ARM Embedded Tutorial - Simple GPIO on and off output

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Easy GPIO input and output

A port consists of 16 pins, which is why the lower 16 bits are used in most registers.

Exception: CRx and BSRR, with BSRR you can use the upper 16 bits to set pins.

Port access registers

The following registers are available for GPIO access:

  • CRL - Configuration Register Low
  • CRH - Configuration Register High
  • IDR - Input Data Register
  • ODR - Output Data Register
  • BSRR - Bit Set Reset Register
  • BRR - Bit Reset Register
  • LCKR - Port Configuration Lock Register

With all registers except CRL and CRH, each bit corresponds to a microcontroller pin.

Switch on GPIO

Before you can access the GPIO, you have to switch it on. Without doing this, no access is possible! This is done as follows for the PortA - PortC. 

  RCC.APB2ENR := RCC.APB2ENR or (% 111 shl 2);

Direct port manipulation

Before you can switch a GPIO pin to HIGH, you have to configure it as an output. This is done via the CRL and CRH registers.

I usually choose Pin 13 of Port C for these mini-examples, because the LED of the STM32F103C is connected to this pin.

Attention: The LED lights up as the anode is connected to the power supply.

CRx - Set pin function

One nibble (4 bit) is available for the function setting. Pin 13 of Port C on output. Since 13 is higher than 7, the HIGH register CRH is used for this. 13 - 8 = 5; It is the fifth nibble in CHR. Pin0 - Pin7 would be configured with CHL. 

  PortC.CRH := $00300000;

IDR - Read out entire port

  // Is Pin 13 HIGH?
  Result := PortC.ODR and (1 shl 13) > 0;

ODR - Write entire port

With this function, the whole port is written at once. 

  // Only pin13 is set to HIGH.
  PortC.ODR := 1 shl 13; 

  // All pin on LOW.
  PortC.ODR := 0;

BSRR - Single pin to LOW

With this function, you can set individual pins to LOW. 

  // Only set pin13 to LOW, the others remain unchanged.
  PortC.BSRR := 1 shl 13;

BSRR - Change the entire port using the bit mask

With this function, you can set individual pins to LOW. 

  // Only set Pin13 to LOW, PIN12 to HIGH, the others remain unchanged.
  PortC.BSRR := (1 shl 13) + ((1 shl 12) shl 16);

BRR - Single pin to HIGH

With this function, individual pins can be set to HIGH. 

  // Only set pin13 to HIGH, the others remain unchanged.
  PortC.BSRR := 1 shl 13;

The upper 16 bits of BSRR correspond to the 16 bits of BRR.

See also

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