bare metal rust code for STM32F103

blue_pill.rs

#![no_std]
#![no_main]

use panic_rtt_target as _;
use cortex_m_rt::entry;
use core::ptr;

#[entry]
fn main() -> ! {
    // Set up named variables for the register addresses we'll use. These are defined in the STM32's
    // reference manual (RM): rm0008. You can find base addresses under the Memory and bus architecture chapter, and the offsets (Things we | the base addresses with), under the RCC and GPIO chapters, registers sections.

    let RCC_AHB2ENR = 0x4002_1000 | 0x18;
    let GPIOC_BASE = 0x4001_1000;

    // You can find these offsets under each RM register table: `Address Offset`
    let GPIOC_CRH = GPIOC_BASE | 0x04;
    let GPIOC_BSRR = GPIOC_BASE | 0x10;
    

    let gpiocen_field = 4;
    let output_pin_num = 13;  // A variable to indicate with GPIO pin number we're using. Ie PC13
    

    unsafe {
        // Set the GPIOCEN field of the RCC register block, AHB2_ENR register. This enables the peripheral clock
        // to all pins on the GPIOC port.
        ptr::write_volatile(RCC_AHB2ENR as *mut u32, 1 << gpiocen_field);

        // Set the mode of PC13 to Output, The hex values 0b01 is defined in the GPIO registers section of the RM
        // We Multiply the pin numbers by 4, since each field of this register is 4 bits wide. Also note that we  perform a register read, as not to override previous settings. 

        let mut existing_val = ptr::read_volatile(GPIOC_CRH as *const u32);
        // We use binary arithmetic here to 0 out the fields we’ll write to. Modifying single-bit fields is simpler.
        existing_val = existing_val & !(0b1111 <<  ((output_pin_num -8) * 4) );  //-8 being the offset for CRH register

        ptr::write_volatile(
            GPIOC_CRH as *mut u32, 
            existing_val | (0b0001 << ((output_pin_num -8) * 4) )  // output mode @ 10MHz max speed; general purpose push-pull
        );

        // check schematic for this bit inverse

        // light up LED; RESET PC13 (0V)
        ptr::write_volatile(GPIOC_BSRR as *mut u32, 1 << (output_pin_num + 16));
        
        // turn off LED; SET PC13 (3.3V)
        //ptr::write_volatile(GPIOC_BSRR as *mut u32, 1 << (output_pin_num));


    }
    loop {}
}