Hello:

There are some ideas how ICD works here. I haven't proved it 'cause I don't
need to.
When you look at registers of PIC16F877 for instance you can see two
reserved bytes at addresses 18Eh and 18Fh. Reading carefully Microchips'
manual about the ICD you should notice three things. First there is one
breakpoint only and it's clear the address must be saved somewhere in the
RAM memory. Above mentioned bytes serve for this. There is probably a
hardware mechanism (get ready when you select the "debug" bit in a
configuration word) initiates an interrupt (rather a reset) when the program
counter reaches the value saved in mentioned registers.
Second: You are not allowed to write anything to program address 0 but NOP
instruction. It suggests the reset of a processor being in a debug mode
doesn't read a value saved there but anything else. May be a jump at the
beginning of a small program saved to the highest memory you are not allowed
to use (The thirt thing). I bet there is a very small program reading the
serial communication, handle registers etc.... Don't forget you aren't
allowed to use a couple of registers being in debug mode.

Working for Microchip I would consider such scheme as the easiest way how to
improve the developing of projects very significantly. I thank them for it.
It shorts a development time of our projects very much so the cost of a ICD
equipment can be forgotten.

I would be glad if someone prove my thoughts and tell me if I was wrong.

Regards

Pokorny


-----Original Message-----
From: M. Adam Davis [mailto:adampic@UBASICS.COM]
Sent: 15. eervence 2001 4:16
Subject: Re: [PIC]: In-Circuit Debugging


The interface between the target and the ICD, and the interface between
the ICD and the computer are not published by Microchip, and I doubt
anyone has spent much time discovering them.  Microchip, however, has
released several bits of information so you can either 1) find out
yourself or 2) make your own based on the chip's built in dubugging
capabilities.

To find out the protocols Microchip uses, you could check out the
program running on the ICD chip, which is included in HEX within MPLAB
distributions.  Some have disassembled it, and unless your state falls
under UCITA you have the right to disassemble and inspect the available
code for purposes of interoperability, though tread carefully there, and
remember, IANAL.

To build your own, check out
http://www.microchip.com/Download/lit/pline/picmicro/families/16f87x/51242a.
pdf
which goes into deail about the on-chip debugging capabilities,
including breakpoints, halting the chip externally, etc.  It doesn't
provide code, but using that and some simple serial routines (available
everywhere, search the archives or piclist) you'll be able to whip out a
simple debugger in very little time.

There is also code to run the Picstart plus, which may be related in
some of the protocols used.

I hope this helps!

-Adam

S Parkis wrote:

>Re PIC16F87X, is the "debug executive program" <--> target PIC
>communications interface documentation available?  I'm looking for the
>sort of information one would need to design a simple debug tool to
>utilize the chip's onboard debugging capabilities.
>
>The documentation I've found indicates the debug data path is RB.6 and
>RB.7, but says nothing about data content, format, transmission
>scheme...
>
>
>(Apologies for previous msg w/out [PIC] preface---what a PITA
>'feature' IMHO]
>
>
>Regards,
>
>Steve
>
>--
>http://www.piclist.com hint: The PICList is archived three different
>ways.  See http://www.piclist.com/#archives for details.
>
>
>
>
>

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