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rgdeja
01-24-2011, 07:30 PM
The 14 Pin Port J2205 on Series 3 (BCM7401 based) and Series 4 (BCM7413) is the EJTAG Port.

PIN 1 - TRST_N
PIN 3 - TDI
PIN 5 - TDO
PIN 7 - TMS
PIN 9 - TCK
PIN 11 -
PIN 13 - N/C

PIN 2 - GND
PIN 4 - GND
PIN 6 - GND
PIN 8 - GND
PIN 10 - GND
PIN 12 - N/C
PIN 14 - VCC

Square pad is PIN 1. on Series 4 the JTAG port is in the cable card slot
VCC is 3.3V so you need 3.3V compatible JTAG Adapter.

Also R2201 (4.7K) must be tied to GND to enable JTAG.

Hope this helps, if there are enough interest I can work on the JTAG and post results here.

Omikron
03-07-2011, 04:29 AM
The BCM CPU used on the Series4 platform has a "factory mode" that is disabled at time of production. Once this is disabled by blowing a fuse on the CPU, it also kills JTAG.

Currently the only known way of re-enabling JTAG is to somehow obtain a "virgin" CPU and doing some advanced BGA rework to replace the CPU on the board. Even then, the TiVo software is programmed to detect the presence of factory mode and it does "things" in response to this.

All that said, if you do somehow get JTAG to do something useful on the Series4 please don't feel shy about posting it. :-)