[E3-hacking] JTAG/Anyone got a broken E3?

David J. Singer doc at deadvirgins.org.uk
Thu May 11 18:14:10 BST 2006

Matt Callow wrote:
> But TMS and TCK are not at the edge of the BGA, so the same method can't 
> be used to find them. It would seem sensible to assume that they appear 
> as more of the 8 pads labeled R40, but I've found that 3 appear to be 
> connected to Vss, 1 to Vdd and the other 2 are connected together!

Ah, but notice that R40 isn't an 8-pin connector, it's an unpopulated 8-pin, 
4-way resistor pack.  I guess it's there to provide the necessary pull-ups 
and pull-downs on the JTAG pins, hence the reason that the pads on the left 
hand side are connected to the power rails.

I'd hazard a guess that the JTAG "TAP" port, if it exists, is somewhere else 
on the board.  A viable approach to finding it might be to look for candidate 
connectors/pads and check for continuity with the TDI and TDO pins on R40 you 
already know.  Chances are, if you find such a "connector", it'll have the 
remaining JTAG pins on it too and it might then just be a case of trial and 
error with suitable JTAG hardware...

David J. Singer
"Time flies like an arrow, fruit flies like a banana"

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