Re: Hardware debuggers for PPC74xx G4 CPUs

Jon Smirl wrote:

On 11/13/07, Benjamin Herrenschmidt [off-list ref] wrote:

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That's why Dominic wants to get OpenOCD running on the PowerPC. All we
need is the programming documentation for controlling the CPU via the
debug hardware.

Note that this is basically different for every CPU around.

I'd like to get it for the MPC5200 because of the project I am working
on, an open source audio device. It would be nice if there was a cheap
hardware debugger available for hackers to use on it. Maybe one of the
Freescale developers will see this and send me the right docs.

Is it radically different? Dominic has been able to support every ARM
7/9 chip he can get his hands on without too much trouble once the
core support was written. I don't think he has ARM 11 working yet.

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Obviously this documentation exist, all of the commercial vendors had
to have it to develop their debuggers. Maybe it is already out there
and we just don't know where to look.

Ben.

DISCLAIMER: Extrapolating grossly from almost no knowledge!

My understanding is that the Freescale PPC debugger interface is based 
on the JTAG interface using a proprietary command set.  Basically, if 
you do their magic BDM (JTAG extension) command, you get into an 
internal scan chain that allows you to read/write the processor 
internals (registers).

The problems are many...
* The documentation is only available under NDA, a problem for open 
source debuggers.
* The scan chain is different on every processor, and may be different 
on different revisions of the same processor.
* If you mess up with JTAG, you will probably burn up the CPU.  Very 
literally.  I've seen it done.  Twice.  (Thankfully not my screwup, and 
it wasn't a PPC so it deserved to die. ;-)  The internal scan chain is 
probably safer, but YMMV.

gvb