External clock oscillator and MCU

Question

I am creating this post as a second part to a recent post I have made: STM32F446 & External Oscillator

I have read other post about using a crystal that runs at 8 MHz rather than 25 MHz. Such as posts like this: Rectifying failed MCU crystal oscillator design

Could someone explain to me why is it preferable to use an 8 MHz crystal rather 25 Mhz? My project's sampling frequency is 44.1 kHz using I2S for the CODEC-MCU comms, I thought that having a 25 Mhz external clock would be fast enough to sample the signal by the guitar.

EDIT: How do you calculate the crystal frequency needed to hit the sampling frequency 44.1 kHz using I2S.

Answer 1

The 8MHz crystal uses less power and is easier to route traces for since you aren't dealing with the faster rise/fall times required for higher frequency signals. The PLL in the MCU can then step up the 8MHz to whatever frequency you actually need.

You need very exacting frequencies for some applications though where getting close woth the PLL is not enough and it needs to be an integer multiple so can be cleanly divided down.

Answer 2

Using a 25 MHz crystal will gain nothing while it makes things just more complex. And there is an application note which says that if you use the internal bootloader for CAN or USB, user must use low frequency values than high for the bootloader to autodetect the frequency more reliably.

The MCU has a PLL so if you want to run the MCU at 180 MHz via the PLL, or generate a master audio clock with one of the audio PLLs, the clock will have to be divided down to 1 MHz anyway before it is input to the PLL, so regarding the MCU speed or audio frequency, it does not matter frequency-wise if the crystal frequency is 8 MHz or 25 MHz.

So because either frequency ends up being fed to PLL, it does not matter which one of the two is used. You can reach 44.1 kHz sampling rate that has no error larger than 0.04% just as well with any clock that is multiple of 1 MHz.

But for the oscillator that runs the crystal, is easier to find and select a crystal that satisfies all parameters that the MCU oscillator hardware circuit requires. Almost any 8 MHz crystal is suitable, as it allows for larger ESR for example, so the crystal oscillator will start up and run more stably. Finding a 25 MHz crystal with low enough ESR and load capacitance rating to allow good gain margin is much much more difficult, and there are far fewer places from where to buy them, if they even stock it.

If you need to hit 44100 Hz more precisely for some reason, then you should consider a crystal which can provide the required clock via PLL or simply use another oscillator just for the audio master clock to get the exact rate.