Other projects • RP2040 Boot Instability on Power-Up

# RP2040 Boot Instability on Power-Up (with W25Q16JVUXIQTR SPI Flash)

## System Configuration
- **MCU**: Raspberry Pi RP2040
- **SPI Flash**: W25Q16JVUXIQTR (2MB)
- **Firmware**: QMK-based (`.uf2` image)
- **Power Source**: USB (connected to PC)
- **Repository (source + schematic)**:
https://github.com/jys923/sonocap_v2_2/ ... onocap_v2/

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## config.h Settings

```c
#define RP2040_FLASH_GENERIC_03H
#define PICO_XOSC_STARTUP_DELAY_MULTIPLIER 64
```

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## Observed Symptoms

- When the **PC powers on** and supplies USB power to the RP2040, it **occasionally fails to boot**.
- Two main failure patterns:
1. **No LED activity at all** – appears that firmware doesn't start.
2. **LEDs light up and stay on longer than expected** (not the intended 200ms flash), and the board becomes unresponsive (no key input, no USB HID).
- When **USB is unplugged and reconnected**, the board boots **correctly** every time.

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## LED Boot Behavior

```c
void keyboard_post_init_kb(void) {
if (!boot_flash_done) {
for (uint8_t i = 0; i < MATRIX_CNT; i++) update_led(i, 255, true);
boot_flash_done = true;
}
}
```

- **Normal case**: All LEDs turn on briefly (~200ms), then turn off.
- **Failure case**:
- Sometimes: **no LED flash at all**.
- Other times: **LEDs flash but remain on too long** and the board remains unresponsive.

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## GDB Debug Information (on failure)

```
pc = 0xfffffffe
lr = 0xfffffff1
sp = 0x20040290
x/8i $pc → movs r0, r0 (endless loop)
```

- `pc = 0xfffffffe` indicates a fallback condition, often due to boot vector fetch failure.
- May imply **XIP failure** during external flash access.
- Stack pointer is within a valid SRAM range.

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## Verified Behavior

| Scenario | Result |
|----------------------------------|----------------------|
| Unplug and replug USB | Always boots fine |
| GPIO-only custom firmware | Boots reliably |
| QMK-based firmware | Boot occasionally fails |
| Same board/flash/schematic | Only QMK shows issue |

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## Suspected Causes

- RP2040 **boot ROM fails to read vector table** from SPI flash at cold start.
- Timing issue between flash response and RP2040 XIP engine?
- Flash not ready fast enough during QMK's early initialization?
- QMK boot sequence may touch memory or peripherals too early.
- USB + flash power sequencing?

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## Request for Help

Any insights or similar experiences would be very appreciated.
I'm happy to share additional logs, boot2 binaries, oscilloscope traces, or minimal repro cases if needed.

Thank you!

Statistics: Posted by jys923 — Thu Jun 05, 2025 2:34 am

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