GPIO3_21 usage differences on Beaglebone Black vs. original Beaglebone

It appears that Pin P9-25 no longer defaults to GPIO3_21 on the BBB and now appears to have a ~20MHz clock? I cannot see this from Table 11 on the SRM. Where can I find these default GPIO differences between the two versions?



I will need to add a table of differences in the SRM to make these differences clearer. Default modes do not necessarily indicate a change in the HW, only ina usage,. As SW can change these it is not something that will prevent the pin from performing the old function as a function of the cape SW being loaded. That has always been the caseā€¦

If you read the schematic there is a note that states this as well:

Oscillator can be disabled via SW
for power down modes or if
GPIO3_21 needs to be used.


I am adding this to the SRM:

  • GPIO3_21 has a 25.576 MHZ clock on it.
  • This is required by the HDMI Framer for Audio purposes. We needed to run a clock into the processor to generate the correct clock frequency. The pin on the processor was already routed to the expansion header. In order not to remove this feature on the expansion header, it was left connected. In order to use the pin as a GPIO pin, you need to disable the clock. While this disables audio to the HDMI, the fact that you want to use this pin for something else, does the same thing.
    Is this a clear explanation?


Thanks Gerald. That is very clear. Are there any other GPIO pins that should be avoided? Dave.

Well, depends. eMMC are ones if you want to use eMMC. That is discussed in the manual and on the WIKI. LCD, that drives the HDMI. If you want to keep HDMI, stay away from those, also discussed in te manual and Wiki, But all of these can be used if you give up eMMC and HDMI. Unfortunately, I have been unable to add features like this without using some of the pins!