Use BSRR and BRR registers to directly operate the STM32 I/O ports

Each GPIO port of STM32 has two special registers, GPIOx_BSRR and GPIOx_BRR registers, through which the corresponding GPIOx port can be directly set to '1' or '0'.

Each of the upper 16 bits of GPIOx_BSRR corresponds to each bit of port x. If a certain position in the upper 16 bits is '1', the corresponding bit of port x is cleared to '0'; if the position in the register is '0', it has no effect on the corresponding bit. Each of the

lower 16 bits of GPIOx_BSRR also corresponds to each bit of port x. If a certain position in the lower 16 bits is '1', the corresponding port bit is set to '1'; if the position in the register is '0', it has no effect on the corresponding port.

Simply put, the upper 16 bits of GPIOx_BSRR are called clear registers, and the lower 16 bits of GPIOx_BSRR are called set registers. Another register GPIOx_BRR has only the lower 16 bits valid, and has the same function as the upper 16 bits of GPIOx_BSRR.

Give an example to illustrate how to use these two registers and the advantages they embody. For example, the 16 IOs of GPIOE are all set to output, and each operation only needs to change the lower 8 bits of data while keeping the upper 8 bits unchanged. Assuming that the new 8 bits of data are in the variable Newdata,

this requirement can be achieved by operating these two registers. There are two functions GPIO_SetBits() and GPIO_ResetBits() in the STM32 firmware library that use these two registers to operate the port.

The above requirements can be achieved as follows:

GPIO_SetBits(GPIOE, Newdata & 0xff);
GPIO_ResetBits(GPIOE, (~Newdata & 0xff));

You can also directly operate these two registers:

GPIOE->BSRR = Newdata & 0xff;
GPIOE->BRR = ~Newdata & 0xff;

Of course, you can also complete the operation of 8 bits at a time:

GPIOE->BSRR = (Newdata & 0xff) | (~Newdata & 0xff)<<16;

From the last operation, it can be seen that using the BSRR register, 8 port bits can be modified at the same time.

If the BRR and BSRR registers are not used, the above requirements need to be implemented as follows:

GPIOE->ODR = GPIOE->ODR & 0xff00 | Newdata;

Using the BRR and BSRR registers can easily and quickly implement operations on certain specific bits of the port without affecting the status of other bits.

For example, if you want to quickly flip bit 7 of GPIOE, you can do:

GPIOE->BSRR = 0x80; // set to '1'
GPIOE->BRR = 0x80; // set to '0'If

you use the conventional 'read-modify-write' method:

GPIOE->ODR = GPIOE->ODR | 0x80; // set to '1'
GPIOE->ODR = GPIOE->ODR & 0xFF7F; // set to '0'Some

people ask whether the upper 16 bits of BSRR are redundant. Please see the following example:

If you want to set bit 7 of GPIOE to '1' and bit 6 to '0' in one operation, it is very convenient to use BSRR:
  GPIOE->BSRR = 0x4080;

If there is no upper 16 bits of BSRR, it will be divided into two operations, resulting in the changes of bit 7 and bit 6 being out of sync!
  GPIOE->BSRR = 0x80;
  GPIOE->BRR = 0x40;