Make software BP work with ATMEL ICE

[friggeri]

Using an ATMEL ICE, I was able to set software breakpoints on an atmega328p, the mcu would properly break but avarice would display "ignoring break event" and I would then have to manually interrupt from avr-gdb.

Running in debug mode, I noticed that the particular payload differed slightly from what avarice is currently expecting. For example:

command "set BP" [0x12, 0x43]
0E 00 16 00 12 43 00 9E 00 00 00
Received 0x81 0x11 0x00 0x06 0x0e 0x16
read:  0e 16 00 12 80 00

Got message seqno 22 (command_sequence == 22)
response: 12 80 00

command "go" [0x12, 0x32]
0E 00 17 00 12 32 00
Received 0x81 0x11 0x00 0x06 0x0e 0x17
read:  0e 17 00 12 80 00

Got message seqno 23 (command_sequence == 23)
response: 12 80 00
Waiting for input.
read:  0e 00 7d 00 12 40 4f 00 00 00 01 00 00 85
Event serial 0x007d
ignoring break event

The breakpoint is set at 0000009E, so we're expecting to see a 0000004F in the breakpoint event, which we do. If we look at the last message, the relevant part of the payload is:

12                   AVR_ISP
40                   EVT3_BREAK
4f 00 00 00          b4 encoding for 0000004F
01 00 00 85

The code postulates that bytes 6 and 7 are the break status register, I'm going to makes a wild guess that in the case of this particular programmer byte 6 might be the low byte and byte 7 the high byte (b2 encoding), which is consistent with a software break.

[maxgerhardt]

This is 100% needed when I tested this in my XPlained Mini 328P. @dl8dtl could we merge this?