ev3-pru1: add PWMs for EV3 LEDs

This adds PWMs so that we can adjust the brighness of the LEDs on the
EV3. The 366Hz duty cycle was selected for code efficency since we can
reuse the same timer as the tacho.

This increases the firmware size by 112 bytes.

Issue: ev3dev/ev3dev#1160

ev3-pru1/main.c

@@ -37,10 +37,19 @@ enum ev3_pru_tacho {
 	NUM_EV3_PRU_TACHO
 };
 
+enum ev3_pru_pwm {
+	EV3_PRU_PWM_0,
+	EV3_PRU_PWM_1,
+	EV3_PRU_PWM_2,
+	EV3_PRU_PWM_3,
+	NUM_EV3_PRU_PWM
+};
+
 struct ev3_pru_tacho_remote_data {
 	uint32_t position[NUM_EV3_PRU_TACHO][EV3_PRU_TACHO_RING_BUF_SIZE];
 	uint32_t timestamp[NUM_EV3_PRU_TACHO][EV3_PRU_TACHO_RING_BUF_SIZE];
 	uint32_t head[NUM_EV3_PRU_TACHO];
+	uint8_t pwm[NUM_EV3_PRU_PWM];
 };
 
 /* end Linux */
@@ -88,11 +97,11 @@ static uint8_t tacho_state[NUM_EV3_PRU_TACHO];
 static uint32_t tacho_prev_timestamp[NUM_EV3_PRU_TACHO];
 static uint32_t tacho_counts[NUM_EV3_PRU_TACHO];
 
-static void update_tacho_state(enum ev3_pru_tacho idx, uint8_t new_state)
+static void update_tacho_state(enum ev3_pru_tacho idx, uint8_t new_state, uint32_t now)
 {
 	uint8_t current_state = tacho_state[idx] & 0x3;
 	enum direction new_dir = UNKNOWN;
-	uint32_t now, elapsed;
+	uint32_t elapsed;
 
 	switch (current_state) {
 		case 0x0:
@@ -131,7 +140,6 @@ static void update_tacho_state(enum ev3_pru_tacho idx, uint8_t new_state)
 	tacho_state[idx] = new_state;
 	tacho_counts[idx] += new_dir;
 
-	now = TIMER64P0.TIM34;
 	elapsed = now - tacho_prev_timestamp[idx];
 
 	// if there was a change in count or if count hasn't changed for 50ms
@@ -146,7 +154,18 @@ static void update_tacho_state(enum ev3_pru_tacho idx, uint8_t new_state)
 	}
 }
 
-int main(void) {
+static void update_pwm(enum ev3_pru_pwm pwm, uint8_t pwm_count, uint32_t gpio_flags)
+{
+	if (pwm_count < REMOTE_DATA.pwm[pwm]) {
+		GPIO.SET_DATA67 = gpio_flags;
+	}
+	else {
+		GPIO.CLR_DATA67 = gpio_flags;
+	}
+}
+
+int main(void)
+{
 	volatile uint8_t *status;
 	struct pru_rpmsg_transport transport;
 	uint16_t src, dst, len;
@@ -166,10 +185,23 @@ int main(void) {
 	while (true) {
 		// wait for the ARM to kick us
 		while (!(__R31 & HOST_INT)) {
-			update_tacho_state(EV3_PRU_TACHO_A, TACHO_STATE(A));
-			update_tacho_state(EV3_PRU_TACHO_B, TACHO_STATE(B));
-			update_tacho_state(EV3_PRU_TACHO_C, TACHO_STATE(C));
-			update_tacho_state(EV3_PRU_TACHO_D, TACHO_STATE(D));
+			uint32_t now = TIMER64P0.TIM34;
+			uint8_t pwm_count;
+
+			update_tacho_state(EV3_PRU_TACHO_A, TACHO_STATE(A), now);
+			update_tacho_state(EV3_PRU_TACHO_B, TACHO_STATE(B), now);
+			update_tacho_state(EV3_PRU_TACHO_C, TACHO_STATE(C), now);
+			update_tacho_state(EV3_PRU_TACHO_D, TACHO_STATE(D), now);
+
+			// this gives us a PWM period of 366Hz with a max duty
+			// cycle of 256 counts (24MHz / 256 / 256)
+			pwm_count = now >> 8;
+
+			// update PWM outputs
+			update_pwm(EV3_PRU_PWM_0, pwm_count, 1 << 13);	// LED0, GPIO 6[13]
+			update_pwm(EV3_PRU_PWM_1, pwm_count, 1 << 7);	// LED1, GPIO 6[7]
+			update_pwm(EV3_PRU_PWM_2, pwm_count, 1 << 14);	// LED2, GPIO 6[14]
+			update_pwm(EV3_PRU_PWM_3, pwm_count, 1 << 12);	// LED3, GPIO 6[12]
 		}
 
 		// clear the interrupt