Question

Suppose that TCA0 has been set up as follows: TCA0.SINGLE.CTRLA CLKSEL field has been set to 0x6 (DIV256) and that TCA0.SINGLE.PER has been set to 15265. What time-period has been selected?

this is in c
code below>
/* Use a struct to make the association between PORTs and bits connected to the LED array more explicit */
struct LED_BITS
{
PORT_t *LED_PORT;
uint8_t bit_mapping;
};
struct LED_BITS LED_Array[10] = {
{&PORTC, PIN5_bm}, {&PORTC, PIN4_bm}, {&PORTA, PIN0_bm}, {&PORTF, PIN5_bm}, {&PORTC, PIN6_bm}, {&PORTB, PIN2_bm}, {&PORTF, PIN4_bm}, {&PORTA, PIN1_bm}, {&PORTA, PIN2_bm}, {&PORTA, PIN3_bm}
};
void CLOCK_init (void);
void InitialiseLED_PORT_bits(void);
void TCA0_init_bits(void);
void Set_Clear_Ports(uint8_t set);
void TogglePorts(void);
int main(void)
{
CLOCK_init();
/* set LED Array D0-D9 to all outputs */
InitialiseLED_PORT_bits();
Set_Clear_Ports(0);
TCA0_init_bits();
sei();
while(1)
{
}
return 1;
}
void CLOCK_init (void)
{
/* Disable CLK_PER Prescaler */
ccp_write_io( (void *) &CLKCTRL.MCLKCTRLB , (0 << CLKCTRL_PEN_bp));
/* If set from the fuses during device programming, the CPU will now run at 20MHz (default is /6) */
}
void InitialiseLED_PORT_bits()

PORTC.DIR = PIN6_bm
void TCA0_init_bits(void)
{
TCA0.SINGLE.INTCTRL = 0b00000001; /* Counter overflow interrupt option */
TCA0.SINGLE.CTRLB = 0b00000000; /* Normal Mode selected - TOP value in PER register */
TCA0.SINGLE.EVCTRL = 0b00000000; /* TCA0 can count events from the EVENT module - disable this option */
/* Now set the PER register to 3125, our top value */
/* 20MHz/64 = 312.5kHz => T=3.2us. Time = 3125*3.2us = 10ms */
TCA0.SINGLE.PER = 3125; /* The TCA0 counter will overflow when it reaches the PER value */
TCA0.SINGLE.CTRLA = 0b00001011; /* Prescale set to /64 and enable TCA0 (start count) */
}
/* Function to set or clear all LED port bits, 1 - set, 0 - clear */
void Set_Clear_Ports(uint8_t set) {
uint8_t i;
for (i = 0; i <= 9; i += 1)
{
if (set)
LED_Array[i].LED_PORT->OUTSET = LED_Array[i].bit_mapping;
else
LED_Array[i].LED_PORT->OUTCLR = LED_Array[i].bit_mapping;
}
}
void Toggle_Ports(void)
{
uint8_t i;
for (i = 0; i <= 9; i += 1)
{
LED_Array[i].LED_PORT->OUTTGL = LED_Array[i].bit_mapping;
}
}
ISR(TCA0_OVF_vect)
{
Toggle_Ports();
/* The interrupt flag has to be cleared manually */
TCA0.SINGLE.INTFLAGS = 0b00000001; /* Writing 1 to the flag bit clears it */
}

Answer 1

The time-period for TCA0 with CLKSEL set to DIV256 and PER set to 15265 is approximately 195.072 milliseconds, calculated assuming a common 20 MHz base clock frequency.

Step-by-step explanation:

The student question pertains to the configuration and calculation of the time-period for Timer/Counter Type A (TCA) in an embedded system environment, specifically for the TCA0 module. The TCA0.SINGLE.CTRLA register's CLKSEL field being set to 0x6 corresponds to a clock prescaler value of DIV256, and the TCA0.SINGLE.PER set to 15265 defines the counter period.

To calculate the time-period that has been selected, we need to consider the base clock frequency, which, in the absence of a specified microcontroller clock speed in the question, is commonly 16 MHz or 20 MHz for many microcontrollers. Assuming a typical clock frequency of 20 MHz, we can calculate the time-period using the formula:

Time-period = (PER value + 1) × Prescaler × Base clock period.

Given a PER value of 15265, a prescaler of 256, and a base clock period of 1 / 20 MHz, the time-period would be (15265 + 1) × 256 × (1 / 20,000,000) seconds. This results in a final time-period of approximately 0.195072 seconds or 195.072 milliseconds.