i2s capabilities on nrf52840

[ms360]

I am attempting to interface a tas2110 with an nrf52840. 
I just became aware of some nrf52 limitations and wanted to get the latest info.

My requirements are as follows
"The TDM serial audio port supports up to 16 32-bit time slots at 44.1/48 kHz, 8 32-bit time slots at a 88.2/96 kHz sample rate and 4 32-bit time slots at a 176.4/192 kHz sample rate. The device supports 2 time slots at 32 bits in width and 4 or 8 time slots at 16, 24 or 32 bits in width. Valid SBCLK to FSYNC ratios are 64, 96, 128, 192, 256, 384 and 512."

Then I saw this post
https://devzone.nordicsemi.com/f/nordic-q-a/15713/i2s-32-bit-word-size/59992

Can the nrf52 output more than 2 time slots? 
I tried to do 4 channels and it looks like it only supports left and right. 

[Susheel Nuguru]

The nRF52840 I2S peripheral cannot generate TDM frames beyond stereo. You can only drive a two-channel (left/right) mode of the TAS2110. 

This is shown in the CONFIG.CHANNELS register where you can either select stereo (left and right) or , left-only or right-only. This is hard limitation for now. I do not think any of our product have multislot TDM available with I2S, this most likely is to keep to power consumption low while supporting the audio usecase.

[ms360]

Thanks Susheel - one more detail - 

2 channels on the TAS2110 requires 32 bit time slots. 
Is that not possible on nrf52840 ?

[Susheel Nuguru]

I am not sure what you mean by 32 bit time slots.

nRF52840 can generate 32 bit time slots for 2 channels. Slot length is set by the BCLK-LRCK RATIO. With stereo it should be something like

slot_bits = RATIO / 2

The I2S peripheral’s SWIDTH supports 8/16/24-bit samples only. You can get 32 bit slots by padding a 24 bit sample to 32 bits. that you can achieve using this

CHANNELS = STEREO
SWIDTH = 24
RATIO = 64 → 32-bit slots

If the TAS2110 requires true 32 bit sample payloads without padding, you cannot do it with nRF52840.

[ms360]

I think I would need sign extension to make that work. 
Will have to explore other options. thanks .