mcp23017: implement pin toggling

Also add an example for using multiple devices.

examples/mcp23017-multiple/main.go

@@ -0,0 +1,45 @@
+// This example demonstrates putting several mcp23017 devices together into
+// a single virtual I/O array.
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/mcp23017"
+)
+
+func main() {
+	err := machine.I2C0.Configure(machine.I2CConfig{
+		Frequency: machine.TWI_FREQ_400KHZ,
+	})
+	if err != nil {
+		panic(err)
+	}
+	// Assume the devices are at addresses 0x20, 0x21
+	dev, err := mcp23017.NewI2CDevices(machine.I2C0, 0x20, 0x21)
+	if err != nil {
+		panic(err)
+	}
+	// Configure pin 0 for input and all the others for output.
+	if err := dev.SetModes([]mcp23017.PinMode{
+		mcp23017.Input | mcp23017.Pullup,
+		mcp23017.Output,
+	}); err != nil {
+		panic(err)
+	}
+	input := dev.Pin(0)
+	// Make a mask that represents all the output pins.
+	// Note that this leverages the driver behaviour which replicates the highest bit in
+	// the last slice element (1 in this case) to all other pins
+	outputMask := mcp23017.PinSlice{^mcp23017.Pins(1 << 0)} // All except pin 0
+	inputVal, err := input.Get()
+	if err != nil {
+		panic(err)
+	}
+	println("input value: ", inputVal)
+	// Set the values of all the output pins.
+	err = dev.SetPins(mcp23017.PinSlice{0b1011011_01101110, 0b11111101_11100110}, outputMask)
+	if err != nil {
+		panic(err)
+	}
+}