ARM Technical Support Knowledge Articles

JTAG programming problems on revision C PB926 when Logic Tiles present

Applies to: PB926


[Updated 4-May-07]


In some RevC PB926EJ-S systems, it is not possible to program logic or Flash memory on the baseboard or attached Logic Tiles with Progcards when:

  • Two or more Virtex II Logic Tiles are mounted on the baseboard
  • One or more Virtex 4 Logic Tiles are mounted on the baseboard


With reference to this PDF file, it can be seen that the inverting buffer and 33 ohm resistor combination (U61, R377) provides the TCK signal to the ARM926EJ-S DevChip. In CONFIG mode an analog switch bypasses the D_TCK / D_RTCK loop that would normally connect up all the 'virtual TAP controllers' in the custom logic in the Logic Tile FPGAs. In CONFIG mode, the FPGAs do not contain an active design so this signal path is not used. Unfortunately, these unused RTCK return paths from any stacked Logic Tiles remain connected to the Dev Chip's TCK signal and interfere with its correct operation by loading the signal and causing reflections.


  • This is a marginal problem which does not occur on all PB926EJ-S boards.
  • This problem only occurs when the system is in CONFIG mode. DEBUG mode JTAG operation is not affected


1. Use a newer baseboard - This problem has been rectified on PB926EJ-S boards with PCB revisions D and E (HPI-0117D, HPI-0117E). On these boards, an extra analog switch (U91) has been inserted, which isolates the D_RTCK stub from the DevChip TCK signal when the system is in CONFIG mode.

2. Use a different baseboard - If you have more than one RevC PB926EJ-S board available, you may find that another board does not exhibit this problem.

3. Workaround for affected RevC boards - It is sometimes possible to resolve this problem by increasing the drive strength of the DevChip TCK signal. This can be done by making a careful short circuit (e.g. a fine wire link) around R377 (33 ohm) on the underside of the baseboard. This is about 15mm from the 20-way JTAG connector. Click on the thumbnail for an illustration of the resistor site:

Segment of PB926EJ-S gerber plot showing location of R377 (Thumbnail)

How do I arrange for this workaround to be applied?

The quickest and recommended solution would be to have the modification performed locally by someone who has the necessary soldering skills and equipment - R377 is less than 2mm long and about 1mm wide. This modification should only be performed by someone experienced in working with small surface mount PCB technology. A very fine soldering iron tip and minimal contact time must be used to avoid damaging the PCB pads.

If it is not possible for the modification to be performed locally, the baseboard can be sent back to ARM in the UK to have the work done, free of charge. Please note that we will only apply the modification to boards that exibit the problem. This workaround is not required for baseboards that are not used with Logic Tiles or do not demonstrate the symptoms listed above.

To have ARM perform the modification, please use the existing returns process, which is described here. Please familiarize yourself with the procedure and submit an online fault report by following this link.

How much time is needed for this modification?

If the modification can be performed on-site, by a competent person, the modification should take substantially less than 1 hour. If the board is shipped to ARM (UK) to have the modification done, this may take several weeks including transit time.

Will I invalidate the warranty?

If the modification is performed by the end user or his representative and the board subsequently fails and must be sent to ARM for repair, we will honor any remaining warranty - provided the fault is not related to the R377 modification.

I have applied the modification, but the JTAG still does not work in CONFIG mode. What do I do now?

The only remaining solution is to use an alternative baseboard - either one of a newer PCB revision, or a RevC board that does not demonstrate this problem.

Attachments: img17240.png , workaround pb926ej-s jtag.pdf , 17235.gif

Article last edited on: 2009-01-20 17:15:44

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