How to read or write peripheral registers

1 Article purpose[edit]

This article provides the basic information needed to start using the Devmem tool, which allows the reading and writing of peripheral registers.

2 Introduction[edit]

The following table provides a brief description of the tool, as well as its availability depending on the software packages:

Yes: this tool is either present (ready to use or to be activated), or can be integrated and activated on the software package.

No: this tool is not present and cannot be integrated, or it is present but cannot be activated on the software package.

Tool STM32MPU Embedded Software distribution STM32MPU Embedded Software distribution for Android™
Name Category Purpose Starter Package Developer Package Distribution Package Starter Package Developer Package Distribution Package
Devmem Tracing tools Used for reading and writing in peripheral registers Yes Yes Yes No No No

3 Installing Devmem on your target board[edit]

You can install Devmem either with the Starter Package, the Developer Package or the Distribution Package. For the Starter Package, board internet access is required.

3.1 Using the STM32MPU Embedded Software distribution[edit]

3.1.1 Starter Package[edit]

Enter the following commands to install Devmem:

Board $> sudo apt-get update
Board $> sudo apt-get install devmem2

For more information about apt-get, see the Package_repository_for_OpenSTLinux_distribution article.

3.1.2 Developer Package[edit]

Enter the following commands:

  • Download sources :
PC $> wget http://free-electrons.com/pub/mirror/devmem2.c 
  • Compile the application :
PC $> make devmem2
  • Install it on the board:
PC $> scp devmem2 root@<board ip address>:/usr/bin

3.1.3 Distribution Package[edit]

Enter the following commands:

  • Build the package with bitbake:
PC $> bitbake devmem2
  • Add it to the targeted image:
PC $> echo 'IMAGE_INSTALL_append += "devmem2"' >> meta-st/meta-st-openstlinux/recipes-st/images/st-image-weston.bbappend
  • Rebuild the image:
PC $> bitbake st-image-weston

You must then Flash the generated image onto your board using STM32CubeProgrammer.

4 Getting started[edit]

To use Devmem, enter the following command:

Board $> devmem2 [address] [type] [data]

Where:

  • [address] corresponds to the register address you want to read
  • [type] corresponds to the desired output value size (b, h, w or l, respectively byte, halfword, word or long). Assigning a type is not necessary if you only read the register - the default output type is a word.
  • [data] corresponds to the data value you wish to write to the register, assigning a type is mandatory in this case . If data is not assigned, the register is only read.

5 To go further[edit]

5.1 Checking UART4 baud rate[edit]

The UART baud rate is generated from the UART clock frequency, with the USARTDIV parameter field in the USART_BRR register. By default in the OpenSTLinux Starter Package delivery, the UART4 baud rate is set to 115 000, the clock frequency CLOCK_UART4_K value is 64 000 000 Hz and USARTDIV is equal to 556. These values are known, and the USARTDIV value is retrieved by using Devmem to read the STM32MP1 registers.

On the STM32MP157 Reference Manual, we see that the base address of UART4 is 0x4001 0000 and the USARTDIV value is located at offset 0x0C.

Board $> devmem2 0x4001000C
/dev/mem opened.
Memory mapped at address 0xb6fc300c.
Read at address 0x4001000C (0xb6fc300c): 0x0000022C

This is equal to 556 decimal.

5.2 Driving LEDs with Devmem[edit]

In this example, we check the output value of GPIO port A pins 13 and 14 corresponding to LEDs 6 and 5 on DK2. According to the Reference Manual, the base address for GPIOA is 0x5000 2000 and the output value of the LEDs is located at the offset 0x14.

However, the GPIO clock must first be enabled for the results to be readable from the registers. Otherwise the resulting output value is 0:

Board $> devmem2 0x50002014
/dev/mem opened.
Memory mapped at address 0xb6f43014.
Read at address 0x50002014 (0xb6f43014): 0x00000000

The clock is named RCC_MP_AHB4ENSETR and is located at address 0x5000 0A28.

Board $> devmem2 0x50000A28
/dev/mem opened.
Memory mapped at address 0xb6fdca28.
Read at address 0x50000A28 (0xb6fdca28): 0x00000000

In this register, the first eleven bits (from LSB to MSB) allow GPIOA to GPIOK respectively to be enabled (1 enable, 0 disable). In this case only GPIOA needs to be enabled, so we write 0x1 to the register.

Board $> devmem2 0x50000A28 w 0x1
/dev/mem opened.
Memory mapped at address 0xb6fdca28.
Read at address 0x50000A28 (0xb6fdca28): 0x00000000
Write at address 0x50000A28 (0xb6fdca28): 0x00000001, read back 0x00000001

We now check the content of the register corresponding to GPIOA:

Board $> devmem2 0x50002014
/dev/mem opened.
Memory mapped at address 0xb6f43014.
Read at address 0x50002014(0xb6f43014): 0x00002400

In this case only LED 5 is enabled. The following table shows the register output value as a function of LED 5 and 6 states.

LED 5 enabled LED 5 disabled
LED 6 enabled 0x400 0x4400
LED 6 disabled 0x2400 0x6400

For example to enable both LEDs, set the value of the register to 0x400:

Board $> devmem2 0x50002014 w 0x400
/dev/mem opened.
Memory mapped at address 0xb6f43014.
Read at address 0x50002014 (0xb6f43014): 0x00002400
Write at address 0x50002014 (0xb6f43014): 0x00000400, readback 0x00000400

The change can be seen on your board.

6 Restricted access[edit]

Keep in mind that the Devmem tool only allows access to peripheral registers. Also all the registers in M4 and A7 cores are secured and attempting to read them causes the board to reboot.

When trying to access a register, make sure the related peripheral has not been isolated on the MCU side, otherwise access is impossible due to access rights.