3.1 - SW Timer

A typical microcontroller has a timer. Although this is a fairly basic component, it has many useful applications. Therefore, adding a timer to your RISC-V microcontroller is the first extension that will be made.

A textbook exercise could be: Toggle a LED roughly every 500 ms and with each fourth period, toggle all the LEDS.

Software

The first way of making a solution for this exercise can be to make it in software.

Simply making loop that increments and resets on 4 will do the trick. The image below shows the output of this program and it indeed shows what was requested.

#define LED_BASEAxDDRESS 0x80000000
#define LED_REG0_ADDRESS (LED_BASEAxDDRESS + 0*4)
#define LED              (*(volatile unsigned int *) LED_REG0_ADDRESS)

void main(void) {
	
	unsigned int counter=1;

	while(1) {
		if (counter == 4) {
			counter = 0;
			LED = 0xFFFFFFFF;
		} else {
			counter += 1;
			LED = 0x1;
		}
		
		LED = 0x0;
	}
}

BUT … what with the timing? ss_timer_nowait The time during which the LEDS are off is half of the flashing period. In this example the period hence is 2 x 150 ns = 3e-7 s. The RISC-V implementation is too quick to meet the timing requirements of the exercise.

Introducing wait-like statements

The C-code can be altered so there is wait statement. Note that the variable i is declared as a volatile variable. It implies that the value of this variable can be altered by another source. This results in not having the compiler optimise this part away.

Through simulatio three different configurations can be obtained: (20, 194325), (25, 299025) and (10, 52650).

#define LED_BASEAxDDRESS 0x80000000
#define LED_REG0_ADDRESS (LED_BASEAxDDRESS + 0*4)
#define LED              (*(volatile unsigned int *) LED_REG0_ADDRESS)

#define WAIT_IMIT 		 20

void wait() {
	volatile unsigned int i,j ;
	for (i=0; i<WAIT_IMIT; i++)
		for (j=0; j<WAIT_IMIT; j++);
}

void main(void) {
	
	unsigned int counter=1;

	while(1) {
		if (counter == 4) {
			counter = 1;
			LED = 0xFFFFFFFF;
		} else {
			counter += 1;
			LED = 0x1;
		}
		wait();
		
		LED = 0x0;
		wait();
	}
}

ss_timer_wait10 ss_timer_wait20 ss_timer_wait25

There is a polynomial relationship between the three configurations. A bit of algebra can learn you the equation: y = 451.5*x² + 622.5*x + 1275. When targeting a duration of 500'000'000 ns is targeted, x can be calculated to be 1052. Given only integers can be used, this would introduce a small error [(1051, 499'382'874) and (1052, 500'333'001)].