aod: RTC based backlight control

mark9064 · mark9064 · commit e3afe0764ff1 · 2024-06-19T17:03:33.000+01:00
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
@@ -40,7 +40,6 @@ set(SDK_SOURCE_FILES
         "${NRF5_SDK_PATH}/integration/nrfx/legacy/nrf_drv_clock.h"
         "${NRF5_SDK_PATH}/modules/nrfx/drivers/src/nrfx_clock.c"
         "${NRF5_SDK_PATH}/modules/nrfx/drivers/src/nrfx_gpiote.c"
-        "${NRF5_SDK_PATH}/modules/nrfx/drivers/src/nrfx_timer.c"
         "${NRF5_SDK_PATH}/modules/nrfx/soc/nrfx_atomic.c"
         "${NRF5_SDK_PATH}/modules/nrfx/drivers/src/nrfx_saadc.c"
 
diff --git a/src/components/brightness/BrightnessController.cpp b/src/components/brightness/BrightnessController.cpp
@@ -2,8 +2,14 @@
 #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);
@@ -13,55 +19,56 @@ void BrightnessController::Init() {
   nrf_gpio_pin_clear(PinMap::LcdBacklightMedium);
   nrf_gpio_pin_clear(PinMap::LcdBacklightHigh);
 
-  timer = NRFX_TIMER_INSTANCE(1);
-  nrfx_timer_config_t timerCfg = {.frequency = NRF_TIMER_FREQ_1MHz,
-                                  .mode = NRF_TIMER_MODE_TIMER,
-                                  .bit_width = NRF_TIMER_BIT_WIDTH_32,
-                                  .interrupt_priority = 6,
-                                  .p_context = nullptr};
-  // Callback will never fire, use empty expression
-  APP_ERROR_CHECK(nrfx_timer_init(&timer, &timerCfg, [](auto, auto) {
-  }));
-  nrfx_timer_extended_compare(&timer,
-                              NRF_TIMER_CC_CHANNEL1,
-                              nrfx_timer_us_to_ticks(&timer, 1000),
-                              NRF_TIMER_SHORT_COMPARE1_CLEAR_MASK,
-                              false);
+  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 val) {
+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 (val > 2000) {
-    val -= 2000;
+  if (rawBrightness > 2 * timerPeriod) {
+    rawBrightness -= 2 * timerPeriod;
     pin = PinMap::LcdBacklightHigh;
-  } else if (val > 1000) {
-    val -= 1000;
+  } else if (rawBrightness > timerPeriod) {
+    rawBrightness -= timerPeriod;
     pin = PinMap::LcdBacklightMedium;
   } else {
     pin = PinMap::LcdBacklightLow;
   }
-  if (val == 1000 || val == 0) {
+  if (rawBrightness == timerPeriod || rawBrightness == 0) {
     if (lastPin != UNSET) {
-      nrfx_timer_disable(&timer);
-      nrfx_timer_clear(&timer);
+      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 (val == 0) {
+    if (rawBrightness == 0) {
       nrf_gpio_pin_set(pin);
     } else {
       nrf_gpio_pin_clear(pin);
     }
   } else {
-    uint32_t newThreshold = nrfx_timer_us_to_ticks(&timer, val);
+    // 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) {
-        nrfx_timer_disable(&timer);
-        nrfx_timer_clear(&timer);
+        RTC->TASKS_STOP = 1;
+        nrf_delay_us(rtcStopTime);
         nrf_ppi_channel_disable(ppiBacklightOff);
         nrf_ppi_channel_disable(ppiBacklightOn);
         nrfx_gpiote_out_uninit(lastPin);
@@ -73,33 +80,25 @@ void BrightnessController::ApplyBrightness(uint16_t val) {
       APP_ERROR_CHECK(nrfx_gpiote_out_init(pin, &gpioteCfg));
       nrfx_gpiote_out_task_enable(pin);
       nrf_ppi_channel_endpoint_setup(ppiBacklightOff,
-                                     nrfx_timer_event_address_get(&timer, NRF_TIMER_EVENT_COMPARE0),
+                                     reinterpret_cast<uint32_t>(&RTC->EVENTS_COMPARE[0]),
                                      nrfx_gpiote_out_task_addr_get(pin));
       nrf_ppi_channel_endpoint_setup(ppiBacklightOn,
-                                     nrfx_timer_event_address_get(&timer, NRF_TIMER_EVENT_COMPARE1),
+                                     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 {
-      // Pause the timer, check where it is before changing the threshold
-      // If we have already triggered CC0 and then move CC0 into the future
-      // without triggering CC1 first, the modulation will be inverted
-      // (e.g on 100us off 900us becomes off 100us on 900us)
-      nrfx_timer_pause(&timer);
-      uint32_t currentCycle = nrfx_timer_capture(&timer, NRF_TIMER_CC_CHANNEL2);
-      uint32_t oldThreshold = nrfx_timer_capture_get(&timer, NRF_TIMER_CC_CHANNEL0);
-      // If the new threshold now in future and we have already triggered the old threshold, switch backlight back on
-      // If we haven't yet triggered CC0 and then move CC0 into the past, we get the same issue: switch the backlight off
-      if ((currentCycle >= oldThreshold && newThreshold > currentCycle) || (currentCycle < oldThreshold && newThreshold <= currentCycle)) {
-        nrfx_gpiote_out_task_trigger(pin);
-      }
-    }
-    nrfx_timer_compare(&timer, NRF_TIMER_CC_CHANNEL0, newThreshold, false);
-    if (lastPin != pin) {
-      nrfx_timer_enable(&timer);
-    } else {
-      nrfx_timer_resume(&timer);
+      // 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) {
@@ -122,16 +121,16 @@ void BrightnessController::Set(BrightnessController::Levels level) {
   switch (level) {
     default:
     case Levels::High:
-      ApplyBrightness(3000);
+      ApplyBrightness(3 * timerPeriod);
       break;
     case Levels::Medium:
-      ApplyBrightness(2000);
+      ApplyBrightness(2 * timerPeriod);
       break;
     case Levels::Low:
-      ApplyBrightness(1000);
+      ApplyBrightness(timerPeriod);
       break;
     case Levels::AlwaysOn:
-      ApplyBrightness(100);
+      ApplyBrightness(timerPeriod / 10);
       break;
     case Levels::Off:
       ApplyBrightness(0);
diff --git a/src/components/brightness/BrightnessController.h b/src/components/brightness/BrightnessController.h
@@ -3,7 +3,6 @@
 #include <cstdint>
 
 #include "nrf_ppi.h"
-#include "nrfx_timer.h"
 #include "nrfx_gpiote.h"
 
 namespace Pinetime {
@@ -26,7 +25,15 @@ namespace Pinetime {
       Levels level = Levels::High;
       static constexpr uint8_t UNSET = UINT8_MAX;
       uint8_t lastPin = UNSET;
-      nrfx_timer_t timer;
+      // 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
diff --git a/src/sdk_config.h b/src/sdk_config.h
@@ -4678,7 +4678,7 @@
 // <q> NRFX_TIMER1_ENABLED  - Enable TIMER1 instance
 
 #ifndef NRFX_TIMER1_ENABLED
-  #define NRFX_TIMER1_ENABLED 1
+  #define NRFX_TIMER1_ENABLED 0
 #endif
 
 // <q> NRFX_TIMER2_ENABLED  - Enable TIMER2 instance
@@ -6319,7 +6319,7 @@
 // <e> TIMER_ENABLED - nrf_drv_timer - TIMER periperal driver - legacy layer
 //==========================================================
 #ifndef TIMER_ENABLED
-  #define TIMER_ENABLED 1
+  #define TIMER_ENABLED 0
 #endif
 // <o> TIMER_DEFAULT_CONFIG_FREQUENCY  - Timer frequency if in Timer mode
 
@@ -6383,7 +6383,7 @@
 // <q> TIMER1_ENABLED  - Enable TIMER1 instance
 
 #ifndef TIMER1_ENABLED
-  #define TIMER1_ENABLED 1
+  #define TIMER1_ENABLED 0
 #endif
 
 // <q> TIMER2_ENABLED  - Enable TIMER2 instance
