I am trying to source a 12.288MHz clock for an STM32L433 but having difficulties finding something.
The clock is required for I2S playback with a bluetooth receiver (BT401).
I have tried using the internal MSI RC to generate the 12.288MHz clock for SAI, but the BT401 is misbehaving so it might be because the MSI RC clock has too much jitter.
I was wondering if a ASFL1-12.288MHZ-EC-T would be suitable for the clock source for the SAI external clock input....
https://uk.rs-online.com/web/p/crystal-oscillators/2403583
Below is the setup...
The I2S amplifier (MAX98357A) seems to work fine with the 12.288MHz clock generated by the internal MSI RC and PLL.
But the BT401 either requires multiple attempts to start playing and sometimes plays just a screeching sound for a few seconds then stops. So I need to rule out whether the issue is caused by clock jitter by trying a more accurate clock source.
I am currently using the STM32L433 Nucleo board which has a footprint for an external HSE crystal, but I cannot find a crystal for that footpring with a 12.288MHz frequency.
The ASFL1-12.288MHZ-EC-T seems to be an all in one device, you just provide power and it generates the output clock. So its not like a crystal which needs a special drive circuit ?
I have read the application note AN2867 Recommended resonators for STM32 MCUs/MPUs, but its complicated and just confused me.
@MM..1 wrote:In normal audio systems all parts use one sync clock.
This is a concern and I'm now not sure whether AScha.3 solution will work, because there will be a clock from the BT401 (as a master) and a clock for the STM32 sending the audio to the amp (as a master)
If only the BT401 behaved as a slave, eveything would be working and I would not have all this grief.
I did briefly try this configuration, but the voice prompts have left over data when the finish so its broken and I need to fix it, though I am now unsure whether this configuration will actually work due to the two difference clock sources.
I have also confirmed this with my contact in china who supplies the boards/chips/firmware
Something like this is the BT ?
For doing it like i supposed ->
If only the BT401 behaved as a slave, eveything would be working and I would not have all this grief.
I did briefly try this configuration, but the voice prompts have left over data when the finish so its broken and I need to fix it, though I am now unsure whether this configuration will actually work due to the two difference clock sources.
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Callbacks ...only the transmit are to handle, as we want play audio. At first .
The receive callbacks...forget , for now. BT receive will run , but we dont use it.
Except, later, if it looks "ok" , then we copy in the (transmit) callbacks not zeros for silence, or "voice" for info, but also then one of the input buffers , to "copy" the input from BT to the play buffer.
(Synchronisation is at first not important, we do it later. Get it working at first, even if BT audio might have some dropouts...or else, not important now. )
We create two independent streams : the important, L433 is master, to play to the dac.
The second, from BT , might run...musik, or not...silence. But output always possible, and voice clear.
You should set the (circ.) buffers for the input and output streams to same size, maybe 2x (2x 2K ),
so copy (in output callbacks) from input to output buffer is easy, because same size.
And yes, the two streams are fully independent, but as both run 48k sample rate, no big problem.
Get this working first, then i explain, how to do some simple synchronization for the BT audio.
