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.