ARM - Initial backlight support (#6487)

* Move AVR backlight to own file, add borrowed ARM implementation

* Tiny fix for backlight custom logic

* Remove duplicate board from rebase

* Fix f303 onekey example

* clang-format

* clang-format

* Remove backlight keymap debug

* Initial pass of ARM backlight docs

* Initial pass of ARM backlight docs - resolve todos

* fix rules validation logic

* Add f072 warning

* Add f072 warning

* tidy up breathing in backlight keymap

* tidy up breathing in backlight keymap

* add missing break to backlight keymap

3 files changed, 728 insertions, 0 deletions

diff --git a/quantum/backlight/backlight.c b/quantum/backlight/backlight.c
new file mode 100644
index 0000000000..e26de86bf9
--- /dev/null
+++ b/quantum/backlight/backlight.c
@@ -0,0 +1 @@
+// TODO: Add common code here, for example cie_lightness implementation
diff --git a/quantum/backlight/backlight_arm.c b/quantum/backlight/backlight_arm.c
new file mode 100644
index 0000000000..3f94ccef8e
--- /dev/null
+++ b/quantum/backlight/backlight_arm.c
@@ -0,0 +1,218 @@
+#include "quantum.h"
+#include "backlight.h"
+#include <hal.h>
+#include "debug.h"
+
+// TODO: remove short term bodge when refactoring BACKLIGHT_CUSTOM_DRIVER out
+#ifdef BACKLIGHT_PIN
+
+#    if defined(STM32F0XX) || defined(STM32F0xx)
+#        error "Backlight support for STMF072 is not available. Please disable."
+#    endif
+
+#    if defined(STM32F1XX) || defined(STM32F1xx)
+#        define USE_GPIOV1
+#    endif
+
+// GPIOV2 && GPIOV3
+#    ifndef BACKLIGHT_PAL_MODE
+#        define BACKLIGHT_PAL_MODE 2
+#    endif
+
+// GENERIC
+#    ifndef BACKLIGHT_PWM_DRIVER
+#        define BACKLIGHT_PWM_DRIVER PWMD4
+#    endif
+#    ifndef BACKLIGHT_PWM_CHANNEL
+#        define BACKLIGHT_PWM_CHANNEL 3
+#    endif
+
+static void breathing_callback(PWMDriver *pwmp);
+
+static PWMConfig pwmCFG = {0xFFFF, /* PWM clock frequency  */
+                           256,    /* PWM period (in ticks) 1S (1/10kHz=0.1mS 0.1ms*10000 ticks=1S) */
+                           NULL,   /* No Callback */
+                           {       /* Default all channels to disabled - Channels will be configured durring init */
+                            {PWM_OUTPUT_DISABLED, NULL},
+                            {PWM_OUTPUT_DISABLED, NULL},
+                            {PWM_OUTPUT_DISABLED, NULL},
+                            {PWM_OUTPUT_DISABLED, NULL}},
+                           0, /* HW dependent part.*/
+                           0};
+
+static PWMConfig pwmCFG_breathing = {0xFFFF,             /** PWM clock frequency  */
+                                     256,                /* PWM period (in ticks) 1S (1/10kHz=0.1mS 0.1ms*10000 ticks=1S) */
+                                     breathing_callback, /* Breathing Callback */
+                                     {                   /* Default all channels to disabled - Channels will be configured durring init */
+                                      {PWM_OUTPUT_DISABLED, NULL},
+                                      {PWM_OUTPUT_DISABLED, NULL},
+                                      {PWM_OUTPUT_DISABLED, NULL},
+                                      {PWM_OUTPUT_DISABLED, NULL}},
+                                     0, /* HW dependent part.*/
+                                     0};
+
+// See http://jared.geek.nz/2013/feb/linear-led-pwm
+static uint16_t cie_lightness(uint16_t v) {
+    if (v <= 5243)     // if below 8% of max
+        return v / 9;  // same as dividing by 900%
+    else {
+        uint32_t y = (((uint32_t)v + 10486) << 8) / (10486 + 0xFFFFUL);  // add 16% of max and compare
+        // to get a useful result with integer division, we shift left in the expression above
+        // and revert what we've done again after squaring.
+        y = y * y * y >> 8;
+        if (y > 0xFFFFUL)  // prevent overflow
+            return 0xFFFFU;
+        else
+            return (uint16_t)y;
+    }
+}
+
+void backlight_init_ports(void) {
+    // printf("backlight_init_ports()\n");
+
+#    ifdef USE_GPIOV1
+    palSetPadMode(PAL_PORT(BACKLIGHT_PIN), PAL_PAD(BACKLIGHT_PIN), PAL_MODE_STM32_ALTERNATE_PUSHPULL);
+#    else
+    palSetPadMode(PAL_PORT(BACKLIGHT_PIN), PAL_PAD(BACKLIGHT_PIN), PAL_MODE_ALTERNATE(BACKLIGHT_PAL_MODE));
+#    endif
+
+    pwmCFG.channels[BACKLIGHT_PWM_CHANNEL - 1].mode           = PWM_OUTPUT_ACTIVE_HIGH;
+    pwmCFG_breathing.channels[BACKLIGHT_PWM_CHANNEL - 1].mode = PWM_OUTPUT_ACTIVE_HIGH;
+    pwmStart(&BACKLIGHT_PWM_DRIVER, &pwmCFG);
+
+    backlight_set(get_backlight_level());
+    if (is_backlight_breathing()) {
+        breathing_enable();
+    }
+}
+
+void backlight_set(uint8_t level) {
+    // printf("backlight_set(%d)\n", level);
+    if (level == 0) {
+        // Turn backlight off
+        pwmDisableChannel(&BACKLIGHT_PWM_DRIVER, BACKLIGHT_PWM_CHANNEL - 1);
+    } else {
+        // Turn backlight on
+        if (!is_breathing()) {
+            uint32_t duty = (uint32_t)(cie_lightness(0xFFFF * (uint32_t)level / BACKLIGHT_LEVELS));
+            // printf("duty: (%d)\n", duty);
+            pwmEnableChannel(&BACKLIGHT_PWM_DRIVER, BACKLIGHT_PWM_CHANNEL - 1, PWM_FRACTION_TO_WIDTH(&BACKLIGHT_PWM_DRIVER, 0xFFFF, duty));
+        }
+    }
+}
+
+uint8_t backlight_tick = 0;
+
+void backlight_task(void) {}
+
+#    define BREATHING_NO_HALT 0
+#    define BREATHING_HALT_OFF 1
+#    define BREATHING_HALT_ON 2
+#    define BREATHING_STEPS 128
+
+static uint8_t  breathing_period  = BREATHING_PERIOD;
+static uint8_t  breathing_halt    = BREATHING_NO_HALT;
+static uint16_t breathing_counter = 0;
+
+bool is_breathing(void) { return BACKLIGHT_PWM_DRIVER.config == &pwmCFG_breathing; }
+
+static inline void breathing_min(void) { breathing_counter = 0; }
+
+static inline void breathing_max(void) { breathing_counter = breathing_period * 256 / 2; }
+
+void breathing_interrupt_enable(void) {
+    pwmStop(&BACKLIGHT_PWM_DRIVER);
+    pwmStart(&BACKLIGHT_PWM_DRIVER, &pwmCFG_breathing);
+    chSysLockFromISR();
+    pwmEnablePeriodicNotification(&BACKLIGHT_PWM_DRIVER);
+    pwmEnableChannelI(&BACKLIGHT_PWM_DRIVER, BACKLIGHT_PWM_CHANNEL - 1, PWM_FRACTION_TO_WIDTH(&BACKLIGHT_PWM_DRIVER, 0xFFFF, 0xFFFF));
+    chSysUnlockFromISR();
+}
+
+void breathing_interrupt_disable(void) {
+    pwmStop(&BACKLIGHT_PWM_DRIVER);
+    pwmStart(&BACKLIGHT_PWM_DRIVER, &pwmCFG);
+}
+
+void breathing_enable(void) {
+    breathing_counter = 0;
+    breathing_halt    = BREATHING_NO_HALT;
+    breathing_interrupt_enable();
+}
+
+void breathing_pulse(void) {
+    if (get_backlight_level() == 0)
+        breathing_min();
+    else
+        breathing_max();
+    breathing_halt = BREATHING_HALT_ON;
+    breathing_interrupt_enable();
+}
+
+void breathing_disable(void) {
+    // printf("breathing_disable()\n");
+    breathing_interrupt_disable();
+    // Restore backlight level
+    backlight_set(get_backlight_level());
+}
+
+void breathing_self_disable(void) {
+    if (get_backlight_level() == 0)
+        breathing_halt = BREATHING_HALT_OFF;
+    else
+        breathing_halt = BREATHING_HALT_ON;
+}
+
+void breathing_toggle(void) {
+    if (is_breathing())
+        breathing_disable();
+    else
+        breathing_enable();
+}
+
+void breathing_period_set(uint8_t value) {
+    if (!value) value = 1;
+    breathing_period = value;
+}
+
+void breathing_period_default(void) { breathing_period_set(BREATHING_PERIOD); }
+
+void breathing_period_inc(void) { breathing_period_set(breathing_period + 1); }
+
+void breathing_period_dec(void) { breathing_period_set(breathing_period - 1); }
+
+/* To generate breathing curve in python:
+ * from math import sin, pi; [int(sin(x/128.0*pi)**4*255) for x in range(128)]
+ */
+static const uint8_t breathing_table[BREATHING_STEPS] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 17, 20, 24, 28, 32, 36, 41, 46, 51, 57, 63, 70, 76, 83, 91, 98, 106, 113, 121, 129, 138, 146, 154, 162, 170, 178, 185, 193, 200, 207, 213, 220, 225, 231, 235, 240, 244, 247, 250, 252, 253, 254, 255, 254, 253, 252, 250, 247, 244, 240, 235, 231, 225, 220, 213, 207, 200, 193, 185, 178, 170, 162, 154, 146, 138, 129, 121, 113, 106, 98, 91, 83, 76, 70, 63, 57, 51, 46, 41, 36, 32, 28, 24, 20, 17, 15, 12, 10, 8, 6, 5, 4, 3, 2, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+
+// Use this before the cie_lightness function.
+static inline uint16_t scale_backlight(uint16_t v) { return v / BACKLIGHT_LEVELS * get_backlight_level(); }
+
+static void breathing_callback(PWMDriver *pwmp) {
+    (void)pwmp;
+    uint16_t interval = (uint16_t)breathing_period * 256 / BREATHING_STEPS;
+    // resetting after one period to prevent ugly reset at overflow.
+    breathing_counter = (breathing_counter + 1) % (breathing_period * 256);
+    uint8_t index     = breathing_counter / interval % BREATHING_STEPS;
+
+    if (((breathing_halt == BREATHING_HALT_ON) && (index == BREATHING_STEPS / 2)) || ((breathing_halt == BREATHING_HALT_OFF) && (index == BREATHING_STEPS - 1))) {
+        breathing_interrupt_disable();
+    }
+
+    uint32_t duty = cie_lightness(scale_backlight(breathing_table[index] * 256));
+
+    chSysLockFromISR();
+    pwmEnableChannelI(&BACKLIGHT_PWM_DRIVER, BACKLIGHT_PWM_CHANNEL - 1, PWM_FRACTION_TO_WIDTH(&BACKLIGHT_PWM_DRIVER, 0xFFFF, duty));
+    chSysUnlockFromISR();
+}
+
+#else
+
+__attribute__((weak)) void backlight_init_ports(void) {}
+
+__attribute__((weak)) void backlight_set(uint8_t level) {}
+
+__attribute__((weak)) void backlight_task(void) {}
+
+#endif
diff --git a/quantum/backlight/backlight_avr.c b/quantum/backlight/backlight_avr.c
new file mode 100644
index 0000000000..445698f47c
--- /dev/null
+++ b/quantum/backlight/backlight_avr.c
@@ -0,0 +1,509 @@
+#include "quantum.h"
+#include "backlight.h"
+#include "debug.h"
+
+#if defined(BACKLIGHT_ENABLE) && (defined(BACKLIGHT_PIN) || defined(BACKLIGHT_PINS))
+
+// This logic is a bit complex, we support 3 setups:
+//
+//   1. Hardware PWM when backlight is wired to a PWM pin.
+//      Depending on this pin, we use a different output compare unit.
+//   2. Software PWM with hardware timers, but the used timer
+//      depends on the Audio setup (Audio wins over Backlight).
+//   3. Full software PWM, driven by the matrix scan, if both timers are used by Audio.
+
+#    if (defined(__AVR_AT90USB646__) || defined(__AVR_AT90USB647__) || defined(__AVR_AT90USB1286__) || defined(__AVR_AT90USB1287__) || defined(__AVR_ATmega16U4__) || defined(__AVR_ATmega32U4__)) && (BACKLIGHT_PIN == B5 || BACKLIGHT_PIN == B6 || BACKLIGHT_PIN == B7)
+#        define HARDWARE_PWM
+#        define ICRx ICR1
+#        define TCCRxA TCCR1A
+#        define TCCRxB TCCR1B
+#        define TIMERx_OVF_vect TIMER1_OVF_vect
+#        define TIMSKx TIMSK1
+#        define TOIEx TOIE1
+
+#        if BACKLIGHT_PIN == B5
+#            define COMxx1 COM1A1
+#            define OCRxx OCR1A
+#        elif BACKLIGHT_PIN == B6
+#            define COMxx1 COM1B1
+#            define OCRxx OCR1B
+#        elif BACKLIGHT_PIN == B7
+#            define COMxx1 COM1C1
+#            define OCRxx OCR1C
+#        endif
+#    elif (defined(__AVR_AT90USB646__) || defined(__AVR_AT90USB647__) || defined(__AVR_AT90USB1286__) || defined(__AVR_AT90USB1287__) || defined(__AVR_ATmega16U4__) || defined(__AVR_ATmega32U4__)) && (BACKLIGHT_PIN == C4 || BACKLIGHT_PIN == C5 || BACKLIGHT_PIN == C6)
+#        define HARDWARE_PWM
+#        define ICRx ICR3
+#        define TCCRxA TCCR3A
+#        define TCCRxB TCCR3B
+#        define TIMERx_OVF_vect TIMER3_OVF_vect
+#        define TIMSKx TIMSK3
+#        define TOIEx TOIE3
+
+#        if BACKLIGHT_PIN == C4
+#            if (defined(__AVR_ATmega16U4__) || defined(__AVR_ATmega32U4__))
+#                error This MCU has no C4 pin!
+#            else
+#                define COMxx1 COM3C1
+#                define OCRxx OCR3C
+#            endif
+#        elif BACKLIGHT_PIN == C5
+#            if (defined(__AVR_ATmega16U4__) || defined(__AVR_ATmega32U4__))
+#                error This MCU has no C5 pin!
+#            else
+#                define COMxx1 COM3B1
+#                define OCRxx OCR3B
+#            endif
+#        elif BACKLIGHT_PIN == C6
+#            define COMxx1 COM3A1
+#            define OCRxx OCR3A
+#        endif
+#    elif (defined(__AVR_ATmega16U2__) || defined(__AVR_ATmega32U2__)) && (BACKLIGHT_PIN == B7 || BACKLIGHT_PIN == C5 || BACKLIGHT_PIN == C6)
+#        define HARDWARE_PWM
+#        define ICRx ICR1
+#        define TCCRxA TCCR1A
+#        define TCCRxB TCCR1B
+#        define TIMERx_OVF_vect TIMER1_OVF_vect
+#        define TIMSKx TIMSK1
+#        define TOIEx TOIE1
+
+#        if BACKLIGHT_PIN == B7
+#            define COMxx1 COM1C1
+#            define OCRxx OCR1C
+#        elif BACKLIGHT_PIN == C5
+#            define COMxx1 COM1B1
+#            define OCRxx OCR1B
+#        elif BACKLIGHT_PIN == C6
+#            define COMxx1 COM1A1
+#            define OCRxx OCR1A
+#        endif
+#    elif defined(__AVR_ATmega32A__) && (BACKLIGHT_PIN == D4 || BACKLIGHT_PIN == D5)
+#        define HARDWARE_PWM
+#        define ICRx ICR1
+#        define TCCRxA TCCR1A
+#        define TCCRxB TCCR1B
+#        define TIMERx_OVF_vect TIMER1_OVF_vect
+#        define TIMSKx TIMSK
+#        define TOIEx TOIE1
+
+#        if BACKLIGHT_PIN == D4
+#            define COMxx1 COM1B1
+#            define OCRxx OCR1B
+#        elif BACKLIGHT_PIN == D5
+#            define COMxx1 COM1A1
+#            define OCRxx OCR1A
+#        endif
+#    elif defined(__AVR_ATmega328P__) && (BACKLIGHT_PIN == B1 || BACKLIGHT_PIN == B2)
+#        define HARDWARE_PWM
+#        define ICRx ICR1
+#        define TCCRxA TCCR1A
+#        define TCCRxB TCCR1B
+#        define TIMERx_OVF_vect TIMER1_OVF_vect
+#        define TIMSKx TIMSK1
+#        define TOIEx TOIE1
+
+#        if BACKLIGHT_PIN == B1
+#            define COMxx1 COM1A1
+#            define OCRxx OCR1A
+#        elif BACKLIGHT_PIN == B2
+#            define COMxx1 COM1B1
+#            define OCRxx OCR1B
+#        endif
+#    else
+#        if !defined(BACKLIGHT_CUSTOM_DRIVER)
+#            if !defined(B5_AUDIO) && !defined(B6_AUDIO) && !defined(B7_AUDIO)
+// Timer 1 is not in use by Audio feature, Backlight can use it
+#                pragma message "Using hardware timer 1 with software PWM"
+#                define HARDWARE_PWM
+#                define BACKLIGHT_PWM_TIMER
+#                define ICRx ICR1
+#                define TCCRxA TCCR1A
+#                define TCCRxB TCCR1B
+#                define TIMERx_COMPA_vect TIMER1_COMPA_vect
+#                define TIMERx_OVF_vect TIMER1_OVF_vect
+#                if defined(__AVR_ATmega32A__)  // This MCU has only one TIMSK register
+#                    define TIMSKx TIMSK
+#                else
+#                    define TIMSKx TIMSK1
+#                endif
+#                define TOIEx TOIE1
+
+#                define OCIExA OCIE1A
+#                define OCRxx OCR1A
+#            elif !defined(C6_AUDIO) && !defined(C5_AUDIO) && !defined(C4_AUDIO)
+#                pragma message "Using hardware timer 3 with software PWM"
+// Timer 3 is not in use by Audio feature, Backlight can use it
+#                define HARDWARE_PWM
+#                define BACKLIGHT_PWM_TIMER
+#                define ICRx ICR1
+#                define TCCRxA TCCR3A
+#                define TCCRxB TCCR3B
+#                define TIMERx_COMPA_vect TIMER3_COMPA_vect
+#                define TIMERx_OVF_vect TIMER3_OVF_vect
+#                define TIMSKx TIMSK3
+#                define TOIEx TOIE3
+
+#                define OCIExA OCIE3A
+#                define OCRxx OCR3A
+#            else
+#                pragma message "Audio in use - using pure software PWM"
+#                define NO_HARDWARE_PWM
+#            endif
+#        else
+#            pragma message "Custom driver defined - using pure software PWM"
+#            define NO_HARDWARE_PWM
+#        endif
+#    endif
+
+#    ifndef BACKLIGHT_ON_STATE
+#        define BACKLIGHT_ON_STATE 0
+#    endif
+
+void backlight_on(uint8_t backlight_pin) {
+#    if BACKLIGHT_ON_STATE == 0
+    writePinLow(backlight_pin);
+#    else
+    writePinHigh(backlight_pin);
+#    endif
+}
+
+void backlight_off(uint8_t backlight_pin) {
+#    if BACKLIGHT_ON_STATE == 0
+    writePinHigh(backlight_pin);
+#    else
+    writePinLow(backlight_pin);
+#    endif
+}
+
+#    if defined(NO_HARDWARE_PWM) || defined(BACKLIGHT_PWM_TIMER)  // pwm through software
+
+// we support multiple backlight pins
+#        ifndef BACKLIGHT_LED_COUNT
+#            define BACKLIGHT_LED_COUNT 1
+#        endif
+
+#        if BACKLIGHT_LED_COUNT == 1
+#            define BACKLIGHT_PIN_INIT \
+                { BACKLIGHT_PIN }
+#        else
+#            define BACKLIGHT_PIN_INIT BACKLIGHT_PINS
+#        endif
+
+#        define FOR_EACH_LED(x)                                 \
+            for (uint8_t i = 0; i < BACKLIGHT_LED_COUNT; i++) { \
+                uint8_t backlight_pin = backlight_pins[i];      \
+                { x }                                           \
+            }
+
+static const uint8_t backlight_pins[BACKLIGHT_LED_COUNT] = BACKLIGHT_PIN_INIT;
+
+#    else  // full hardware PWM
+
+// we support only one backlight pin
+static const uint8_t backlight_pin = BACKLIGHT_PIN;
+#        define FOR_EACH_LED(x) x
+
+#    endif
+
+#    ifdef NO_HARDWARE_PWM
+__attribute__((weak)) void backlight_init_ports(void) {
+    // Setup backlight pin as output and output to on state.
+    FOR_EACH_LED(setPinOutput(backlight_pin); backlight_on(backlight_pin);)
+
+#        ifdef BACKLIGHT_BREATHING
+    if (is_backlight_breathing()) {
+        breathing_enable();
+    }
+#        endif
+}
+
+__attribute__((weak)) void backlight_set(uint8_t level) {}
+
+uint8_t backlight_tick = 0;
+
+#        ifndef BACKLIGHT_CUSTOM_DRIVER
+void backlight_task(void) {
+    if ((0xFFFF >> ((BACKLIGHT_LEVELS - get_backlight_level()) * ((BACKLIGHT_LEVELS + 1) / 2))) & (1 << backlight_tick)) {
+        FOR_EACH_LED(backlight_on(backlight_pin);)
+    } else {
+        FOR_EACH_LED(backlight_off(backlight_pin);)
+    }
+    backlight_tick = (backlight_tick + 1) % 16;
+}
+#        endif
+
+#        ifdef BACKLIGHT_BREATHING
+#            ifndef BACKLIGHT_CUSTOM_DRIVER
+#                error "Backlight breathing only available with hardware PWM. Please disable."
+#            endif
+#        endif
+
+#    else  // hardware pwm through timer
+
+#        ifdef BACKLIGHT_PWM_TIMER
+
+// The idea of software PWM assisted by hardware timers is the following
+// we use the hardware timer in fast PWM mode like for hardware PWM, but
+// instead of letting the Output Match Comparator control the led pin
+// (which is not possible since the backlight is not wired to PWM pins on the
+// CPU), we do the LED on/off by oursleves.
+// The timer is setup to count up to 0xFFFF, and we set the Output Compare
+// register to the current 16bits backlight level (after CIE correction).
+// This means the CPU will trigger a compare match interrupt when the counter
+// reaches the backlight level, where we turn off the LEDs,
+// but also an overflow interrupt when the counter rolls back to 0,
+// in which we're going to turn on the LEDs.
+// The LED will then be on for OCRxx/0xFFFF time, adjusted every 244Hz.
+
+// Triggered when the counter reaches the OCRx value
+ISR(TIMERx_COMPA_vect) { FOR_EACH_LED(backlight_off(backlight_pin);) }
+
+// Triggered when the counter reaches the TOP value
+// this one triggers at F_CPU/65536 =~ 244 Hz
+ISR(TIMERx_OVF_vect) {
+#            ifdef BACKLIGHT_BREATHING
+    if (is_breathing()) {
+        breathing_task();
+    }
+#            endif
+    // for very small values of OCRxx (or backlight level)
+    // we can't guarantee this whole code won't execute
+    // at the same time as the compare match interrupt
+    // which means that we might turn on the leds while
+    // trying to turn them off, leading to flickering
+    // artifacts (especially while breathing, because breathing_task
+    // takes many computation cycles).
+    // so better not turn them on while the counter TOP is very low.
+    if (OCRxx > 256) {
+        FOR_EACH_LED(backlight_on(backlight_pin);)
+    }
+}
+
+#        endif
+
+#        define TIMER_TOP 0xFFFFU
+
+// See http://jared.geek.nz/2013/feb/linear-led-pwm
+static uint16_t cie_lightness(uint16_t v) {
+    if (v <= 5243)     // if below 8% of max
+        return v / 9;  // same as dividing by 900%
+    else {
+        uint32_t y = (((uint32_t)v + 10486) << 8) / (10486 + 0xFFFFUL);  // add 16% of max and compare
+        // to get a useful result with integer division, we shift left in the expression above
+        // and revert what we've done again after squaring.
+        y = y * y * y >> 8;
+        if (y > 0xFFFFUL)  // prevent overflow
+            return 0xFFFFU;
+        else
+            return (uint16_t)y;
+    }
+}
+
+// range for val is [0..TIMER_TOP]. PWM pin is high while the timer count is below val.
+static inline void set_pwm(uint16_t val) { OCRxx = val; }
+
+#        ifndef BACKLIGHT_CUSTOM_DRIVER
+__attribute__((weak)) void backlight_set(uint8_t level) {
+    if (level > BACKLIGHT_LEVELS) level = BACKLIGHT_LEVELS;
+
+    if (level == 0) {
+#            ifdef BACKLIGHT_PWM_TIMER
+        if (OCRxx) {
+            TIMSKx &= ~(_BV(OCIExA));
+            TIMSKx &= ~(_BV(TOIEx));
+            FOR_EACH_LED(backlight_off(backlight_pin);)
+        }
+#            else
+        // Turn off PWM control on backlight pin
+        TCCRxA &= ~(_BV(COMxx1));
+#            endif
+    } else {
+#            ifdef BACKLIGHT_PWM_TIMER
+        if (!OCRxx) {
+            TIMSKx |= _BV(OCIExA);
+            TIMSKx |= _BV(TOIEx);
+        }
+#            else
+        // Turn on PWM control of backlight pin
+        TCCRxA |= _BV(COMxx1);
+#            endif
+    }
+    // Set the brightness
+    set_pwm(cie_lightness(TIMER_TOP * (uint32_t)level / BACKLIGHT_LEVELS));
+}
+
+void backlight_task(void) {}
+#        endif  // BACKLIGHT_CUSTOM_DRIVER
+
+#        ifdef BACKLIGHT_BREATHING
+
+#            define BREATHING_NO_HALT 0
+#            define BREATHING_HALT_OFF 1
+#            define BREATHING_HALT_ON 2
+#            define BREATHING_STEPS 128
+
+static uint8_t breathing_period = BREATHING_PERIOD;
+static uint8_t breathing_halt = BREATHING_NO_HALT;
+static uint16_t breathing_counter = 0;
+
+#            ifdef BACKLIGHT_PWM_TIMER
+static bool breathing = false;
+
+bool is_breathing(void) { return breathing; }
+
+#                define breathing_interrupt_enable() \
+                    do {                             \
+                        breathing = true;            \
+                    } while (0)
+#                define breathing_interrupt_disable() \
+                    do {                              \
+                        breathing = false;            \
+                    } while (0)
+#            else
+
+bool is_breathing(void) { return !!(TIMSKx & _BV(TOIEx)); }
+
+#                define breathing_interrupt_enable() \
+                    do {                             \
+                        TIMSKx |= _BV(TOIEx);        \
+                    } while (0)
+#                define breathing_interrupt_disable() \
+                    do {                              \
+                        TIMSKx &= ~_BV(TOIEx);        \
+                    } while (0)
+#            endif
+
+#            define breathing_min()        \
+                do {                       \
+                    breathing_counter = 0; \
+                } while (0)
+#            define breathing_max()                                 \
+                do {                                                \
+                    breathing_counter = breathing_period * 244 / 2; \
+                } while (0)
+
+void breathing_enable(void) {
+    breathing_counter = 0;
+    breathing_halt = BREATHING_NO_HALT;
+    breathing_interrupt_enable();
+}
+
+void breathing_pulse(void) {
+    if (get_backlight_level() == 0)
+        breathing_min();
+    else
+        breathing_max();
+    breathing_halt = BREATHING_HALT_ON;
+    breathing_interrupt_enable();
+}
+
+void breathing_disable(void) {
+    breathing_interrupt_disable();
+    // Restore backlight level
+    backlight_set(get_backlight_level());
+}
+
+void breathing_self_disable(void) {
+    if (get_backlight_level() == 0)
+        breathing_halt = BREATHING_HALT_OFF;
+    else
+        breathing_halt = BREATHING_HALT_ON;
+}
+
+void breathing_toggle(void) {
+    if (is_breathing())
+        breathing_disable();
+    else
+        breathing_enable();
+}
+
+void breathing_period_set(uint8_t value) {
+    if (!value) value = 1;
+    breathing_period = value;
+}
+
+void breathing_period_default(void) { breathing_period_set(BREATHING_PERIOD); }
+
+void breathing_period_inc(void) { breathing_period_set(breathing_period + 1); }
+
+void breathing_period_dec(void) { breathing_period_set(breathing_period - 1); }
+
+/* To generate breathing curve in python:
+ * from math import sin, pi; [int(sin(x/128.0*pi)**4*255) for x in range(128)]
+ */
+static const uint8_t breathing_table[BREATHING_STEPS] PROGMEM = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 17, 20, 24, 28, 32, 36, 41, 46, 51, 57, 63, 70, 76, 83, 91, 98, 106, 113, 121, 129, 138, 146, 154, 162, 170, 178, 185, 193, 200, 207, 213, 220, 225, 231, 235, 240, 244, 247, 250, 252, 253, 254, 255, 254, 253, 252, 250, 247, 244, 240, 235, 231, 225, 220, 213, 207, 200, 193, 185, 178, 170, 162, 154, 146, 138, 129, 121, 113, 106, 98, 91, 83, 76, 70, 63, 57, 51, 46, 41, 36, 32, 28, 24, 20, 17, 15, 12, 10, 8, 6, 5, 4, 3, 2, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+
+// Use this before the cie_lightness function.
+static inline uint16_t scale_backlight(uint16_t v) { return v / BACKLIGHT_LEVELS * get_backlight_level(); }
+
+#            ifdef BACKLIGHT_PWM_TIMER
+void breathing_task(void)
+#            else
+/* Assuming a 16MHz CPU clock and a timer that resets at 64k (ICR1), the following interrupt handler will run
+ * about 244 times per second.
+ */
+ISR(TIMERx_OVF_vect)
+#            endif
+{
+    uint16_t interval = (uint16_t)breathing_period * 244 / BREATHING_STEPS;
+    // resetting after one period to prevent ugly reset at overflow.
+    breathing_counter = (breathing_counter + 1) % (breathing_period * 244);
+    uint8_t index = breathing_counter / interval % BREATHING_STEPS;
+
+    if (((breathing_halt == BREATHING_HALT_ON) && (index == BREATHING_STEPS / 2)) || ((breathing_halt == BREATHING_HALT_OFF) && (index == BREATHING_STEPS - 1))) {
+        breathing_interrupt_disable();
+    }
+
+    set_pwm(cie_lightness(scale_backlight((uint16_t)pgm_read_byte(&breathing_table[index]) * 0x0101U)));
+}
+
+#        endif  // BACKLIGHT_BREATHING
+
+__attribute__((weak)) void backlight_init_ports(void) {
+    // Setup backlight pin as output and output to on state.
+    FOR_EACH_LED(setPinOutput(backlight_pin); backlight_on(backlight_pin);)
+
+    // I could write a wall of text here to explain... but TL;DW
+    // Go read the ATmega32u4 datasheet.
+    // And this: http://blog.saikoled.com/post/43165849837/secret-konami-cheat-code-to-high-resolution-pwm-on
+
+#        ifdef BACKLIGHT_PWM_TIMER
+    // TimerX setup, Fast PWM mode count to TOP set in ICRx
+    TCCRxA = _BV(WGM11);  // = 0b00000010;
+    // clock select clk/1
+    TCCRxB = _BV(WGM13) | _BV(WGM12) | _BV(CS10);  // = 0b00011001;
+#        else  // hardware PWM
+    // Pin PB7 = OCR1C (Timer 1, Channel C)
+    // Compare Output Mode = Clear on compare match, Channel C = COM1C1=1 COM1C0=0
+    // (i.e. start high, go low when counter matches.)
+    // WGM Mode 14 (Fast PWM) = WGM13=1 WGM12=1 WGM11=1 WGM10=0
+    // Clock Select = clk/1 (no prescaling) = CS12=0 CS11=0 CS10=1
+
+    /*
+    14.8.3:
+    "In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM [..]."
+    "In fast PWM mode the counter is incremented until the counter value matches either one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 = 14), or the value in OCRnA (WGMn3:0 = 15)."
+    */
+    TCCRxA = _BV(COMxx1) | _BV(WGM11);             // = 0b00001010;
+    TCCRxB = _BV(WGM13) | _BV(WGM12) | _BV(CS10);  // = 0b00011001;
+#        endif
+    // Use full 16-bit resolution. Counter counts to ICR1 before reset to 0.
+    ICRx = TIMER_TOP;
+
+    backlight_init();
+#        ifdef BACKLIGHT_BREATHING
+    if (is_backlight_breathing()) {
+        breathing_enable();
+    }
+#        endif
+}
+
+#    endif  // hardware backlight
+
+#else  // no backlight
+
+__attribute__((weak)) void backlight_init_ports(void) {}
+
+__attribute__((weak)) void backlight_set(uint8_t level) {}
+
+#endif  // backlight
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