Hi,
I'm reverse engineering a SIP phone so I can reposess it and develop open source
firmware for it. I think I found why JTAG was failing.
It appears that my TRST is wired to GND through 12 Ohm resistor. As a result,
I cannot reset the chip into JTAG mode -- it needs Vtref on TRST to function,
AFAIK.
Is this a well-known pattern? Any more hints from experience?
Thanks,
-Rick
Do you mean TRST is tied on your target board to GND via 12 Ohm pull-down ?
Using a pull-down to TRST is practiced to make sure the TAP go-and-stay
in reset state at the powerup of your target.
TI add pull-down on TRST on all their boards.
But a 12 Ohm is a bit too smaller to strong !
Are you sure it is not 1K2 Ohm ?
If you have a Amontec JTAGkey or JTAGkey-2 you may drives TRST as
push-pull or open-drain.
If a pull-down is on the TRST signal at the target board, only the
push-pull mode of TRST must be selected.
If push-pull driver for TRST the VTREF *could* be used to driver the
high level of the push-pull .
In the case of Amontec JTAGkey and JTAGkey-3 dongles, the VTREF is used
to drive the push-pull buffer, but we are working on a new JTAGkey-3
using the VTREF as a true high-impedance reference votage input , then
we use the VTREF as input to a voltage follower to drive the output
buffers .
Regards,
Laurent Gauch
www.amontec.com
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