Using audio clock (22.5792MHz or 24.576MHz) as STM system clock

What board? What STM32? Lots of combinations, lets try and narrow it down a bit.

Most STM32 can take external sources in range 4-50 MHz, or crystals upto 25 or 26 MHz, these don't have to be integers. The part can clock from HSI also, and PLL can be used for I2S. The I2S clock can be feed from a different pin as I recall. Clock sources can be transitioned between HSI, HSE and PLL, you have to stop the PLL to reconfigure it, so switch to a clock that is still running to keep the synchronous machine running.

You'd need to define HSE_VALUE correctly, and fiddle with PLL settings (simple gear ratios)

Thanks Clive

Here's the board

https://aealicdn.com/kf/HTB1yQHxeoo09KJjSZFDq6z9npXaM.jpg

- it's as simple as it gets - STM32F4 read SD card into memory chip & outputs I2S audio signals

Other side of board is just screen & control buttons

As you can see just two audio clocks on pcb so STM is switching between these as system clock depending on the samplerate of the audio file.

The benefit is that the system clock & I2S clock are always synchronised so no clock crossing domain issues

If the F04 is powerful enough (& it is for this simple job) then I don't see the need for a different uC

You probably mean 'F40x - 'F04x is from an entirely different line (and that would certainly pose a challenge, but probably wouldn't be impossible to use). 

The audio playback is not something processing intensive; and while as I said I haven't use SD cards yet, I wouldn't anticipate those be processing intensive either. Unless the audio needs substantial processing (DSP or (de)compression), it's mostly just moving data from one pile to other, and leave the hardware (DMA) do the rest. OTOH some of the 'F0xx lack enough I2S-related resources, and also the dedicated SD-card interface.

The presented board is based on 'F407 but that was probably not a very thoroughly though out choice, or those chips were at hand already for some reason. One of the telling signs is that the somewhat cheaper 'F405 is identical except it does not have the ETH interface (which is not used on this board).

This board may also predate the 'F446 I am talking about. I tend to optimize my designs.

I know a SYNCH clock ISN'T needed but that is a design choice that works for me, for various reasons.

Could you please elaborate? The last 7 years I am moving audio and similar data to and from the STM32s and I haven't found a single reason for synchronizing the data stream with the system clock, yet. I don't pretend I know every possible usage case, of course; and I am curious.

Thanks,

JW