Question about serial interfaces on RAK3172


I plan to build up a LoRa GPS tracker with an RAK3172 module and custom firmware which is programmed in STM32CubeIDE. Because I want to use the LPUART for the GPS module to use UART-based wakeups I decided to use UART1 with the bootloader to upload the firmware.

According to the bootloader documentation from STM, this should be possible but I want to be sure.

I also added all the stuff into CubeIDE (no BSP currently) and chose 9600 baud for the LPUART and 115200 baud for UART1 for programming.

Can I use this setup with your manual about using custom firmware concerning the different UART pins? If so is it possible to use this setup with one of my RAK3272S boards (RAK3172 SiP)?

Thanks for answering!

Welcome back to forum @kampi ,

If you plan to use UART1 for bootloading, you must use PA9 and PA10 and not the UART1 mapping of RAK3172.

PB6 and PB7 is our mapping of UART1 peripheral. But those pins cannot be used for the STM32 UART bootloader.

The UART bootloader pins are hardwired inside the chip and can’t be mapped.

You can check STM32WLExxx reference manual for more details. But here’s a screenshot of the section showing the pins that you can use.

It should work with the RAK3172-SIP but you are missing a number of features in the CubeIDE IOC config including the RF switches and the HSE clock. These are different between the RAK3172 and the RAK3172-SIP. The -SIP uses different RFSWITCH pins and I recall uses a TCXO. It is minor stuff to fix though.

Hi @carlrowan,

thanks for the response. Okay so I understand it wrong because I thought the same limitations for the pins (i. e. capabilities for interfaces like PB6 for UART) are also used by the bootloader. But it looks different. I will change this! So be on the right page: I have to use PA9 (SCL and Bootloader TX) and PA10 (SDA and Bootloader TX)

Hi @Hagrid
the HSE clock is on the bottom right. The RFSWITCH pins are missing, that´s true. I haven´t added the BSP stuff so far :slight_smile:

You are correct.

PA9 - TX
PA10 - RX

I just confirmed on the RAK3272 Breakout Board. These pins are not exposed so I have to solder directly on pins.


Hi @carlrowan,

thank you for checking it out.

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