ARM Embedded Tutorial - Simple Timer

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Simple timer function

In this example, a simple timer for an STM32F103C is used. This microcontroller has 4 timers and you can use timers 2-4 very easily. Timer 1 has several options, so it is more complicated to use.

Program head

program Timer_Blink;

uses
  cortexm3;

Constants and identifiers

const
  TIM_CR1_CEN : uInt16 = $0001;            // Counter enable
  RCC_APB1ENR_TIM2EN : uInt32 = $00000001; // TIM2 clock enabled

  TIM_DIER_CC1IE : uInt16 = $0002;         // Capture / Compare 1 interrupt enable
  TIM_SR_CC1IF : uInt16 = $0002;           // Capture / Compare 1 interrupt flag
  TIM2_IRQn = 28;                          // TIM2 global interrupt

  // For easy access to the LED PC13.
  Type
    TLed = bitpacked array[0..15] of boolean;

var
  LedC : TLed absolute GPIOC.ODR;

Interrupt function

procedure Timer2_Interrupt; public name 'TIM2_global_interrupt';  interrupt;
  begin
    TIM2.SR  := TIM2.SR and not TIM_SR_CC1IF; // Clear CC1IF
    LedC[13] := not LedC[13];                 // Toggle LED PC13
  end;

Main program and initialization

With TIM2.PSC and TIM2.ARR you can set the speed of the timer, the smaller the values, the faster it runs.

  begin
    // Turn on ports
    RCC.APB2ENR := RCC.APB2ENR or (%111 shl 2);

    // Switch ports to output
    GPIOC.CRL := $33333333;
    GPIOC.CRH := $33333333;

    // --- timer ---
    // RCC enable for Timer2
    RCC.APB1ENR := RCC.APB1ENR or RCC_APB1ENR_TIM2EN;

    // Various default settings
    TIM2.CR1   := $0000;  // Control register 1;  Wait with timer enable until settings complete
    TIM2.CR2   := $0000;  // Control register 2
    TIM2.CCMR1 := $0000;  // Capture / compare mode register
    TIM2.CCER  := $0000;  // Capture / compare enable register

    // Prescaler
    TIM2.PSC := 1000;

    // AutoReload value
    TIM2.ARR := 1000;

    // DMA and interrupt control register
    TIM2.DIER := TIM_DIER_CC1IE;  // Compare 1 interrupt enable

    // Enable counter
    TIM2.CR1 := TIM2.CR1 or TIM_CR1_CEN;

    // NVIC configuration
    NVIC.IP[TIM2_IRQn] := $20;    // IRQ priority, right 4 bits always to be zero
    NVIC.ISER[TIM2_IRQn shr 5] := 1 shl (TIM2_IRQn and $1F);  // Enable interrupt

    // --- main loop ---
    while True do begin
      // Do something
    end;
 end.

Multiple timers

You can use 3 simple timers in parallel. These are Timer2 to Timer4. Timer1 is used differently. If you want to use timers 3 and 4, you simply have to replace TIM2 with TIM3 and/or TIM4.

See also

• ARM Embedded Tutorials - Overview