Split PWM enable from setting compare value

This allows more flexibility in choosing the brightness of blinking,
currently used only to flicker at the current brightness for battery
warning.

Sadly due to additional calls to set low intensity for config mode this
saves only 2 bytes of flash memory, but with some other parameters it is
possible to save up to 6 bytes more.

rampinglight.c

@@ -30,6 +30,7 @@
 
 #define FLASH_TIME 20
 #define FLICKER_TIME 2
+#define FLASH_PWM 40  // TODO Using TURBO_PWM saves 6 bytes due to optimization
 
 #define BAT_LOW  141  // ~3.2 V
 #define BAT_CRIT 120  // ~2.7 V
@@ -120,18 +121,27 @@ void delay_s(void) {
   delay_10ms(100);
 }
 
+/**
+ * Enable PWM output with currently set value.
+ */
+void enable_output(void) {
+  TCCR0A |= (1 << COM0B1);
+}
+
+/**
+ * Disable PWM output and keep current value.
+ */
+void disable_output(void) {
+  TCCR0A &= ~(1 << COM0B1);
+}
+
 /**
  * Set output, i.e. compare register.
  *
  * @param pwm Raw PWM level.
  */
-static inline void set_pwm(uint8_t pwm) {
+static void set_pwm(const uint8_t pwm) {
   OCR0B = pwm;
-  if (!pwm) {
-    TCCR0A &= ~(1 << COM0B1);
-  } else {
-    TCCR0A |= (1 << COM0B1);
-  }
 }
 
 /**
@@ -140,34 +150,34 @@ static inline void set_pwm(uint8_t pwm) {
  *
  * @param level Index in ramp_values or fixed_values depending on fixed_mode
  */
-void set_level(uint8_t level) {
+void set_level(const uint8_t level) {
   if (level == 0) {
-    set_pwm(0);
+    disable_output();
   } else {
     if (options.fixed_mode) {
       set_pwm(fixed_values[level - 1]);
     } else {
       set_pwm(ramp_values[level - 1]);
     }
+    enable_output();
   }
 }
 
 /**
- * Blink or strobe the LED on low intensity.
+ * Blink or strobe the LED on the current intensity. After this function
+ * returns the output will be disabled!
  *
  * @param count Number of flashes.
  * @param speed Duration of a single flash in n*10 ms.
  */
 void blink(uint8_t count, const uint8_t speed) {
-  const uint8_t old_pwm = OCR0B;
   while (count--) {
-    set_pwm(40);
+    enable_output();
     delay_10ms(speed);
-    set_pwm(0);
+    disable_output();
     delay_10ms(speed);
     delay_10ms(speed);
   }
-  set_pwm(old_pwm);
 }
 
 /**
@@ -265,7 +275,7 @@ void restore_state(void) {
  * @param new_opts New options
  * @param flashes  Number of flashes
  */
-void toggle_option(uint8_t new_opts, uint8_t flashes) {
+void toggle_option(const uint8_t new_opts, const uint8_t flashes) {
   blink(flashes, FLASH_TIME);
   uint8_t old_options = options.raw;
   options.raw = new_opts;
@@ -465,6 +475,10 @@ int main(void) {
         break;
 
       case kTurbo:
+        // assert(output_is_on);
+        // Turbo mode is only entered by user action which means the driver was
+        // restarted which in turn means that the output was enabled during
+        // initialization
         set_pwm(TURBO_PWM);
         break;
 
@@ -473,7 +487,7 @@ int main(void) {
 
 #ifdef BATTCHECK
       case kBattcheck:
-        set_pwm(0);
+        disable_output();
 
         const uint8_t voltage = battery_voltage();
 
@@ -482,16 +496,18 @@ int main(void) {
           --i;
         }
 
+        set_pwm(FLASH_PWM);
         blink(i+1, FLASH_TIME);
         delay_s();
         break;
 #endif  // ifdef BATTCHECK
 
       case kConfig:
-        set_level(0);
+        disable_output();
+        set_pwm(FLASH_PWM);
         delay_s();
 
-        state = kDefault;
+        state = kDefault;  // Exit config mode after one iteration
 
         // This assumes that the bit field starts at the least significant bit
         uint8_t opts = options.raw;
@@ -500,6 +516,8 @@ int main(void) {
         toggle_option(opts ^ 0b00000100, 3);  // Freeze on high
         toggle_option(opts ^ 0b00001000, 4);  // Start with high
 
+        set_level(output);  // Restore previous level
+
         break;
     }
 
@@ -508,7 +526,8 @@ int main(void) {
       // TODO Take several measurements for noise filtering?
       const uint8_t voltage = battery_voltage();
       if (voltage <= BAT_CRIT) {
-        set_pwm(0);
+        disable_output();
+        set_pwm(FLASH_PWM);
         while (1) {
           // Do not go to sleep, but flash every few seconds to notify the user
           // that the flashlight is still turned on but the battery is dying.
@@ -519,7 +538,9 @@ int main(void) {
           delay_s();
         }
       } else if (voltage <= BAT_LOW) {
+        // Flicker with same brightness as current mode
         blink(16, FLICKER_TIME);
+        enable_output();
       }
     }
 #endif  // ifdef LOW_VOLTAGE_PROTECTION