Initialize OCR1A to FAN02_MIN_DUTY and add PWM documentation

src/bus/pwm.c

@@ -7,17 +7,6 @@ int PWM_Init(void)
 	// PD5: PWM NF-A8 Fan Peltier Cold Side
 	// PD7: PWM NF-R8 Fan Heating Element
 
-	DDRD |= BIT(PD4) | BIT(PD5); // | BIT(PD7);
-
-	// TIMER1: Fast mode, non-inverting, top=ICR1, prescale /1
-
-	TCCR1A = BIT(WGM11) | BIT(COM1A1) | BIT(COM1B1);
-	TCCR1B = BIT(WGM12) | BIT(WGM13) | BIT(CS10);
-	ICR1 = PWM_CYCLE_TOP; // 8000 MHz / 25000 KHz
-
-	OCR1A = FAN01_MIN_DUTY;
-	OCR1B = FAN02_MIN_DUTY;
-
 	// ATMega32A does not have more than two outputs for the
 	// 16-bit timer and the other 8-bit timers don't have modes
 	// where the value of TOP can be changed. We can only get
@@ -27,14 +16,91 @@ int PWM_Init(void)
 	// 8-bit this gives us a really low duty step size of 2.5%.
 	// Ideal would be two 16-bit timers with two outputs each.
 
+	DDRD |= BIT(PD4) | BIT(PD5) | BIT(PD7);
+	// PORTD &= ~BIT(PD7); // Turn off PD7
+
+	// TCCR1A Timer1 Control Register A
+	// 7       6       5       4       3      2      1      0
+	// COM1A1  COM1A0  COM1B1  COM1B0  FOC1A  FOC1B  WGM11  WGM10
+
+	// TCCR1B Timer1 Control Register B
+	// 7       6       5       4       3      2      1      0
+	// CNC1    ICES1   –       WGM13   WGM12  CS12   CS11   CS10
+
+	// The COM1A1:0 and COM1B1:0 control the Output
+	// Compare pins (OC1A and OC1B respectively) behavior.
+	// If one or both of the COM1A1:0 bits are written to
+	// one, the OC1A output overrides the normal port
+	// functionality of the I/O pin it is connected to.
+
+	// If one or both of the COM1B1:0 bit are written to
+	// one, the OC1B output overrides the normal port
+	// functionality of the I/O pin it is connected to.
+	// However, note that the Data Direction Register
+	// (DDR) bit corresponding to the OC1A or OC1B pin
+	// must be set in order to enable the output driver.
+
+	// COM1B1:0: Compare Output Mode, Fast PWM
+	// COM1A1/   COM1A0/   Description
+	// COM1B1    COM1B0
+	// 0         0         OC1A/OC1B disconnected.
+	// 0         1         WGM13:0 = 15: Toggle OC1A on match,
+	//                     OC1B disconnected. For other WGM13:0
+	//                     settings OC1A/OC1B disconnected.
+	// 1         0         Clear OC1A/OC1B on match and set at
+	//                     BOTTOM (non-inverting).
+	// 1         1         Set OC1A/OC1B on match and clear at
+	//                     BOTTOM (inverting mode).
+
+	// A special case occurs when OCR1A/OCR1B equals TOP
+	// and COM1A1/COM1B1 is set. In this case the compare
+	// match is ignored, but the set or clear is done at
+	// BOTTOM.
+
+	// FOC1A/FOC1B: Force Output Compare for Compare unit A/B
+	// For ensuring compatibility with future devices, these
+	// bits must be set to zero when TCCR1A is written when
+	// operating in a PWM mode.
+
+	// WGM11:0: Waveform Generation Mode
+	// Combined with the WGM13:2 bits found in the TCCR1B
+	// Register, these bits control the counting sequence of
+	// the counter, the source for maximum (TOP) counter
+	// value, and what type of waveform generation to be used.
+	// See page 115 of the ATMega32A data sheet for all modes.
+
+	// Mode  WGM13 WGM12 WGM11 WGM10  Timer Mode   TOP
+	// 14    1     1     1     0      Fast PWM     ICR1
+
+	// CS12:0: Clock Select
+
+	// CS12  CS11  CS10  Description
+	// 0     0     0     No clock source.
+	// 0     0     1     clk I/O /1 no prescaling.
+	// 0     1     0     clk I/O /8 from prescaler.
+	// 0     1     1     clk I/O /64 from prescaler.
+	// 1     0     0     clk I/O /256 from prescaler.
+	// 1     0     1     clk I/O /1024 from prescaler.
+	// 1     1     0     External clock on T1 falling edge.
+	// 1     1     1     External clock on T1 rising edge.
+
+	// TIMER1: Fast mode, non-inverting, top=ICR1, prescale /1
+
+	TCCR1A = BIT(WGM11) | BIT(COM1A1) | BIT(COM1B1);
+	TCCR1B = BIT(WGM12) | BIT(WGM13) | BIT(CS10);
+	ICR1 = PWM_CYCLE_TOP; // 8000 MHz / 25000 KHz
+
+	OCR1B = FAN01_MIN_DUTY; // PD4
+	OCR1A = FAN02_MIN_DUTY; // PD5
+
 	// TIMER2: Fast mode, non-inverting, top=0xFF, prescale /8
+	// TOP set to 8000000 (f_cpu) / 8 (prescale) / 25000 (f_pwm) - 1
 
-	// Top set to 8000000 (f_cpu) / 8 (prescale) / 25000 (f_pwm) - 1
 	// TCCR2 = BIT(WGM20) | BIT(WGM21) | BIT(COM21) | BIT(CS21);
-	// OCR2 = 40 - 1; // XXX: OCR2A=top OCR2B=duty
+	// OCR2 = 40 - 1; // XXX: OCR2A=TOP OCR2B=duty
 
-	TCCR2 = 0;
+	TCCR2 = 0; // Normal operation
-	OCR2 = 0;
+	OCR2 = FAN03_MIN_DUTY; // PD7
 
 	return 0;
 }

src/main.c

@@ -35,7 +35,7 @@ int Init(void)
 	Info("Initializing...");
 
 	// The watchdog timer is clocked from a separate
-	// on-chip oscillator which runs at 1MHz. Eight
+	// on-chip oscillator which runs at 1 MHz. Eight
 	// different clock cycle periods can be selected
 	// to determine the reset period. If the reset
 	// period expires, the chip resets and executes