machine/avr: implement raw GPIO access for bitbanged drivers

src/machine/machine_atmega.go

@@ -23,23 +23,6 @@ func (p GPIO) Configure(config GPIOConfig) {
 	}
 }
 
-// Set changes the value of the GPIO pin. The pin must be configured as output.
-func (p GPIO) Set(value bool) {
-	if value { // set bits
-		if p.Pin < 8 {
-			*avr.PORTD |= 1 << p.Pin
-		} else {
-			*avr.PORTB |= 1 << (p.Pin - 8)
-		}
-	} else { // clear bits
-		if p.Pin < 8 {
-			*avr.PORTD &^= 1 << p.Pin
-		} else {
-			*avr.PORTB &^= 1 << (p.Pin - 8)
-		}
-	}
-}
-
 // Get returns the current value of a GPIO pin.
 func (p GPIO) Get() bool {
 	if p.Pin < 8 {
@@ -51,6 +34,14 @@ func (p GPIO) Get() bool {
 	}
 }
 
+func (p GPIO) getPortMask() (*avr.RegValue, uint8) {
+	if p.Pin < 8 {
+		return avr.PORTD, 1 << p.Pin
+	} else {
+		return avr.PORTB, 1 << (p.Pin - 8)
+	}
+}
+
 // InitPWM initializes the registers needed for PWM.
 func InitPWM() {
 	// use waveform generation

src/machine/machine_attiny.go

@@ -15,13 +15,8 @@ func (p GPIO) Configure(config GPIOConfig) {
 	}
 }
 
-// Set changes the value of the GPIO pin. The pin must be configured as output.
-func (p GPIO) Set(value bool) {
-	if value { // set bits
-		*avr.PORTB |= 1 << p.Pin
-	} else { // clear bits
-		*avr.PORTB &^= 1 << p.Pin
-	}
+func (p GPIO) getPortMask() (*avr.RegValue, uint8) {
+	return avr.PORTB, 1 << p.Pin
 }
 
 // Get returns the current value of a GPIO pin.

src/machine/machine_avr.go

@@ -13,6 +13,39 @@ const (
 	GPIO_OUTPUT
 )
 
+// Set changes the value of the GPIO pin. The pin must be configured as output.
+func (p GPIO) Set(value bool) {
+	if value { // set bits
+		port, mask := p.PortMaskSet()
+		*port = mask
+	} else { // clear bits
+		port, mask := p.PortMaskClear()
+		*port = mask
+	}
+}
+
+// Return the register and mask to enable a given GPIO pin. This can be used to
+// implement bit-banged drivers.
+//
+// Warning: there are no separate pin set/clear registers on the AVR. The
+// returned mask is only valid as long as no other pin in the same port has been
+// changed.
+func (p GPIO) PortMaskSet() (*avr.RegValue, avr.RegValue) {
+	port, mask := p.getPortMask()
+	return port, *port | avr.RegValue(mask)
+}
+
+// Return the register and mask to disable a given port. This can be used to
+// implement bit-banged drivers.
+//
+// Warning: there are no separate pin set/clear registers on the AVR. The
+// returned mask is only valid as long as no other pin in the same port has been
+// changed.
+func (p GPIO) PortMaskClear() (*avr.RegValue, avr.RegValue) {
+	port, mask := p.getPortMask()
+	return port, *port &^ avr.RegValue(mask)
+}
+
 // InitADC initializes the registers needed for ADC.
 func InitADC() {
 	// set a2d prescaler so we are inside the desired 50-200 KHz range at 16MHz.