Mon Mar 28 00:17:50 EDT 2011

I2C or SPI?

While I2C is only 2-wire, AUX is used on the PK2 to support it.  Maybe
because PGD is used for debug signalling?

I tried both I2C and SPI, and I can't get I2C to do anything.
Probably pin or pullup misconfig.  However, SPI is straightforward.

Getting the SPI to work on the pic seems trivial.  However, there is
still the same problem of target-side signalling; I2C and SPI are not
going to solve that, and probably only complicate matters by using the
AUX pin.

Simplest seems to be to use the ordinary ICSP proto, and use AUX for
client busy signalling.  This is safe, as the pin is dedicated, and
PK2 can switch it to input before doing anything else.

My guess is that the secret ICD debug proto does something similar.