General Purpose Output

• The output buffer is enabled (open drain or push-pull).
• The Schmitt trigger input is active.
• The pull-up and -down resistors are active, according to PUPDR.
• Input data is sampled every AHB clock.
• Input data register holds I/O state.
• Output data register holds last written value.

Make sure the peripheral is enabled:

• Enable GPIOx in RCC→AHBENR[0].

Configure the MODE and PUPDR as well as the output type and speed register,
for every pin that should act as digital output:

• Write 0x01 to the corresponding fields in GPIOx→MODE.
• Configure pull-up / -down in GPIOx→PUPDR.
• Configure output type in GPIOx→OTYPER.
  (in conjunction with GPIOx→PUPDR)
• Configure output speed in GPIOx→OSPEEDR.