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[Bug]: RAK12500 GPS not detected on RAK19001 — firmware only probes UART, never I2C #9904

@mowaxuk

Description

@mowaxuk

Category

Other

Hardware

Rak4631

Is this bug report about any UI component firmware like InkHUD or Meshtatic UI (MUI)?

  • Meshtastic UI aka MUI colorTFT
  • InkHUD ePaper
  • OLED slide UI on any display

Firmware Version

2.7.15.567b8ea

Description

Hardware:

Base board: RAK19001 (Dual IO, 6 sensor slots A-F)
Core: RAK4630 (nRF52840)
GPS module: RAK12500 (u-blox ZOE-M8Q) in Sensor Slot A
SD card: RAK15002 in IO slot (SPI, no I2C conflict)
Firmware: 2.7.15.567b8ea VANILLA

Describe the bug:
The RAK12500 GPS module is not detected by Meshtastic firmware. gps_mode is set to ENABLED but gps_enabled persistently reports false and position returns empty {}.
Serial debug output via PuTTY (115200 baud) shows the firmware probing for GPS over UART only, cycling through all baud rates and chip families before giving up:

DEBUG | [GPS] Trying $PDTINFO (Unicore Family)...
DEBUG | [GPS] Trying $PCAS06,1*1A (ATGM33xx Family)...
DEBUG | [GPS] Trying $PAIR021*39 (Airoha Family)...
DEBUG | [GPS] Trying $PQTMVERNO*58 (LC86)...
DEBUG | [GPS] Trying $PCAS06,0*1B (L76K)...
DEBUG | [GPS] Trying $PMTK605*31 (MTK Family)...
WARN  | [GPS] No GNSS Module (baudrate 9600)
WARN  | [GPS] Give up on GPS probe and set to 9600

The firmware never attempts I2C detection. The u-blox ZOE-M8Q communicates over I2C, so it is never found.

Hardware confirmed working:
Flashing the RAK WisBlock Hardware Test firmware (WB_HW_Test_V1.1.11_H) immediately detects and communicates with the RAK12500, producing valid GNSS output. This confirms the module, wiring, and slot are all correct — the issue is specific to Meshtastic firmware.

Expected behaviour:
Meshtastic should probe for GPS over I2C on the RAK19001 sensor slots, as the RAK12500 uses the u-blox ZOE-M8Q which communicates over I2C. The 3V3_S power rail (controlled by IO2/GPIO34) may also not be being enabled by firmware, preventing the module from powering up before UART probing begins.

**

Image

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RAK4630 on RAK19001 base board
RAK12500 in Sensor Slot A
Flash Meshtastic 2.7.15 VANILLA
Enable GPS in position config
Observe serial debug — GPS probe attempts UART only and fails

Additional context:
GPS pin settings are all at 0 (rx_gpio, tx_gpio, gps_en_gpio) which is understood to be correct for RAK WisBlock builds per the RAK forum. This issue has also been discussed at https://forum.rakwireless.com/t/issue-with-rak19001-and-rak12500/16109

Relevant log output

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