How to configure the GPIO pins on an STM32L152?

Question

I got an STM32L discovery board that contains an stm32l152rb microprocessor. I try to program that board in assembly without any library at all. So far I succeeded in writing linker scripts and a basic routine that copies the .data section into the RAM and zeroes out the bss section. But apart from that I am a little bit lost.

As a tiny experiment to understand how configuring GPIo works, I want to make a program for the board that lights the green LED connected to GPIO port B, pin 7. Reading the datasheet, I conclude that I need to do the following two things:

1. Configure GPIO port B, pin 7 as an output pin
2. Set said pin to high

But at this place, I figuratively don't see the forest for the trees. I am not sure which of the multiple IO modes avaible I should choose and all these acronyms confuse me. I would really appreciate somebody giving me detailed information, instruction and possibly example assembly on how to do this.

Answer 1

Before using GPIO module you must do the following:

1. Reset the GPIO registers:
   • RCC module, register AHBRSTR, bit GPIOBRST set to 1 - rst high
   • RCC module, register AHBRSTR, bit GPIOBRST set to 0 - rst low
2. Enable GPIO registers clock:
   • RCC module, register AHBENR, bit GPIOBEN set to 1 - Enable clock

After that you can start using GPIO registers

1. GPIO module, register GPIOB_MODER, bitfield [15:14] set to 01 - General purpose output mode
2. GPIO module, register GPIOB_OTYPER, bit 7 set to 0 - Output push-pull
3. GPIO module, register GPIOB_OSPEEDR, bitfield [15:14] set to 00 - 400 kHz Very low speed
4. GPIO module, register GPIOB_PUPDR, bitfield [15:14] set to 00 - No pull-up, pull-down

Now you can use either GPIOB_BSRR (set/reset) register or GPIOB_ODR (output) register:

• GPIO module, register GPIOB_BSRR, bit 7 set to 1 - Set GPIOB pin 7 (high)
• GPIO module, register GPIOB_BSRR, bit 23 set to 1 - ReSet GPIOB pin 7 (low)