ARM Technical Support Knowledge Articles

Why doesn't the UART0 serial port work on my revision E PB926?

Applies to: PB926


[New 21 September 2007]

Boards affected:

PB926EJ-S RevD (HBI-0118D)
PB926EJ-S RevE (HBI-0118E)


Serial port 0 either does not work at all, or suffers from data corruption.


The signal ground pin (pin 5) of the UART0 DB9 connector (J10) is not connected to the GND net on the PCB. If the PB926 board and the serial equipment connected to J10 do not share a common GND connection, the serial port will not work properly.

If suspected, this condition can be verified by temporarily shorting the PB926EJ-S GND connection together with the GND connection on the attached serial equipment. There are numerous GND test loops fitted to the Versatile PCBs. Care should be taken that no connection is made to any other signal while performing this test, else permanent damage may result to the PB926EJ-S or attached equipment.

Temporary workarounds:

1. Use the temporary ground strap described above. This should not be used as a permanent solution, since it presents a risk that other signals in the system may accidentally be shorted to GND, should the ground strap become detached.
2. Use a different serial port. It is only the UART0 port that has the missing GND signal connection

Permanent fix:

If the missing ground connection is found to be the problem, a permanent fix can be applied to the PB926EJ-S board; either by the end user or by sending the board back to ARM to have the work performed under warranty.

To permanently connect pin 5 of J10 to GND, apply a short across capacitor C197. This can be done by removing the capacitor and replacing it with a 0-Ohm link, or by soldering a very fine piece of wire across the C197 site.

C197 can be located on the bottom side of the PCB, directly under the UART0 connector site. It will be necessary to remove the PCB from the aluminum tray by removing eight Philips screws. Please refer to the PCB gerber file clip below for the relative location:

PB926 gerber clip for FAQ18590 (thumbnail) (Click on thumbnail for larger image)

This fix works because there are several inductors (e.g. L22) that connect the GND and GND_EARTH (Chassis) nets together on the PCB. Shorting C197 connects J10 pin 5 to the connector Chassis, which is in turn connected to GND via these inductors. This can be seen in the schematic clip below:

PB926 UART0 schematic for FAQ18590 (thumbnail) (Click on thumbnail for larger image)

How do I arrange for this fix 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 - C197 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 exhibit the problem. This workaround is not required for baseboards on which the UART0 connector is not used.

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 any user modification.

Attachments: img18593.gif , img18591.gif , 18593.gif , 18591.gif

Article last edited on: 2009-01-20 17:40:54

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