> I'm trying to add support for ATmega406 to the avrdude.conf but
> apparently, this MCU does not support the "SPI Serial Instruction
> Programming Set", so I don't know what to enter in the "pgm_enable",
> "chip_erase", "read", "write", "loadpage", etc. sections.
I think you could simply omit those in that case. The JTAG ICE code
will never reference them anyway.
> Can someone assist me on that? I already have the "JTAG Instruction
> Programming Set" of the ATmega406, but those instructions are split
> into a TDI and a TDO part.
AVRDUDE doesn't talk JTAG by itself, it always relies on tools like
the JTAGICEmkII or AVR Dragon for that. Thus, the low-level JTAG
instructions are not needed here. (The communication with the ICE or
Dragon is described in appnote AVR067, this is what AVRDUDE uses.)