+#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
+ 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