Incorrect Pin Configuration

Oct 13, 2025 by wbrum.org 131 views

I2S(INPUT_PULLUP) Causes TDM to Output Only Stereo: A Puzzling Issue on RP2040/RP2350

Introduction

The I2S (Inter-IC Sound) protocol is a widely used audio interface in embedded systems, allowing for the transmission of high-quality audio signals between devices. However, when using the I2S(INPUT_PULLUP) configuration on the RP2040/RP2350 boards, a peculiar issue arises: the TDM (Time-Division Multiplexing) output is reduced to stereo, with channels (0,2) and (1,3) being "mixed" together. In this article, we will delve into the possible causes of this issue and explore potential solutions.

Understanding I2S and TDM

Before we dive into the issue, let's briefly review the basics of I2S and TDM. I2S is a serial communication protocol that transmits audio data between devices. It consists of three main signals: BCLK (Bit Clock), MCLK (Master Clock), and DIN (Data In) or DOUT (Data Out). TDM is a technique used to multiplex multiple audio channels onto a single I2S stream, allowing for the transmission of multiple channels over a single wire.

The Issue: I2S(INPUT_PULLUP) Causes TDM to Output Only Stereo

When using the I2S(INPUT_PULLUP) configuration, the I2S hardware is initialized with the input pin set to INPUT_PULLUP. This configuration is typically used when the I2S device is acting as a receiver, rather than a transmitter. However, when attempting to get both input and output samples, the TDM output is reduced to stereo, with channels (0,2) and (1,3) being "mixed" together.

I2S i2s(INPUT_PULLUP);
i2s.setSysClk(SAMPLERATE); // optimize sysclock for sample rate
i2s.setMCLK(PIN_MCLK); // set mclk pin
i2s.setMCLKmult(512); // set mclk multiplier (512 for 32bit 8ch 4i4o TDM on AK4619) 
i2s.setBCLK(PIN_BCLK); // set bclk pin
i2s.setDIN(PIN_DIN);  // set data in pin
i2s.setDOUT(PIN_DOUT); // set data out pin
i2s.setBitsPerSample(32); // 32 bits per channel
i2s.setTDMFormat(); // we're using TDM, not standard I2S
i2s.setTDMChannels(4); // AUDIO_CHANNELS channels (4i4o)

Comparison with I2S(OUTPUT) Configuration

For comparison, when using the I2S(OUTPUT) configuration, the I2S hardware is initialized with the output pin set to OUTPUT. This configuration is typically used when the I2S device is acting as a transmitter. In this case, the TDM output appears to work fine, with all four channels being transmitted correctly.

I2S i2s(OUTPUT);
i2s.setSysClk(SAMPLERATE); // optimize sysclock for sample rate
i2s.setMCLK(PIN_MCLK); // set mclk pin
i2s.setMCLKmult(512); // set mclk multiplier (512 for 32bit 8ch 4i4o TDM on AK4619) 
i2s.setBCLK(PIN_BCLK); // set bclk pin
i2s.setDIN(PIN_DIN);  // set data in pin
i2s.setDOUT(PIN_DOUT); // set data out pin
i2s.setBitsPerSample(32); // 32 bits per channel
i2s.setTDMFormat(); // we're using TDM, not standard I2S
i2s.setTDMChannels(4); // AUDIO_CHANNELS channels (4i4o)

Possible Causes and Solutions

Based on the observed behavior, there are several possible causes for this issue:

Incorrect Pin Configuration: The pin configuration for the I2S device may be incorrect, leading to the TDM output being reduced to stereo.
Clock Signal Issues: The clock signal (MCLK or BCLK) may be malfunctioning, causing the TDM output to be reduced to stereo.
Data Signal Issues: The data signal (DIN or DOUT) may be malfunctioning, causing the TDM output to be reduced to stereo.

To resolve this issue, we can try the following:

Verify Pin Configuration: Double-check the pin configuration for the I2S device to ensure that it is correct.
Check Clock Signal: Verify that the clock signal (MCLK or BCLK) is functioning correctly.
Check Data Signal: Verify that the data signal (DIN or DOUT) is functioning correctly.

Conclusion

In conclusion, the I2S(INPUT_PULLUP) configuration on the RP2040/RP2350 boards causes the TDM output to be reduced to stereo, with channels (0,2) and (1,3) being "mixed" together. This issue is likely caused by incorrect pin configuration, clock signal issues, or data signal issues. By verifying the pin configuration, checking the clock signal, and checking the data signal, we can resolve this issue and ensure that the TDM output is transmitted correctly.

Recommendations

Based on our analysis, we recommend the following:

Use I2S(OUTPUT) Configuration: Use the I2S(OUTPUT) configuration instead of I2S(INPUT_PULLUP) to ensure that the TDM output is transmitted correctly.
Verify Pin Configuration: Double-check the pin configuration for the I2S device to ensure that it is correct.
Check Clock Signal: Verify that the clock signal (MCLK or BCLK) is functioning correctly.
Check Data Signal: Verify that the data signal (DIN or DOUT) is functioning correctly.

By following these recommendations, we can ensure that the TDM output is transmitted correctly and that the I2S device functions as expected.
I2S(INPUT_PULLUP) Causes TDM to Output Only Stereo: A Q&A Article

Introduction

In our previous article, we explored the issue of I2S(INPUT_PULLUP) causing the TDM output to be reduced to stereo on the RP2040/RP2350 boards. We discussed the possible causes of this issue and provided recommendations for resolving it. In this article, we will answer some frequently asked questions (FAQs) related to this issue.

Q: What is the difference between I2S(OUTPUT) and I2S(INPUT_PULLUP) configurations?

A: The main difference between I2S(OUTPUT) and I2S(INPUT_PULLUP) configurations is the direction of data flow. I2S(OUTPUT) is used when the I2S device is acting as a transmitter, while I2S(INPUT_PULLUP) is used when the I2S device is acting as a receiver.

Q: Why does I2S(INPUT_PULLUP) cause the TDM output to be reduced to stereo?

A: The exact reason for this issue is not clear, but it is likely caused by incorrect pin configuration, clock signal issues, or data signal issues. We recommend verifying the pin configuration, checking the clock signal, and checking the data signal to resolve this issue.

Q: Can I use I2S(INPUT_PULLUP) with TDM output?

A: While it is technically possible to use I2S(INPUT_PULLUP) with TDM output, it is not recommended. The TDM output may be reduced to stereo, which can cause issues with audio playback.

Q: How can I verify the pin configuration for the I2S device?

A: To verify the pin configuration for the I2S device, you can refer to the datasheet for the I2S device or consult with the manufacturer. You can also use a logic analyzer or oscilloscope to verify the pin configuration.

Q: How can I check the clock signal for the I2S device?

A: To check the clock signal for the I2S device, you can use a logic analyzer or oscilloscope to verify the clock signal. You can also refer to the datasheet for the I2S device or consult with the manufacturer.

Q: How can I check the data signal for the I2S device?

A: To check the data signal for the I2S device, you can use a logic analyzer or oscilloscope to verify the data signal. You can also refer to the datasheet for the I2S device or consult with the manufacturer.

Q: Can I use a different I2S device that supports TDM output?

A: Yes, you can use a different I2S device that supports TDM output. However, you will need to verify that the new device is compatible with your system and that it supports the required features.

Q: How can I troubleshoot issues with the I2S device?

A: To troubleshoot issues with the I2S device, you can refer to the datasheet for the I2S device or consult with the manufacturer. You can also use a logic analyzer or oscilloscope to verify the pin configuration, clock signal, and data signal.

Conclusion

Recommendations

Based on our analysis, we recommend the following:

Use I2S(OUTPUT) Configuration: Use the I2S(OUTPUT) configuration instead of I2S(INPUT_PULLUP) to ensure that the TDM output is transmitted correctly.
Verify Pin Configuration: Double-check the pin configuration for the I2S device to ensure that it is correct.
Check Clock Signal: Verify that the clock signal (MCLK or BCLK) is functioning correctly.
Check Data Signal: Verify that the data signal (DIN or DOUT) is functioning correctly.

By following these recommendations, we can ensure that the TDM output is transmitted correctly and that the I2S device functions as expected.