Solar Cluster: Debugging an ATTiny24A, programmer upgrade

So, debugging the ATTiny24A, one big problem I’ve got is understanding what the ADC is seeing in each channel. There’s no serial output, no LCD, just a handful of LEDs and a PWM output. Not good enough.

The ICSP header though, necessarily exposes the pins needed to do SPI and I²C. Could that do? I’d need something to do the transfers with.

The programmer I’ve used to date has been a Olimex STK500v2 clone (the tiny one built into a DB25 backshell), which works well, but it has one nit: I haven’t figured out a way to do raw SPI transfers with it. It might be possible, I’m not sure.

I immediately thought of the Raspberry Pi. The other option I had close on hand was a Freetronics LeoStick. One I’d have to write programming firmware for — which may be worth doing some day. The other, I can just install from repositories. But how does one interface the two?

Adafruit have this tutorial on doing exactly that. HOWEVER, they wire the Pi straight up to the AVR. Fine if they’re both 3.3V, but trouble if the AVR is running at 5V like mine. I’d expect this to release magic smoke!

So, a level shifter is needed. I happened to have a Freetronics one laying around which gave me 4 channels, good enough. I just had to figure out what pins to use. For reasons unexplained, Adafruit seem to pick weird and wonderful pins that are not close together. Another guide, suggested using the standard SPI pins. I more or less went this route, but used GPIO channel 22 instead for reset, so I could use the one female header to connect to them all.

The connector was a spare that came with the LeoStick: they come with two 13-pin ones. I cut it with a hacksaw to give me two 3-pin headers and a 6-pin header. The 3-pin headers were glued together to give me a 2×3 pin header, and the other was soldered to the level converter. Two pins had to be swapped, annoyingly, but otherwise wiring it up was straightforward.

I just ran some off-cut CAT5e cable to the ICSP connector, keeping the lead length short so as to prevent clock skew.

The configuration file for AVRDude looks like this:

# Linux GPIO configuration for avrdude.
# Change the lines below to the GPIO pins connected to the AVR.
  id    = "pi";
  desc  = "Use the Linux sysfs interface to bitbang GPIO lines";
  type  = "linuxgpio";
  reset = 22;
  sck   = 11;
  mosi  = 10;
  miso  = 9;

I can flash my ATTiny24A from the Pi now with the following command:

$ sudo avrdude -p t24 -c pi …arguments…

So with that done, I should be able to use a simple Python script to read and write bytes via bit-banged SPI via the ICSP header, and implement some firmware to react via SPI.