Clock sources on expansion headers

I’m trying to design a peripheral cape that requires a max 16 MHz clock input. What are my options for a clock source on the P8-P9 headers? I looked at the TI datasheet for the mysterious CLKOUT2 pin on P9 - they do not recommend using the output because it has an unspecified jitter, although the BBB uses CLKOUT1 as an input for the HDMI Framer. Couple questions - what output does one see on CLKOUT2? (I don’t have a high bandwidth DSO handy). I’m wondering if I should bother with the clock, or just use the on chip clock found on the peripheral IC.

Then I thought I should set up a PWM at 50% duty as a clock source. However, I’m unable to locate the maximum PWM frequency that can be set up. Any pointers? Any clever ideas?


My suggestion is that you consider an external oscillator due to the issue as described in the TRM. But, you copuld also design it such that with a resisotr swap, you could use either one. I am not sure how good the CLKOUT2 frequency is at 16MHz. A lot of this depends on what exactly your application s and how you are using it.


I was thinking about using an I2S DAC such as a cheap TDA1543 (9Mhz clock input max, could not find a MAX5556 around here which is OK with up to 25Mhz clock input). I want to be able to get no-so-high-end audio without the HDMI interface for an SDR project.

The “SPI1_SCLK - P9_31 - Bit clock” pin on the expansion board is CLKOUT1, which is used for the HDMI Framer Processor. It’s at 24.576MHz, and it would be good for a MAX5556 without a divider for my purpose.

This CLKOUT1 is constantly on (I see it on my DSO right now, since it’s a cheap 25Mhz one, the pulses look almost like sine waves), so I think it could be used with a frequency divider just as they do with the SN74AUC1G74 to provide the 12MHz clock signal for the TDA199988.