esp32: configure the I/O matrix for GPIO pins

Only some pins (notably including GPIO2 aka machine.LED) have GPIO for
the default function 1. Other pins (such as GPIO 15) had a different
function by default. Function 3 means GPIO for all the pins, so always
use that when configuring a pin to use as a GPIO pin.

In the future, the mux configuration will need to be updated for other
functions such as SPI, I2C, etc.

src/machine/machine_esp32.go

@@ -20,18 +20,43 @@ type PinMode uint8
 const (
 	PinOutput PinMode = iota
 	PinInput
+	PinInputPullup
+	PinInputPulldown
 )
 
 // Configure this pin with the given configuration.
 func (p Pin) Configure(config PinConfig) {
-	if config.Mode == PinOutput {
+	var muxConfig uint32 // The mux configuration.
+
+	// Configure this pin as a GPIO pin.
+	const function = 3 // function 3 is GPIO for every pin
+	muxConfig |= (function - 1) << esp.IO_MUX_GPIO0_MCU_SEL_Pos
+
+	// Make this pin an input pin (always).
+	muxConfig |= esp.IO_MUX_GPIO0_FUN_IE
+
+	// Set drive strength: 0 is lowest, 3 is highest.
+	muxConfig |= 2 << esp.IO_MUX_GPIO0_FUN_DRV_Pos
+
+	// Select pull mode.
+	if config.Mode == PinInputPullup {
+		muxConfig |= esp.IO_MUX_GPIO0_FUN_WPU
+	} else if config.Mode == PinInputPulldown {
+		muxConfig |= esp.IO_MUX_GPIO0_FUN_WPD
+	}
+
+	// Configure the pad with the given IO mux configuration.
+	p.mux().Set(muxConfig)
+
+	switch config.Mode {
+	case PinOutput:
 		// Set the 'output enable' bit.
 		if p < 32 {
 			esp.GPIO.ENABLE_W1TS.Set(1 << p)
 		} else {
 			esp.GPIO.ENABLE1_W1TS.Set(1 << (p - 32))
 		}
-	} else {
+	case PinInput, PinInputPullup, PinInputPulldown:
 		// Clear the 'output enable' bit.
 		if p < 32 {
 			esp.GPIO.ENABLE_W1TC.Set(1 << p)
@@ -97,6 +122,89 @@ func (p Pin) Get() bool {
 	}
 }
 
+// mux returns the I/O mux configuration register corresponding to the given
+// GPIO pin.
+func (p Pin) mux() *volatile.Register32 {
+	// I have no idea whether there is any pattern in the GPIO <-> pad mapping.
+	// I couldn't find it.
+	switch p {
+	case 36:
+		return &esp.IO_MUX.GPIO36
+	case 37:
+		return &esp.IO_MUX.GPIO37
+	case 38:
+		return &esp.IO_MUX.GPIO38
+	case 39:
+		return &esp.IO_MUX.GPIO39
+	case 34:
+		return &esp.IO_MUX.GPIO34
+	case 35:
+		return &esp.IO_MUX.GPIO35
+	case 32:
+		return &esp.IO_MUX.GPIO32
+	case 33:
+		return &esp.IO_MUX.GPIO33
+	case 25:
+		return &esp.IO_MUX.GPIO25
+	case 26:
+		return &esp.IO_MUX.GPIO26
+	case 27:
+		return &esp.IO_MUX.GPIO27
+	case 14:
+		return &esp.IO_MUX.MTMS
+	case 12:
+		return &esp.IO_MUX.MTDI
+	case 13:
+		return &esp.IO_MUX.MTCK
+	case 15:
+		return &esp.IO_MUX.MTDO
+	case 2:
+		return &esp.IO_MUX.GPIO2
+	case 0:
+		return &esp.IO_MUX.GPIO0
+	case 4:
+		return &esp.IO_MUX.GPIO4
+	case 16:
+		return &esp.IO_MUX.GPIO16
+	case 17:
+		return &esp.IO_MUX.GPIO17
+	case 9:
+		return &esp.IO_MUX.SD_DATA2
+	case 10:
+		return &esp.IO_MUX.SD_DATA3
+	case 11:
+		return &esp.IO_MUX.SD_CMD
+	case 6:
+		return &esp.IO_MUX.SD_CLK
+	case 7:
+		return &esp.IO_MUX.SD_DATA0
+	case 8:
+		return &esp.IO_MUX.SD_DATA1
+	case 5:
+		return &esp.IO_MUX.GPIO5
+	case 18:
+		return &esp.IO_MUX.GPIO18
+	case 19:
+		return &esp.IO_MUX.GPIO19
+	case 20:
+		return &esp.IO_MUX.GPIO20
+	case 21:
+		return &esp.IO_MUX.GPIO21
+	case 22:
+		return &esp.IO_MUX.GPIO22
+	case 3:
+		return &esp.IO_MUX.U0RXD
+	case 1:
+		return &esp.IO_MUX.U0TXD
+	case 23:
+		return &esp.IO_MUX.GPIO23
+	case 24:
+		return &esp.IO_MUX.GPIO24
+	default:
+		return nil
+	}
+}
+
 var (
 	UART0 = UART{Bus: esp.UART0, Buffer: NewRingBuffer()}
 	UART1 = UART{Bus: esp.UART1, Buffer: NewRingBuffer()}