AVR eeprom debug log

Using text output for debugging is a common technique in both embedded and hosted environments.  In embedded environments the overhead of printf() or Wiring's Serial.print() can be quite large - over 1KB.  A lightweight transmit-only soft UART like my BBUart with some code to convert binary to hex will take 64 bytes, but on an 8-pin AVR, dedicating a pin to a soft UART may be a problem.  For some old parts like the ATtiny13a, the accuracy of the internal RC oscillator can also make UART output problematic.  I recently purchased 5 ATtiny13a's, and running at 3.3V, the oscillator for one of them was closer to 9.2Mhz than the nominal 9.6Mhz specified in the datasheet.

My solution is to use the EEPROM for debug logging.  The code takes only 22 bytes of flash, and the data log can be read using avrdude.  The eelog function will use up to 256 bytes of EEPROM as a circular log buffer.  Just include eelog.h, then make calls eelog() passing a byte to add to the log.  I wrote a test program which logs address 0x3F through 0x00 of the AVR I/O registers.  Then I used avrdude to save the EEPROM in hex form to a file:

Then the file can be viewed in a text editor.  Another option would be to save the EEPROM as a binary file and use a hex editor.  Here's the contents of the ee13.hex file:
:2000000000009D9D000000000000000700002400000000000000000000000000000000007B
:2000200000202002020202000009000000000000000000000000003000000000000000003F
:00000001FF

From the log file, the value of stack pointer low (SPL) is 0x9D, or 2 bytes less than the end of RAM (0x9F).  Considering the call to main() uses 2 bytes, it looks like the eelog function is working as expected.

picobootSTK500 v1 release

I've just released v1.0 of my arduino-compatible picoboot bootloater.  It now includes support for EEPROM reads, and has been tested on an ATmega168p pro mini (the beta release was only tested on a 328p pro mini).  It also fixes a possible bug where the bootloader could hang while writing the non-read-while-write section of the flash.  Since it's been a few months since the beta release, which has been working well on a couple m328p modules, I've decided to bump the release to v1.

The bootloader only takes 224 bytes of flash space, so there's room left to add support for eeprom writes, and possibly auto-baud for the serial in the future.

Hex files for m168 and m328 are included in the github repo, and the Makefile includes a rule to use avrdude for flashing the bootloader.  If you are using the Arduino IDE with an ATmega328p, picoboot is drop-in compatible with the optiboot bootloader used on the Uno, so just select the Uno in the boards menu.  For the ATmega168, modify the board.txt file to support the faster upload speed and extra flash space:

Using picoboot increases the unused flash space by 12.5% compared to the stock bootloader on the ATmega168 boards such as the 168p Pro Mini.