ADAU1787 + nRF5340 bi-directional I2S: codec transmits audio correctly but receives silence/no output

Hello.

I am bringing up a custom board with an ADAU1787 connected to an nRF5340 over I2C + I2S. I2C programming is working correctly, but I am having trouble getting bi-directional I2S working.

My goal:
I want to establish full-duplex I2S between the ADAU1787 and nRF5340:

nRF5340 = I2S master

ADAU1787 = I2S slave

48 kHz

16-bit stereo

Standard I2S format

For now, the intended signal flow for testing is:
Mic → ADAU1787 ADC → I2S → nRF5340 → I2S → ADAU1787 DAC

The setup:
The board has two analog microphones connected to ADC0 and ADC1. As a sanity check, I connected ADC0 straight to DAC1. This works correctly, and I can hear the microphones through the DAC output.

The board has an oscillator that is fed to the codec's MCLKIN. The rest of the pins are connected as follows:
nRF SCK -> ADAU1787 BCLK0
nRF LRCK -> ADAU1787 FSYNC0
nRF SDOUT -> ADAU1787 SDATAI0
ADAU1787 SDATAO0 -> nRF SDIN

The Nordic firmware simply performs a passthrough: I2S RX → log samples → copy to I2S TX. It is set up as such:
.mode = NRF_I2S_MODE_MASTER,
.format = NRF_I2S_FORMAT_I2S,
.alignment = NRF_I2S_ALIGN_LEFT,
.ratio = NRF_I2S_RATIO_128X, // for 48 kHz
.mck_setup = 0x66666000,
.sample_width = NRF_I2S_SWIDTH_16BIT,
.channels = NRF_I2S_CHANNELS_STEREO,
.clksrc = NRF_I2S_CLKSRC_ACLK,
.enable_bypass = false,

The DSP program sets up the SigmaDSP as follows:

ADC0 (microphone) is routed (with a T connection) to DAC1 as a sanity check, and also to SDSP outputs 2 and 3, which are set up as the ASRC output channels, which are, in turn, routed to the SPT0 output channels.
SAI 00 and 01 are routed to ASRCI 0 and 1. ASRC 0 (left channel) is then routed to the DAC0.

The problem:
The ADAU1787 appears to be transmitting audio to the Nordic correctly, but the return path from the Nordic back to the codec does not work.
Symptoms:

Nordic receives valid samples over I2S

Logged samples look reasonable:

near zero when silent

increase when audio is present

left/right channels are identical (expected since only one mic is currently used)

But audio sent back from Nordic over I2S never reaches the DAC

So it seems:
ADAU1787 → Nordic = works
Nordic → ADAU1787 = not working

What I've tested:
I2S clocks are present and stable.
Codec successfully transmits audio to Nordic and the RX samples look correct.
I tried both ASRC path and direct SPT. In both cases I got the same result: output seems to work but input doesn't.
I have verified that the power control registers ASRCI0_EN, ASRCI1_EN, ASRCO0_EN, ASRCO1_EN, SPT0_IN_EN and SPT0_OUT_EN are all enabled.

Questions

Is there anything obviously wrong in the SigmaStudio routing for a slave I2S configuration?

Is the ASRC required for this setup, or should direct SPT routing work?

What can I do to help debug this issue?

Thanks in advance.
Felipe.

adau_1787.dspproj

hardware

ADAU1787

i2s input

i2s

processors and dsp

Processors and Microcontrollers

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