was trying to use a code that was meant for Arduinos of a more recent vintage. And here is what I learned:

The Atmega8 does not have the PCICR (which I believe stands for “pin change interrupt control”) register. This feature allows multiplexing of interrupts so that you can have multiple pins signal the same external interrupt, and the pins that trigger interrupts are programmable. On the Atmega8, you can only use Arduino pins D2 and D3 to directly signal interrupts 0 and 1 respectively.

So the device I was trying to use operates over serial, and the supplied code uses the built-in SoftwareSerial bit-banging serial library to talk to it.
The SoftwareSerial library attaches an interrupt to the configured Arduino RX pin (device TX) to trigger when it goes low, indicating the start of a transmission from the device. This library was only written with the more configurable chips in mind, so didn’t support direct interrupts like on the Atmega8. Google didn’t turn up any obvious fixes other than people running into the same compiler error I initially hit:

error: 'PCICR' was not declared in this scope.

In the end a few short changes are needed to make it work: one to disable digitalPinToPCICR() and related macros (as there is no such register on this device); and another change to attach the proper interrupt inside SoftwareSerial. A patch is here.

To debug this, I hooked up the TX side of a USB-serial adapter to the RX pin on the Arduino, ran screen(1) on the computer to control it, and started mashing keys. Until I got the irq triggering working properly, I also monitored the output with a DMM just to make sure the output was going low when I held down a key. A test sketch for that looks like this:

SoftwareSerial port2(2, 3);

void setup()
{
  Serial.begin(9600);
  port2.begin(9600);
  Serial.println("Listening...");
}

void loop()
{
  while (port2.available()) {
    Serial.write(port2.read());
  }
}

It works fine. I’ll write about what I was doing with it in some future installment.

I have this more-expensive-than-my-car hunk of metal on loan for the next few weeks. It’s a pretty far cry from the old Tektronix analog scopes I trained on back in the day (which, at the time, were also much more expensive than my then-primary mode of transportation: shoes). As a warmup exercise, I hooked it up to the output of my guitar amplifier and played an A, which you can see pictured. You can also see that I’m apparently tuned down a half-step as the frequency measured on the scope is about a semitone shy of 220 Hz.

This was a neat teaching opportunity for my four year-old: I showed Alex visually that sound is just made up of different kinds of waves. Later at bathtime he connected that with the physical waves he was making in the tub: “Just like music!” he said.