Using Periodic Interrupt Mode

A basic use case of the timer is to set it to trigger an interrupt every time it is updated. This mode is useful if a piece of code must be executed repeatedly every few milliseconds. The user must enable the interrupts and set an Interrupt Service Routine (ISR), which will contain the appropriate code. A basic example containing the initialization and an ISR is provided below. The program will toggle a pin every 250 ms using TCA's periodic interrupts. A pin must be configured as an output by setting the corresponding bit of the Direction register before the initialization of the timer as described below. In this case, Port A pin 0 (PA0) was chosen.

1. 1.Setting the corresponding bit in the Interrupt Control register enables the overflow interrupt of TCA.

   TCA0.SINGLE.INTCTRL = TCA_SINGLE_OVF_bm;

   Figure 1. Interrupt Control Register
2. 2.In this mode, no waveform must be generated, so the Waveform Generation bit field in the CTRLB register must be configured accordingly.

   TCA0.SINGLE.CTRLB = TCA_SINGLE_WGMODE_NORMAL_gc;

   Figure 2. CTRLB Register
3. 3.Since the timer may count clock ticks, not events, the CNTEI bit of the EVCTRL register must be set to ‘0’. It is worth mentioning that this is the default value of the CNTEI bit.

   TCA0.SINGLE.EVCTRL &= ~(TCA_SINGLE_CNTEI_bm);

   Figure 3. EVCTRL Register
4. 4.The value written in the Period register represents the number of clock ticks between the moment when the timer starts and the moment when the first interrupt is triggered; and also the number of clock ticks between two consecutive interrupts. It can be deduced from the following equation.
   Note: The value written to the Period register will be one less than the desired count, because the counting starts from ‘0’.
   timeTCAIRQ(s)=TCAperiod⁢+1TCAclock(Hz)⁢

   where the clock of the TCA instance is defined by:  TCAclock(Hz)⁢=fCLK(Hz)TCAprescaler⁢

   and the peripheral clock fCLK=CLK_MAINMain⁢   clock⁢ prescaler

   Combining these equations, the following result is obtained:  

   timeTCAIRQ(s)=(TCAperiod⁢ +1)×TCAprescaler⁢ fCLK(Hz)
   Note: The Period register is 16 bits wide, thus the longest achievable interrupt period with no TCA prescaler is listed below.
   timeTCAIRQ(s)=(TCAperiod⁢+1)×TCApresclaerfCLK(Hz)=(0xFFFF+1)×13333333(Hz)=19,66×10−3s
   Considering the targeted values for this example,

   TCAperiod=timeTCAIRQ(s)TCAprescaler−1=250×10−3(s)×3333333256−1≅3254=0xCB6

   TCA0.SINGLE.PER = 0x0CB6;

5. 5.From the CTRLA register, the prescaler is set to ‘256’. To start the counter, the user must set the Enable bit in the same register.

   TCA0.SINGLE.CTRLA = TCA_SINGLE_CLKSEL_DIV256_gc
                              | TCA_SINGLE_ENABLE_bm; 

   Figure 4. Control A Register
6. 6.After the timer is fully configured, the sei(); macro enables the global interrupts. The peripherals need to always be configured when the global interrupts are disabled in order to avoid problems.
   In the ISR, the output pin is toggled by setting the corresponding bit in the Input register of the port. Also, the Overflow Interrupt flag is cleared.

   Tip: Interrupt flags have to be cleared in software by writing ‘1’ at the respective bit location.

   ISR(TCA0_OVF_vect)
   {
       PORTA.OUTTGL = PIN0_bm;
       
       TCA0.SINGLE.INTFLAGS = TCA_SINGLE_OVF_bm;
   }

   Figure 5. INTCTRL Register
   An interrupt request is generated when the corresponding interrupt source is enabled, and the Interrupt flag is set. As soon as the flag is set, the microcontroller will start executing the code from the ISR written by the user. The interrupt request remains active until the Interrupt flag is cleared. The parameter of the ISR is the interrupt vector. Therefore, the user can specify what interrupt source the ISR corresponds to. A list of the TCA interrupt vectors is provided below. When programming, it is useful to use the autocomplete function of the IDE to identify the desired interrupt vector.

   Figure 6. Available Interrupt Vectors and Sources in Normal Mode
7. 7.The Port A pin 0 (PA0) is set as output by writing a ‘1’ to the corresponding bit in the Direction register of the port. This GPIO is configured only to obtain a visible output, but it has nothing to do with the TCA instance itself in this mode.

   PORTA.DIR |= PIN0_bm;