HP Printer JTAG

Harvesting parts from a HP Officejet 6310 the other day I saw this:

Of course I couldn't leave something like that without at least attempting to find out what they were for! Luckily I hadn't yet destroyed any important bits so it went back together...

And typically, they didn't seem to do anything at all. Maybe because it had no cartridges? Well I certainly wasn't going to buy any, so I thought I'd take a ROM image and if I got bored enough some day I could have a poke around it. But then - I found a connector header covered over with soldermask. As a 2x19 with central ground pins I took a lucky guess at "Mictor 38" layout and got a JTAG connection.

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VTarget = 2.691V
Info: TotalIRLen = 4, IRPrint = 0x01
Info: CP15.0.0: 0x41059461: ARM, Architecure 5TE
Info: CP15.0.1: 0x0F0D20D2: ICache: 4kB (4*32*32), DCache: 4kB (4*32*32)
Info: Cache type: Separate, Write-back, Format B
Found 1 JTAG device, Total IRLen = 4:
 #0 Id: 0x1594602B, IRLen: 04, IRPrint: 0x1, ARM946E-S Core
Found ARM with core Id 0x1594602B (ARM9)
  ETM V1.3: 4 pairs addr.comp, 2 data comp, 8 MM decs, 2 counters, sequencer
Target interface speed: 100 kHz
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...and that's where my knowledge of JTAG debugging ends at the moment. The CPU is some kind of custom HP "top secret" NXP part that doesn't exist, which makes things a bit trickier. This probably isn't the best way to learn about JTAG either. Nevertheless, I'll just keep prodding it until something pops!


<-- "Door open"?! that's the least of your problems haha