Really poor clock quality on the Nucleo-H753ZI board

[John.Sager]

I'm trying to use a Nucleo-H753ZI board for an audio (SPDIF) project and, compared with the Nucleo-F746 board I was using, the clock quality is abysmal - off frequency and with loads of phase noise. It appears to be derived from the HSI on the ST-LINK device (F723) rather than the 25MHz crystal oscillator (why 25MHz, why not 8MHz?). Has anyone else had issues with this? I've raised a support case with ST (case no 145713) but I'm not expecting a ST-LINK software fix anytime soon, even if it is possible.

[waclawek.jan]

> What is the advantage of using a 25 MHz quartz instead of 8, 16, or 24, which make it easier to obtain exact frequencies?

They could not use the 8MHz crystal used in older STLinks, given the 'F723's USB HS_PHY PLL limitations, see RM0431 USBPHYC_PLL1.PLL1SEL.

They could've used 16 or 24MHz, though. They really did not think about usability - probably some young ambitious but unexperienced crew. The ample usage of the unobtainium ARM-pushed connectors instead of the bog standard 100-mil headers, is telling, too.

JW

[John.Sager]

I've tested with 8.333MHz (25/3), and it's probably acceptable for SPDIF, though there is a bit of jitter from the fractional-N PLL I used to get close to 480MHz. However I bought some Kyocera KC3225K 8MHz devices and a friend has mounted one up on a little PCB with some pins, so I'll use that as an external clock.