H7x3 SAI with DMA in MONO mode

[LCE]

Heyho,

using all SAI blocks on H723 / H733 / H735 with stereo audio ADCs via DMA, working smoothly as long as both channels are used.

Now I wanted to implement turning off single channels, and I thought I could do that with the SAI / DMA settings.

I just found the MONO mode & bit and its limitations (RM0468 Rev 3, page 2304):

So only slot 0 (channel 1 of stereo ADC, or in audio terms: LEFT channel) can be used. (This is terrible!)

As far as I understand, the settings in register SLOTR stay the same as for stereo mode (NBSLOT = 1 and SLOTEN = 3).

DMA buffer length is set to half the stereo length.

Problem:

I still see data of channel 2 in the DMA buffer, it's changing all the time: sometimes channel 1 data only, sometimes stereo data (with the usual channel multiplexing, samples: c1 c2 c1 c2 c1 ...).

Anybody any ideas?

Thanks in advance!

[LCE]

It doesn't matter which SAI block is used, SAI 1 A or B or SAI 4 A or B, always the same result.

[AScha.3]

Hi,

i think, you cannot do it this way, because ADC always sending standard L-R-L-R... - you have to set the SAI to this.

If you dont want stereo, but just one channel: why not simply copy only L or R samples from the circular buffer ?

You get, what you want and the SAI runs on the format, the ADC is sending.

 

[LCE]

Hi AScha.3,

copying is no option.

The system is based on DMA transfers in the background - what is absolutely necessary with max. 8 channels at 32 bit / 200 kHz.

At least I found a big bug in switching DMA buffers in DBM mode.

Now I get the 1st of the 2 stereo channels cleanly with the MONO bit.

Maybe I can do some LRCK polarity switching to get channel 2.

[LCE]

So, for the non-master SAI blocks, I can simply switch the FSPOL bit in FRCR to get the 2nd channel in MONO.

But - as expected - I must not do that with the SAI clock master, because then all other stereo channels get switched (L becomes R, R becomes L).

I mean, I could do that, but our software guys would kill me... :D

Conclusion:

- the SAI clock master's channel 1 (or left) must always be turned on (channel 2 / R can be turned off)

- for the SAIs working as clock slaves, the FSPOL bit must be inverted to get only channel 2 / R

[LCE]

Correction:

the above is true - but not necessary.

Sometimes, just don't believe the RM...

That part is wrong, slot 0 is stored only if slot 0 is enabled (as proposed on the previous page).

If SLOTEN in SLOTR is set to 2 (slot 1 is enabled) and MONO bit is set, luckily slot 1 (= 2nd / R channel of stereo) data is given to DMA.

No FSPOL shenanigans needed! :)

[LCE]

Anyway, I would feel safer if someone else can confirm that, maybe some SAI expert from ST ?

[LCE]

Could you please check that, maybe @STOne-32 or @KDJEM.1 ?