sam: return an error when an incorrect PWM pin is used

Previously it would trigger a nil pointer panic.

src/examples/pwm/pwm.go

@@ -28,13 +28,16 @@ func main() {
 	machine.InitPWM()
 
 	red := machine.PWM{redPin}
-	red.Configure()
+	err := red.Configure()
+	checkError(err, "failed to configure red pin")
 
 	green := machine.PWM{greenPin}
-	green.Configure()
+	err = green.Configure()
+	checkError(err, "failed to configure green pin")
 
 	blue := machine.PWM{bluePin}
-	blue.Configure()
+	err = blue.Configure()
+	checkError(err, "failed to configure blue pin")
 
 	var rc uint8
 	var gc uint8 = 20
@@ -52,3 +55,10 @@ func main() {
 		time.Sleep(time.Millisecond * 500)
 	}
 }
+
+func checkError(err error, msg string) {
+	if err != nil {
+		print(msg, ": ", err.Error())
+		println()
+	}
+}

src/machine/machine_atmega328p.go

@@ -79,13 +79,14 @@ func InitPWM() {
 }
 
 // Configure configures a PWM pin for output.
-func (pwm PWM) Configure() {
+func (pwm PWM) Configure() error {
 	switch pwm.Pin / 8 {
 	case 0: // port B
 		avr.DDRB.SetBits(1 << uint8(pwm.Pin))
 	case 2: // port D
 		avr.DDRD.SetBits(1 << uint8(pwm.Pin-16))
 	}
+	return nil
 }
 
 // Set turns on the duty cycle for a PWM pin using the provided value. On the AVR this is normally a

src/machine/machine_atsamd21.go

@@ -1090,9 +1090,12 @@ func InitPWM() {
 }
 
 // Configure configures a PWM pin for output.
-func (pwm PWM) Configure() {
+func (pwm PWM) Configure() error {
 	// figure out which TCCX timer for this pin
 	timer := pwm.getTimer()
+	if timer == nil {
+		return ErrInvalidOutputPin
+	}
 
 	// disable timer
 	timer.CTRLA.ClearBits(sam.TCC_CTRLA_ENABLE)
@@ -1139,12 +1142,19 @@ func (pwm PWM) Configure() {
 		val := pwm.getPMux() & sam.PORT_PMUX0_PMUXO_Msk
 		pwm.setPMux(val | uint8(pwmConfig<<sam.PORT_PMUX0_PMUXE_Pos))
 	}
+
+	return nil
 }
 
 // Set turns on the duty cycle for a PWM pin using the provided value.
 func (pwm PWM) Set(value uint16) {
 	// figure out which TCCX timer for this pin
 	timer := pwm.getTimer()
+	if timer == nil {
+		// The Configure call above cannot have succeeded, so simply ignore this
+		// error.
+		return
+	}
 
 	// disable output
 	timer.CTRLA.ClearBits(sam.TCC_CTRLA_ENABLE)
@@ -1154,7 +1164,7 @@ func (pwm PWM) Set(value uint16) {
 	}
 
 	// Set PWM signal to output duty cycle
-	pwm.setChannel(uint32(value))
+	pwm.setChannel(timer, uint32(value))
 
 	// Wait for synchronization on all channels
 	for timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_CC0 |
@@ -1223,32 +1233,32 @@ func (pwm PWM) getTimer() *sam.TCC_Type {
 }
 
 // setChannel sets the value for the correct channel for PWM on this pin
-func (pwm PWM) setChannel(val uint32) {
+func (pwm PWM) setChannel(timer *sam.TCC_Type, val uint32) {
 	switch pwm.Pin {
 	case 6:
-		pwm.getTimer().CC0.Set(val)
+		timer.CC0.Set(val)
 	case 7:
-		pwm.getTimer().CC1.Set(val)
+		timer.CC1.Set(val)
 	case 8:
-		pwm.getTimer().CC0.Set(val)
+		timer.CC0.Set(val)
 	case 9:
-		pwm.getTimer().CC1.Set(val)
+		timer.CC1.Set(val)
 	case 14:
-		pwm.getTimer().CC0.Set(val)
+		timer.CC0.Set(val)
 	case 15:
-		pwm.getTimer().CC1.Set(val)
+		timer.CC1.Set(val)
 	case 16:
-		pwm.getTimer().CC2.Set(val)
+		timer.CC2.Set(val)
 	case 17:
-		pwm.getTimer().CC3.Set(val)
+		timer.CC3.Set(val)
 	case 18:
-		pwm.getTimer().CC2.Set(val)
+		timer.CC2.Set(val)
 	case 19:
-		pwm.getTimer().CC3.Set(val)
+		timer.CC3.Set(val)
 	case 20:
-		pwm.getTimer().CC2.Set(val)
+		timer.CC2.Set(val)
 	case 21:
-		pwm.getTimer().CC3.Set(val)
+		timer.CC3.Set(val)
 	default:
 		return // not supported on this pin
 	}

src/machine/machine_atsamd51.go

@@ -1216,7 +1216,7 @@ const (
 const period = 0xFFFF
 
 // Configure configures a PWM pin for output.
-func (pwm PWM) Configure() {
+func (pwm PWM) Configure() error {
 	// Set pin as output
 	sam.PORT.GROUP[0].DIRSET.Set(1 << uint8(pwm.Pin))
 	// Set pin to low
@@ -1240,6 +1240,9 @@ func (pwm PWM) Configure() {
 
 	// figure out which TCCX timer for this pin
 	timer := pwm.getTimer()
+	if timer == nil {
+		return ErrInvalidOutputPin
+	}
 
 	// disable timer
 	timer.CTRLA.ClearBits(sam.TCC_CTRLA_ENABLE)
@@ -1264,7 +1267,7 @@ func (pwm PWM) Configure() {
 
 	// Set the initial value
 	// TCCx->CC[tcChannel].reg = (uint32_t) value;
-	pwm.setChannel(0)
+	pwm.setChannel(timer, 0)
 
 	for timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_CC0) ||
 		timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_CC1) {
@@ -1282,12 +1285,19 @@ func (pwm PWM) Configure() {
 	// Wait for synchronization
 	for timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_ENABLE) {
 	}
+
+	return nil
 }
 
 // Set turns on the duty cycle for a PWM pin using the provided value.
 func (pwm PWM) Set(value uint16) {
 	// figure out which TCCX timer for this pin
 	timer := pwm.getTimer()
+	if timer == nil {
+		// The Configure call above cannot have succeeded, so simply ignore this
+		// error.
+		return
+	}
 
 	// Wait for synchronization
 	for timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_CTRLB) {
@@ -1297,7 +1307,7 @@ func (pwm PWM) Set(value uint16) {
 	}
 
 	// TCCx->CCBUF[tcChannel].reg = (uint32_t) value;
-	pwm.setChannelBuffer(uint32(value))
+	pwm.setChannelBuffer(timer, uint32(value))
 
 	for timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_CC0) ||
 		timer.SYNCBUSY.HasBits(sam.TCC_SYNCBUSY_CC1) {
@@ -1329,61 +1339,61 @@ func (pwm PWM) setPinCfg(val uint8) {
 	pwm.Pin.setPinCfg(val)
 }
 
-// setChannel sets the value for the correct channel for PWM on this pin
+// setChannel sets the value for the correct channel for PWM on this pin.
-func (pwm PWM) setChannel(val uint32) {
+func (pwm PWM) setChannel(timer *sam.TCC_Type, val uint32) {
 	switch pwm.Pin {
 	case PA16:
-		pwm.getTimer().CC[0].Set(val)
+		timer.CC[0].Set(val)
 	case PA17:
-		pwm.getTimer().CC[1].Set(val)
+		timer.CC[1].Set(val)
 	case PA14:
-		pwm.getTimer().CC[0].Set(val)
+		timer.CC[0].Set(val)
 	case PA15:
-		pwm.getTimer().CC[1].Set(val)
+		timer.CC[1].Set(val)
 	case PA18:
-		pwm.getTimer().CC[2].Set(val)
+		timer.CC[2].Set(val)
 	case PA19:
-		pwm.getTimer().CC[3].Set(val)
+		timer.CC[3].Set(val)
 	case PA20:
-		pwm.getTimer().CC[0].Set(val)
+		timer.CC[0].Set(val)
 	case PA21:
-		pwm.getTimer().CC[1].Set(val)
+		timer.CC[1].Set(val)
 	case PA23:
-		pwm.getTimer().CC[3].Set(val)
+		timer.CC[3].Set(val)
 	case PA22:
-		pwm.getTimer().CC[2].Set(val)
+		timer.CC[2].Set(val)
 	case PB31:
-		pwm.getTimer().CC[1].Set(val)
+		timer.CC[1].Set(val)
 	default:
 		return // not supported on this pin
 	}
 }
 
 // setChannelBuffer sets the value for the correct channel buffer for PWM on this pin
-func (pwm PWM) setChannelBuffer(val uint32) {
+func (pwm PWM) setChannelBuffer(timer *sam.TCC_Type, val uint32) {
 	switch pwm.Pin {
 	case PA16:
-		pwm.getTimer().CCBUF[0].Set(val)
+		timer.CCBUF[0].Set(val)
 	case PA17:
-		pwm.getTimer().CCBUF[1].Set(val)
+		timer.CCBUF[1].Set(val)
 	case PA14:
-		pwm.getTimer().CCBUF[0].Set(val)
+		timer.CCBUF[0].Set(val)
 	case PA15:
-		pwm.getTimer().CCBUF[1].Set(val)
+		timer.CCBUF[1].Set(val)
 	case PA18:
-		pwm.getTimer().CCBUF[2].Set(val)
+		timer.CCBUF[2].Set(val)
 	case PA19:
-		pwm.getTimer().CCBUF[3].Set(val)
+		timer.CCBUF[3].Set(val)
 	case PA20:
-		pwm.getTimer().CCBUF[0].Set(val)
+		timer.CCBUF[0].Set(val)
 	case PA21:
-		pwm.getTimer().CCBUF[1].Set(val)
+		timer.CCBUF[1].Set(val)
 	case PA23:
-		pwm.getTimer().CCBUF[3].Set(val)
+		timer.CCBUF[3].Set(val)
 	case PA22:
-		pwm.getTimer().CCBUF[2].Set(val)
+		timer.CCBUF[2].Set(val)
 	case PB31:
-		pwm.getTimer().CCBUF[1].Set(val)
+		timer.CCBUF[1].Set(val)
 	default:
 		return // not supported on this pin
 	}

src/machine/machine_generic.go

@@ -90,7 +90,8 @@ func InitPWM() {
 }
 
 // Configure configures a PWM pin for output.
-func (pwm PWM) Configure() {
+func (pwm PWM) Configure() error {
+	return nil
 }
 
 // Set turns on the duty cycle for a PWM pin using the provided value.

src/machine/machine_nrf52.go

@@ -159,7 +159,8 @@ func InitPWM() {
 }
 
 // Configure configures a PWM pin for output.
-func (pwm PWM) Configure() {
+func (pwm PWM) Configure() error {
+	return nil
 }
 
 // Set turns on the duty cycle for a PWM pin using the provided value.

src/machine/machine_nrf52840.go

@@ -104,8 +104,8 @@ func InitADC() {
 }
 
 // Configure configures an ADC pin to be able to read analog data.
-func (a ADC) Configure() {
+func (a ADC) Configure() error {
-	return // no pin specific setup on nrf52840 machine.
+	return nil // no pin specific setup on nrf52840 machine.
 }
 
 // Get returns the current value of a ADC pin in the range 0..0xffff.