PWM start/stop in RTC callback.

I initially tried calling prst_pwm_init() in main(), and just using start/stop in
the callback, but that led to a weird behavior in the PWM output. It was set to
high after the end of the callback.

Callnig uninit/re-calling init() inside the RTC callback fixed, but more investigation
is required.

code/b-parasite/config/prst_config.h

@@ -3,6 +3,9 @@
 
 #include "nrf_gpio.h"
 
+// Built-in LED.
+#define PRST_LED_PIN NRF_GPIO_PIN_MAP(1, 11)
+
 // Deep sleep.
 #define PRST_DEEP_SLEEP_IN_SECONDS 2
 
@@ -11,6 +14,5 @@
 
 // PWM.
 #define PRST_PWM_PIN NRF_GPIO_PIN_MAP(0, 29)
-#define PRST_PWM_FREQUENCY 500000
 
 #endif  // _PRST_CONFIG_H_

code/b-parasite/src/main.c

@@ -19,12 +19,7 @@
 #include "prst/pwm.h"
 #include "prst/rtc.h"
 
-// P0.03
-// #define LED_PIN 3
-#define LED_PIN NRF_GPIO_PIN_MAP(1, 11)
-
-// Environmental sensing.
-#define SERVICE_UUID 0x181a
+#include "prst_config.h"
 
 #define DEAD_BEEF 0xDEADBEEF
 
@@ -40,11 +35,7 @@ static void log_init(void) {
 }
 
 static void leds_init(void) {
-  nrf_gpio_cfg_output(LED_PIN);
-  nrf_gpio_pin_toggle(LED_PIN);
-  nrf_delay_ms(500);
-  nrf_gpio_pin_toggle(LED_PIN);
-  nrf_delay_ms(500);
+  nrf_gpio_cfg_output(PRST_LED_PIN);
   NRF_LOG_INFO("Leds inited");
 }
 
@@ -67,28 +58,29 @@ static uint8_t data;
 static void rtc_callback() {
   NRF_LOG_INFO("rtc callback running...\n");
   NRF_LOG_FLUSH();
-  nrf_gpio_pin_set(LED_PIN);
+  nrf_gpio_pin_set(PRST_LED_PIN);
+  prst_pwm_init();
+  prst_pwm_start();
+  // TODO: ADC.
+  nrf_delay_ms(500);
+  prst_pwm_stop();
   prst_ble_update_adv_data(++data);
   prst_adv_start();
   nrf_delay_ms(300);
   prst_adv_stop();
-  nrf_gpio_pin_clear(LED_PIN);
+  nrf_gpio_pin_clear(PRST_LED_PIN);
 }
 
 int main(void) {
-  // Initialize.
   log_init();
   leds_init();
   power_management_init();
-  prst_pwm_init();
-  prst_pwm_start();
   prst_ble_init();
 
   prst_rtc_set_callback(rtc_callback);
   prst_rtc_init();
 
-  // Enter main loop.
   for (;;) {
     power_manage();
   }
-}
+}

code/b-parasite/src/prst/pwm.c

@@ -8,11 +8,19 @@
 
 #include "prst_config.h"
 
+// Each step in the counter will take 1/16e6 s.
 #define PRST_PWM_BASE_FREQ NRF_PWM_CLK_16MHz
+// We will count up to 16. It will take 1us at 16MHz.
+// With the NRF_PWM_MODE_UP_AND_DOWN count mode, we assume 1us is half the
+// output PWM period (total 2us => 500MHz frequency). We set a duty cycle of
+// 50% below with PRST_PWM_FLIP_AT_COUNT to be half the max count.
+#define PRST_PWM_MAX_COUNT 16
+// We will toggle the PWM output when we reach this count.
+#define PRST_PWM_FLIP_AT_COUNT PRST_PWM_MAX_COUNT / 2
 
 static nrf_drv_pwm_t m_pwm0 = NRF_DRV_PWM_INSTANCE(0);
 
-static nrf_pwm_values_common_t seq_values_[] = {8};
+static nrf_pwm_values_common_t seq_values_[] = {PRST_PWM_FLIP_AT_COUNT};
 static const nrf_pwm_sequence_t seq_ = {
     .values.p_common = seq_values_,
     .length = NRF_PWM_VALUES_LENGTH(seq_values_),
@@ -20,8 +28,6 @@ static const nrf_pwm_sequence_t seq_ = {
     .end_delay = 0};
 
 void prst_pwm_init() {
-  UNUSED_VARIABLE(seq_);
-
   nrf_drv_pwm_config_t const config0 = {
       .output_pins =
           {
@@ -35,16 +41,19 @@ void prst_pwm_init() {
       .base_clock = NRF_PWM_CLK_16MHz,
       // This is the hal COUNTERTOP.
       .count_mode = NRF_PWM_MODE_UP_AND_DOWN,
-      // .top_value = (uint16_t)(1e16 / PRST_PWM_FREQUENCY),
-      .top_value = 16,
+      .top_value = PRST_PWM_MAX_COUNT,
       .load_mode = NRF_PWM_LOAD_COMMON,
       .step_mode = NRF_PWM_STEP_AUTO};
   APP_ERROR_CHECK(nrf_drv_pwm_init(&m_pwm0, &config0, NULL));
 }
 
 void prst_pwm_start() {
+  // Loop forever.
   APP_ERROR_CHECK(
       nrf_drv_pwm_simple_playback(&m_pwm0, &seq_, 1, NRF_DRV_PWM_FLAG_LOOP));
 }
 
-void prst_pwm_stop() {}
+void prst_pwm_stop() {
+  APP_ERROR_CHECK(nrf_drv_pwm_stop(&m_pwm0, /*wait_until_stopped=*/true));
+  nrf_drv_pwm_uninit(&m_pwm0);
+}