The first part of this project aims to build a debugWire debugger with common available hardware like a standard usb-to-serial converter (FT232).
This is implemented in software (python) and is composed of two parts: the debugWire Implementation through serial interface, and a small implementation of a gdb remote target server to be able to debug the target with gdb (avr-gdb).
The second part of this project may consist of developing an arduino-like board capable of isp programming, debug interface and serial converter.
Atmel debugWire is actually supported on the standard avr programmers (Dragon-obsolate, JTAGICE-obsolete, Xplained boards) which are usable only with Atmel Studio.
What if we already have an arduino?
I use a Attiny85 dev board (Digispark usb DevBoard [schematics]) connected to my computer with an FT232 Usb to serial converter and a diode on the tx pin.
TODO Make some images
+---------+ +------------+ +-----------------+
| PC (USB)|--------| FT232 (RXD)|-------+---+------------|(!RST) |
+---------+ | (TXD)|---|<--+ | | |
| | +++ | |
| | | | (R1 10K) | ATTINY85 |
| | +++ | |
| | | | |
| (VCC)|-----------+------------|(VCC) |
| (GND)|------------------------|(GND) |
+------------+ +-----------------+
As the schematic represents, the diode (1N4148) is connected from the rx line, pulled up to vcc with a 10K resistor to the FT232 TX. Note that this setup is good for slow speeds, an improvement can be made using a transistor on the tx line.
The fisrst thing to do is enable DebugWire. This has to be made with an ISP/HV programmer Using an arduino and the default ArduinoISP programmer, I overran the fuse bits to enable DWEN fuse and changed the clock source to 8MHz Internal:
arduino isp is connected as /dev/ttyACM0
First we check for all the connections to be working. this should read the device fingerprint
$avrdude -v -cstk500v1 -P /dev/ttyACM0 -b 19200 -p attiny85
Next we want to read the fuse memory
$avrdude -v -cstk500v1 -P /dev/ttyACM0 -b 19200 -p attiny85 -U lfuse:r:-:h -U hfuse:r:-:h -U efuse:r:-:h -U lock:r:-:h
NB If lock bits are set we need to clear the device first. i don't know how to do it =(
For my setup i choose this fuse values
- LFUSE = 0xE2 -> internal RC 8MHz oscillator
- HFUSE = 0x9D -> Debug wire enable
- EFUSE = 0xFE -> self programming enable
you can use this site to ease up the configuration and here is the fuse programming command. Note: after setting those fuses the isp interface will be unaccessible without a debugWire setup (WIP) You should upload a sketch or a test program before setting the fuses.
avrdude -v -cstk500v1 -P /dev/ttyACM0 -b 19200 -p attiny85 -U lfuse:w:0xE2:m -U hfuse:w:0x9D:m -U efuse:w:0xFE:m
The code used for testing is available in ./avrtest.
after compilation you'll find an eeprom image, flash image and the elf you can use to debug the program.