SAI - how to synchronize the data transmit between 2 blocks

Note that ideally I want to do this with DMA. Maybe there is a way to do it on the DMA level, with the DMA FIFO?

Anyone? :)

Note that there's only a handful of users answering questions here; I don't remember many threads with SAI/TDM. (I used a single block SAI for I2S and then other for SPDIF-Tx, and all this was quite some time ago, I don't really remember the details).

So, without looking into the manual, IMO the blocks are master-slave, and slave won't transmit before master does. So, what I'd do is, start DMA into the slave's data register, make sure the FIFO is full, and then start DMA into the master's data register. They should start transmitting simultaneously when data arrive to master; DMA is there to top up the FIFO as transmitter in SAI drains it.

JW

I tested the DMAPause/Resume and I can confirm it gets out of sync when resuming. BUT it turns out that within one frame the data is still where it should be - so that the data is still sent to the correct audio output (see below).

Though the slave and master can still be one frame out of sync, and can get more and more out of sync each time the HAL_SAI_DMAPause/Resume functions are called.

Notice that in terms of audio one frame out of sync means some audio channels will be 1/fs out of phase, f.x. 0.02ms (with fs=44.1kHz) - I am thinking this should not be audible really?

Here is my test setup:

• I am sending 8 samples with DMA set to Half-Word Circular
• I start transmitting with DMA in the main()
• In the loop I call Pause/Resume when I press a button

The code:

// test data - dummy data very visual on the logic analyzer
static const uint16_t data[8] = { 0b1, 0b101, 0b10101, 0b1010101, 0b101010101, 0b10101010101, 0b1010101010101, 0b101010101010101 };
 
int main(void) {
 
 // [...]
 
 HAL_SAI_Transmit_DMA(SAI_BlockSlave, (uint8_t*)data, 8); // slave first
 HAL_SAI_Transmit_DMA(SAI_BlockMaster, (uint8_t*)data, 8);
 
 while (1) {
 
 // [...]
 
 if (buttonPressed) {
	 if (paused) {
		 HAL_SAI_DMAResume(SAI_BlockSlave); // slave first
		 HAL_SAI_DMAResume(SAI_BlockMaster);
	 }
 	 else {
		 HAL_SAI_DMAPause(SAI_BlockMaster); // master first
		 HAL_SAI_DMAPause(SAI_BlockSlave);
	 }
 
 // [...]
	}
 }
}

When I start the SAIs I get that:

So far so good, you can see both data lines are in sync, and also that the data is in the right place in the frame (since this is TDM, the DAC will send the samples to different outputs according to their position in the frame).

Then I press the button (calling HAL_SAI_DMAPause) and it does not (always) stop at the end of the frame, nor does the two data lines stop at the time (again: not always).

When I press again (calling HAL_SAI_DMAResume), and it seems the DMA/SAI make sure that the data within each frame is at its place, but since one SAI starts transmitting a bit before the other, then the other come one frame later sometimes, like here:

I have no idea what could be DMAPause/Resume. I don't use Cube/HAL and don't intend to debug it; and I also doubt anybody does.

Why don't just simply stop, and start afresh?

Normally, pause in audio is to be performed by filling the Tx buffer with a neutral value (zeros), while clocks and DMA keeps running; and simply ignoring the incoming data.

JW

Hi @SLasn.1 !

Just found your very interesting thread about SAI and dual-line transmit. I'm new to SAI and it appears to be not the largely used library of STM32 users :)

Have you found a better solution to the zeros buffers?

PS: I've posted a related question in this thread, maybe you can confirm this idea can work, it seems quite similar to your use? https://community.st.com/t5/stm32-mcus-products/sai-transmission-to-external-dac-more-serial-data-outputs/m-p/655246 

Thanks!