How to read or write peripheral registers

1. Installation

For reading and writing STM32MP1 registers, we will use Devmem tool. There are two ways for install it on your board:

• If you have an internet access on your board through apt-get
• Else through bitbake

1.1. With apt-get

Just enter the following commands:

 sudo apt-get update
 sudo apt-get install devmem2

1.2. With bitbake

Enter the following commands:

 bitbake devmem2
 echo 'IMAGE_INSTALL_append += "devmem2"' >> meta-st/meta-st-openstlinux/recipes-st/images/st-image-weston.bbappend
 bitbake st-image-Weston

Then you just have to flash the generated image on your board with CubeProgrammer.

2. Use of Devmem

To use Devmem, enter the following command:

 devmem2 [address] [type] [data]

Where:

• [address] corresponds to the register address you want to read
• [type] corresponds to the output value size you want (b, h, w or l, respectively byte, halfword, word or long). Assigning a type is not necessary if you only read the register and the default output will be a word.
• [data] corresponds to the data value you want to set on the register, assigning a type is mandatory in this case . If you don't assign a data, it will only read the register.

3. Examples of Devmem use

3.1. UART4 Baudrate

The Baudrate is defined as the Clock frequency over the Usartdiv. The Clock frequency clock_uart4_k value is 64 000 000 Hz and the Usartdiv is approximatively equal to 115 108. So the UART4 Baudrate is equal to 556. These results are known and thanks to Devmem we will retrieve the Baudrate value by looking at STM32MP1 registers. On the STM32MP157 Reference Manual, we find out that the base address of UART4 is 0x4001 0000 and the Baudrate value is at the offset 0x0C.

 devmem2 0x40010000
0x22C

Which is equal to 556 in decimal.

3.2. GPIO

In this example, we will check the value of GPIO hardware configuration register 10 of the port A. According to the Reference Manual, we find out that the base address for GPIOA is 0x5000 2000 and the hardware configuration register 10 is at the offset 0x3C8. Normally the value in this register must be 0x0000 1240. However, GPIOs' clock have to be enabled at first if we want to see some results in registers. Else it will output 0 has you can see:

 devmem2 0x500023C8
Read at address 0x500023C8 (0xb6f683c8): 0x00000000

This clock is called RCC_MP_AHB4ENSETR and is located at the address 0x5000 0A28.

 devmem2 0x50000A28
Read at address 0x50000A28 (0xb6fdca28): 0x00000000

In this register, the first eleven bits (from LSB to MSB) allow enabling respectively from GPIOA to GPIOK (1 enable, 0 disable). In our case we only need the GPIO A to be enabled so we will write 0x1 in the register.

 devmem2 0x50000A28 w 0x1
Read at address 0x50000A28 (0xb6fdca28): 0x00000000
Write at address 0x50000A28 (0xb6fdca28): 0x00000001, readback 0x00000001

Now let's check again in the GPIOA's register:

 devmem2 0x500023C8
Read at address 0x500023C8 (0xb6fdca28): 0x00001240

General-Purpose Input/Output (A realization of open ended transmission between devices on an embedded level. These pins available on a processor can be programmed to be used to either accept input or provide output to external devices depending on user desires and applications requirements.)

Reset and Clock Control