Add support for 328P hardware backlight on B1/B2 (#6776)

docs/feature_backlight.md

@@ -32,16 +32,18 @@ This feature is distinct from both the [RGB underglow](feature_rgblight.md) and
 
 Hardware PWM is supported according to the following table:
 
-|Backlight Pin|AT90USB64/128|ATmega16/32U4|ATmega16/32U2|ATmega32A|
-|-------------|-------------|-------------|-------------|---------|
-|`B5`         |Timer 1      |Timer 1      |             |         |
-|`B6`         |Timer 1      |Timer 1      |             |         |
-|`B7`         |Timer 1      |Timer 1      |Timer 1      |         |
-|`C4`         |Timer 3      |             |             |         |
-|`C5`         |Timer 3      |             |Timer 1      |         |
-|`C6`         |Timer 3      |Timer 3      |Timer 1      |         |
-|`D4`         |             |             |             |Timer 1  |
-|`D5`         |             |             |             |Timer 1  |
+|Backlight Pin|AT90USB64/128|ATmega16/32U4|ATmega16/32U2|ATmega32A|ATmega328P|
+|-------------|-------------|-------------|-------------|---------|----------|
+|`B1`         |             |             |             |         |Timer 1   |
+|`B2`         |             |             |             |         |Timer 1   |
+|`B5`         |Timer 1      |Timer 1      |             |         |          |
+|`B6`         |Timer 1      |Timer 1      |             |         |          |
+|`B7`         |Timer 1      |Timer 1      |Timer 1      |         |          |
+|`C4`         |Timer 3      |             |             |         |          |
+|`C5`         |Timer 3      |             |Timer 1      |         |          |
+|`C6`         |Timer 3      |Timer 3      |Timer 1      |         |          |
+|`D4`         |             |             |             |Timer 1  |          |
+|`D5`         |             |             |             |Timer 1  |          |
 
 All other pins will use software PWM. If the [Audio](feature_audio.md) feature is disabled or only using one timer, the backlight PWM can be triggered by a hardware timer:
 

quantum/quantum.c

@@ -1104,6 +1104,22 @@ void matrix_scan_quantum() {
 #            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)