Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
How to set bits in C and write to STM32 GPIO registers using Arduino IDE

STM32duino GPIO Registers and Programming

Bit Setting in C

Setting a bit

Use the bitwise OR operator ( | ) to set a bit.

number |= 1 << x;

That will set bit x.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1 << x);

That will clear bit x. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1 << x;

That will toggle bit x.

Checking a bit

To check a bit, shift the number x to the right, then bitwise AND it:

bit = (number >> x) & 1;

That will put the value of bit x into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following:

number ^= (-x ^ number) & (1 << n);

Bit n will be set if x is 1, and cleared if x is 0.

GPIO Registers

The libmaple libraries, on which STM32duino is based, provides access to registers by the syntax:

GPIOA->regs->REG

where REG can be one of the following:

CRH and CRL

CRH is used to set type/and or speed of pins 8-15 of the port.
CRL is used to set type/and or speed of pins 0-7 of the port.
Accessed as a 32 bit word, with 4 bits representing the state of each pin. Out of these 4 bits, the low 2 bits are MODE, and high 2 bits are CNF.

alt text

The 4 bits for each pin can be set to:
0b0011 (binary) or 0x3 (HEX) - Corresponds to setting pin as output, same as pinMode()
0b1000 or 0x8 - Corresponds to setting pin as input, same as pinMode()

Say I want to set PORTA pins 0, 3 and 4 to OUTPUT and 1, 6, 7 to INPUT, and leave pins 2 and 5 in their original state. The code is:

PORTA->regs->CRL = (PORTA->regs->CRL & 0x00F00F00) | 0x88000080 |0x00033003;
//0x00F00F00 is bitmask to retain value of pins 2 and 5 in original state
//0x88000080 is bitmask to set inputs
//0x00033003 is bitmask to set outputs

IDR - Input Data Register

Used to read input of entire 16 pins of port at once. Accessed as a 32 bit word whose lower 16 bits represent each pin. The pins being read must be set to INPUT mode by using CRL/CRH or pinMode() before using this.

Say I want to read pins A2. The code is:

bool result = GPIOA->regs->IDR & 0x0004; //returns true if A2 is HIGH
//0x0004 is 0b0000000000000100

ODR - Output Data Register

Used to write output to entire 16 pins of port at once. Accessed and written as a 32 bit word whose lower 16 bits represent each pin. The pins being read must be set to OUTPUT mode by using CRL/CRH or pinMode() before using this.

Say I want to set pins A2, A12 and A13, and reset (clear) all other pins in the 16 pin bus. The code is:

GPIOA->regs->ODR = 0b0011000000000100; //note,  binary

Now if I want to set and clear A2, A12 and A13 without altering other pins, the code is:

//Set A2, A12, A13 (HIGH)
GPIOA->regs->ODR |= 0b0011000000000100;
//Clear A2, A12, A13 (LOW)
GPIOA->regs->ODR &= ~(0b0011000000000100);

but notice how, if we want to touch only some pins, we have to READ, MASK and WRITE. That's why there is BRR and BSRR

BRR - Bit Reset Register

32 bit word. Lower 16 bits have 1's where bits are to be set to "LOW". Upper 16 bits have 1's where bits are to be set "HIGH". 0's mean ignore

Now, to set and clear A2, A12, A13 while preserving the state of all other pins in the port, the code is:

//Set A2, A12, A13 (HIGH)
GPIOA->regs->BRR = 0b0011000000000100 << 16; //move to upper 16 bits
//Clear A2, A12, A13 (LOW)
GPIOA->regs->BRR = 0b0011000000000100;

BSRR - Bit Set Reset Register

BSRR is like the complement of BRR. It's also a 32 bit word. Lower 16 bits have 1's where bits are to be set to "HIGH". Upper 16 bits have 1's where bits are to be set "LOW". 0's mean ignore

In this case, to set and clear A2, A12, A13 while preserving the state of all other pins in the port, the code is:

//Set A2, A12, A13 (HIGH)
GPIOA->regs->BSRR = 0b0011000000000100;
//Clear A2, A12, A13 (LOW)
GPIOA->regs->BSRR = 0b0011000000000100 << 16; //move to upper 16 bits

Combination of BRR and BSRR

Since BRR and BSRR are opposite of each other, you can use both if you don't want to do the bit shift left operation .

In this case, to set and clear A2, A12, A13 while preserving the state of all other pins in the port, the code is:

//Set A2, A12, A13 (HIGH)
GPIOA->regs->BSRR = 0b0011000000000100; //lower 16 bits
//Clear A2, A12, A13 (LOW)
GPIOA->regs->BRR = 0b0011000000000100; //lower 16 bits

Sources:

@sengiv
Copy link

sengiv commented May 1, 2021

This saved me hours of work, thanks!!

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment