aod: PPI/RTC-based backlight brightness

src/components/brightness/BrightnessController.cpp

@@ -2,38 +2,138 @@
 #include <hal/nrf_gpio.h>
 #include "displayapp/screens/Symbols.h"
 #include "drivers/PinMap.h"
+#include <libraries/delay/nrf_delay.h>
 using namespace Pinetime::Controllers;
 
+namespace {
+  // reinterpret_cast is not constexpr so this is the best we can do
+  static NRF_RTC_Type* const RTC = reinterpret_cast<NRF_RTC_Type*>(NRF_RTC2_BASE);
+}
+
 void BrightnessController::Init() {
   nrf_gpio_cfg_output(PinMap::LcdBacklightLow);
   nrf_gpio_cfg_output(PinMap::LcdBacklightMedium);
   nrf_gpio_cfg_output(PinMap::LcdBacklightHigh);
+
+  nrf_gpio_pin_clear(PinMap::LcdBacklightLow);
+  nrf_gpio_pin_clear(PinMap::LcdBacklightMedium);
+  nrf_gpio_pin_clear(PinMap::LcdBacklightHigh);
+
+  static_assert(timerFrequency == 32768, "Change the prescaler below");
+  RTC->PRESCALER = 0;
+  // CC1 switches the backlight on (pin transitions from high to low) and resets the counter to 0
+  RTC->CC[1] = timerPeriod;
+  // Enable compare events for CC0,CC1
+  RTC->EVTEN = 0b0000'0000'0000'0011'0000'0000'0000'0000;
+  // Disable all interrupts
+  RTC->INTENCLR = 0b0000'0000'0000'1111'0000'0000'0000'0011;
   Set(level);
 }
 
+void BrightnessController::ApplyBrightness(uint16_t rawBrightness) {
+  // The classic off, low, medium, high brightnesses are at {0, timerPeriod, timerPeriod*2, timerPeriod*3}
+  // These brightness levels do not use PWM: they only set/clear the corresponding pins
+  // Any brightness level between the above levels is achieved with efficient RTC based PWM on the next pin up
+  // E.g 2.5*timerPeriod corresponds to medium brightness with 50% PWM on the high pin
+  // Note: Raw brightness does not necessarily correspond to a linear perceived brightness
+
+  uint8_t pin;
+  if (rawBrightness > 2 * timerPeriod) {
+    rawBrightness -= 2 * timerPeriod;
+    pin = PinMap::LcdBacklightHigh;
+  } else if (rawBrightness > timerPeriod) {
+    rawBrightness -= timerPeriod;
+    pin = PinMap::LcdBacklightMedium;
+  } else {
+    pin = PinMap::LcdBacklightLow;
+  }
+  if (rawBrightness == timerPeriod || rawBrightness == 0) {
+    if (lastPin != UNSET) {
+      RTC->TASKS_STOP = 1;
+      nrf_delay_us(rtcStopTime);
+      nrf_ppi_channel_disable(ppiBacklightOff);
+      nrf_ppi_channel_disable(ppiBacklightOn);
+      nrfx_gpiote_out_uninit(lastPin);
+      nrf_gpio_cfg_output(lastPin);
+    }
+    lastPin = UNSET;
+    if (rawBrightness == 0) {
+      nrf_gpio_pin_set(pin);
+    } else {
+      nrf_gpio_pin_clear(pin);
+    }
+  } else {
+    // If the pin on which we are doing PWM is changing
+    // Disable old PWM channel (if exists) and set up new one
+    if (lastPin != pin) {
+      if (lastPin != UNSET) {
+        RTC->TASKS_STOP = 1;
+        nrf_delay_us(rtcStopTime);
+        nrf_ppi_channel_disable(ppiBacklightOff);
+        nrf_ppi_channel_disable(ppiBacklightOn);
+        nrfx_gpiote_out_uninit(lastPin);
+        nrf_gpio_cfg_output(lastPin);
+      }
+      nrfx_gpiote_out_config_t gpioteCfg = {.action = NRF_GPIOTE_POLARITY_TOGGLE,
+                                            .init_state = NRF_GPIOTE_INITIAL_VALUE_LOW,
+                                            .task_pin = true};
+      APP_ERROR_CHECK(nrfx_gpiote_out_init(pin, &gpioteCfg));
+      nrfx_gpiote_out_task_enable(pin);
+      nrf_ppi_channel_endpoint_setup(ppiBacklightOff,
+                                     reinterpret_cast<uint32_t>(&RTC->EVENTS_COMPARE[0]),
+                                     nrfx_gpiote_out_task_addr_get(pin));
+      nrf_ppi_channel_endpoint_setup(ppiBacklightOn,
+                                     reinterpret_cast<uint32_t>(&RTC->EVENTS_COMPARE[1]),
+                                     nrfx_gpiote_out_task_addr_get(pin));
+      nrf_ppi_fork_endpoint_setup(ppiBacklightOn, reinterpret_cast<uint32_t>(&RTC->TASKS_CLEAR));
+      nrf_ppi_channel_enable(ppiBacklightOff);
+      nrf_ppi_channel_enable(ppiBacklightOn);
+    } else {
+      // If the pin used for PWM isn't changing, we only need to set the pin state to the initial value (low)
+      RTC->TASKS_STOP = 1;
+      nrf_delay_us(rtcStopTime);
+      // Due to errata 20,179 and the intricacies of RTC timing, keep it simple: override the pin state
+      nrfx_gpiote_out_task_force(pin, false);
+    }
+    // CC0 switches the backlight off (pin transitions from low to high)
+    RTC->CC[0] = rawBrightness;
+    RTC->TASKS_CLEAR = 1;
+    RTC->TASKS_START = 1;
+    lastPin = pin;
+  }
+  switch (pin) {
+    case PinMap::LcdBacklightHigh:
+      nrf_gpio_pin_clear(PinMap::LcdBacklightLow);
+      nrf_gpio_pin_clear(PinMap::LcdBacklightMedium);
+      break;
+    case PinMap::LcdBacklightMedium:
+      nrf_gpio_pin_clear(PinMap::LcdBacklightLow);
+      nrf_gpio_pin_set(PinMap::LcdBacklightHigh);
+      break;
+    case PinMap::LcdBacklightLow:
+      nrf_gpio_pin_set(PinMap::LcdBacklightMedium);
+      nrf_gpio_pin_set(PinMap::LcdBacklightHigh);
+  }
+}
+
 void BrightnessController::Set(BrightnessController::Levels level) {
   this->level = level;
   switch (level) {
     default:
     case Levels::High:
-      nrf_gpio_pin_clear(PinMap::LcdBacklightLow);
-      nrf_gpio_pin_clear(PinMap::LcdBacklightMedium);
-      nrf_gpio_pin_clear(PinMap::LcdBacklightHigh);
+      ApplyBrightness(3 * timerPeriod);
       break;
     case Levels::Medium:
-      nrf_gpio_pin_clear(PinMap::LcdBacklightLow);
-      nrf_gpio_pin_clear(PinMap::LcdBacklightMedium);
-      nrf_gpio_pin_set(PinMap::LcdBacklightHigh);
+      ApplyBrightness(2 * timerPeriod);
       break;
     case Levels::Low:
-      nrf_gpio_pin_clear(PinMap::LcdBacklightLow);
-      nrf_gpio_pin_set(PinMap::LcdBacklightMedium);
-      nrf_gpio_pin_set(PinMap::LcdBacklightHigh);
+      ApplyBrightness(timerPeriod);
+      break;
+    case Levels::AlwaysOn:
+      ApplyBrightness(timerPeriod / 10);
       break;
     case Levels::Off:
-      nrf_gpio_pin_set(PinMap::LcdBacklightLow);
-      nrf_gpio_pin_set(PinMap::LcdBacklightMedium);
-      nrf_gpio_pin_set(PinMap::LcdBacklightHigh);
+      ApplyBrightness(0);
       break;
   }
 }

src/components/brightness/BrightnessController.h

@@ -2,11 +2,14 @@
 
 #include <cstdint>
 
+#include "nrf_ppi.h"
+#include "nrfx_gpiote.h"
+
 namespace Pinetime {
   namespace Controllers {
     class BrightnessController {
     public:
-      enum class Levels { Off, Low, Medium, High };
+      enum class Levels { Off, AlwaysOn, Low, Medium, High };
       void Init();
 
       void Set(Levels level);
@@ -20,6 +23,25 @@ namespace Pinetime {
 
     private:
       Levels level = Levels::High;
+      static constexpr uint8_t UNSET = UINT8_MAX;
+      uint8_t lastPin = UNSET;
+      // Maximum time (μs) it takes for the RTC to fully stop
+      static constexpr uint8_t rtcStopTime = 46;
+      // Frequency of timer used for PWM (Hz)
+      static constexpr uint16_t timerFrequency = 32768;
+      // Backlight PWM frequency (Hz)
+      static constexpr uint16_t pwmFreq = 1000;
+      // Wraparound point in timer ticks
+      // Defines the number of brightness levels between each pin
+      static constexpr uint16_t timerPeriod = timerFrequency / pwmFreq;
+      // Warning: nimble reserves some PPIs
+      // https://github.com/InfiniTimeOrg/InfiniTime/blob/034d83fe6baf1ab3875a34f8cee387e24410a824/src/libs/mynewt-nimble/nimble/drivers/nrf52/src/ble_phy.c#L53
+      // SpiMaster uses PPI 0 for an erratum workaround
+      // Channel 1, 2 should be free to use
+      static constexpr nrf_ppi_channel_t ppiBacklightOn = NRF_PPI_CHANNEL1;
+      static constexpr nrf_ppi_channel_t ppiBacklightOff = NRF_PPI_CHANNEL2;
+
+      void ApplyBrightness(uint16_t val);
     };
   }
 }

src/displayapp/DisplayApp.cpp

@@ -242,11 +242,17 @@ void DisplayApp::Refresh() {
         RestoreBrightness();
         break;
       case Messages::GoToSleep:
-        while (brightnessController.Level() != Controllers::BrightnessController::Levels::Off) {
+        while (brightnessController.Level() != Controllers::BrightnessController::Levels::Low) {
           brightnessController.Lower();
           vTaskDelay(100);
         }
-        lcd.Sleep();
+        // Don't actually turn off the display for AlwaysOn mode
+        if (settingsController.GetAlwaysOnDisplay()) {
+          brightnessController.Set(Controllers::BrightnessController::Levels::AlwaysOn);
+        } else {
+          brightnessController.Set(Controllers::BrightnessController::Levels::Off);
+          lcd.Sleep();
+        }
         PushMessageToSystemTask(Pinetime::System::Messages::OnDisplayTaskSleeping);
         state = States::Idle;
         break;

src/systemtask/SystemTask.cpp

@@ -102,7 +102,9 @@ void SystemTask::Work() {
   watchdog.Setup(7, Drivers::Watchdog::SleepBehaviour::Run, Drivers::Watchdog::HaltBehaviour::Pause);
   watchdog.Start();
   NRF_LOG_INFO("Last reset reason : %s", Pinetime::Drivers::ResetReasonToString(watchdog.GetResetReason()));
-  APP_GPIOTE_INIT(2);
+  if (!nrfx_gpiote_is_init()) {
+    nrfx_gpiote_init();
+  }
 
   spi.Init();
   spiNorFlash.Init();