SDK • I2C LED driver (IS31FL3731) detected at 0x74, but no PWM / LED output on RP2040

Hi everyone,

I'm working on an RP2040-based board and trying to get the **IS31FL3731** LED matrix driver working, but so far I haven’t had any success getting it to light up.

Here’s what I’ve done and where I’m stuck.

## Summary

- **I2C scan detects the device at 0x74**. So the physical connection seems okay.
- Pull-up resistors are in place.
- I confirmed multiple times: the device responds to an I2C scan.
```
Found I2C device at address 0x74
```
- However, any attempts to configure the device or turn on LEDs fail. No visible response.

## Hardware / Setup

- MCU: RP2040 (Raspberry Pi Pico)
- Supply voltage: 3.3V
- I2C address: 0x74 (default, A0 = GND)
- SDK: Pico C SDK
- Using my own minimal driver, referencing Adafruit’s IS31FL3731 library

## What I tried

- Sent shutdown register (0x0A) = 0x01 (exit shutdown)
- Config register (0x00) = 0x00
- Tried writing to PWM registers (after setting frame/page), no LED output
- Also tried selecting different frames and writing PWM values
- Added small delays between commands just in case

## Repository

My current test code is available here:
[https://github.com/jys923/tinyusb-pico-hid](https://github.com/jys923/tinyusb-pico-hid)

The IS31FL3731 test logic is located in the `main.c` file (see `is31fl3731_init()` and `is31fl3731_set_led_pwm()` functions).

## Questions

- Is there a strict initialization sequence I may be missing?
- Do I need to enable a frame or set something else before writing to PWM?
- Could it be a timing issue or does it require specific delays?

Any help, tips, or working examples for RP2040 or baremetal C would be greatly appreciated.
Again, the I2C scan **does detect the chip at 0x74**, so I believe it’s wired correctly.

Thanks in advance!

Statistics: Posted by jys923 — Mon Jun 30, 2025 6:33 am — Replies 0 — Views 35

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