File content as of revision 11:42d5cbbd0f3e:

#include <mbed.h>
#include <iostream>

using namespace std;

void inline led_init ()
{
    // Led is at PA5.
    // -
    // Set PA5 as AF1 (TIM2_CH1).
    // -
    // Alternate Function Low Register.
    // -
    GPIOA->AFR[0] &= ~GPIO_AFRL_AFRL5;
    GPIOA->AFR[0] |= 1 << GPIO_AFRL_AFRL5_Pos;
    // Port Output Speed Register.
    // -
    GPIOA->OSPEEDR |= GPIO_OSPEEDER_OSPEEDR5;
    // Port Mode Register.
    // -
    GPIOA->MODER &= ~GPIO_MODER_MODER5;
    GPIOA->MODER |= 2 << GPIO_MODER_MODER5_Pos;
}

void inline reset_timer ()
{
    // Peripheral Reset Register.
    //     TIM2 timer reset.
    // -
    RCC->APB1RSTR |= RCC_APB1RSTR_TIM2RST;
    RCC->APB1RSTR &= ~RCC_APB1RSTR_TIM2RST;
}

void inline clock_enable_for_timer ()
{
    reset_timer();
    // Peripheral Clock Enable Register.
    //     TIM2 Timer Clock Enable.
    // -
    RCC->APB1ENR |= RCC_APB1ENR_TIM2EN;
}

void inline timer_setup_pwm ()
{
    const double duty_cycle = 0.6;
    // Capture/ Compare Register 1.
    // -
    TIM2->CCR1 = TIM2->ARR * duty_cycle;
    // Capture/ Compare Mode Register 1.
    //     Output Compare 1 Mode.
    //     Output Compare 1 Preload Enable.
    //     Output Compare 1 Fast Enable.
    // -
    TIM2->CCMR1 = (0b111 << TIM_CCMR1_OC1M_Pos) | TIM_CCMR1_OC1PE | TIM_CCMR1_OC1FE;
    // ^ 0b111 : PWM Mode 2 : On while CCR < ARR, OFF while >=.
    // -
}

void inline timer_enable_pwm ()
{
    timer_setup_pwm();
    // Capture/ Compare Enable Register.
    //     Capture/ Compare 1 Output Enable.
    // -
    TIM2->CCER = TIM_CCER_CC1E;
}

void inline timer_downscale_by (const unsigned short& v)
{
    // Prescaler.
    // -
    TIM2->PSC = v;
}

void inline timer_limit_counter_to (const unsigned int& v)
{
    // Auto-Reload Register.
    // -
    TIM2->ARR = v;
}

void inline timer_set_period_to (const double& T)
{
    // Precondition : T <= 59 (= (2^32 - 1) / 72e6).
    // -
    timer_limit_counter_to(72000000 * T);
    // ^Explanation : By limiting ARR to 72e6, the counter gets filled in 1sec (at 72Mhz).
    // -
}

void inline timer_enable_interrupt ()
{
    // DMA/ Interrupt Enable Register.
    //     Update Interrupt Enable.
    // -
    TIM2->DIER = TIM_DIER_UIE;
}

void inline timer_enable ()
{
    // Control Register 1.
    //     Update Request Source | Counter Enable.
    // -
    TIM2->CR1 = TIM_CR1_URS | TIM_CR1_CEN;
}

void inline timer_clear_status ()
{
    // Status Register.
    // -
    TIM2->SR = 0;
}

/* 
    // I was unable to use the following handler :
    // -
    extern "C" void TIM2_IRQHandler (void)
    {
        cout << "Interrupt hit!" << endl;
        clear_timer_status();
        toggle_LED();
    }
*/

// I couldn 't get TIM2_IRQHandler() to work; probably wrong name. Thus, I 'll use a custom handler
// and attach it using SetVector() from CMSIS : https://arm-software.github.io/CMSIS_5/Core/html/group__NVIC__gr.html.
// -
void interrupt_handler ()
{
    timer_clear_status();
}

void inline interrupt_enable ()
{
//    NVIC_SetPriority(TIM2_IRQn, 250);
    NVIC_SetVector(TIM2_IRQn, reinterpret_cast<uint32_t>(interrupt_handler));
    NVIC_EnableIRQ(TIM2_IRQn);
    // ^Definition of NVIC_SetVector() : https://arm-software.github.io/CMSIS_5/Core/html/group__NVIC__gr.html#gab43c1c59d5c081f1bc725237f4b1f916.
    // -
}

void inline timer_init ()
{
    clock_enable_for_timer();
    timer_set_period_to(0.1);
    timer_enable_interrupt();
    timer_enable_pwm();
    timer_enable();
}

int main ()
{
    cout << "Entered main()." << endl;
    led_init();
    timer_init();
    interrupt_enable();
    cout << "Exiting main().." << endl;
}