machine/nrf: SPI master implementation

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/examples/mcp3008/mcp3008.go

@@ -0,0 +1,55 @@
+// Connects to an MCP3008 ADC via SPI.
+// Datasheet: https://www.microchip.com/wwwproducts/en/en010530
+package main
+
+import (
+	"errors"
+	"machine"
+	"time"
+)
+
+// CS_PIN is the pin used for Chip Select (CS). Change to whatever is in use on your board.
+const CS_PIN = 3
+
+var (
+	tx          []byte
+	rx          []byte
+	val, result uint16
+	cs          machine.GPIO
+)
+
+func main() {
+	cs = machine.GPIO{CS_PIN}
+	cs.Configure(machine.GPIOConfig{Mode: machine.GPIO_OUTPUT})
+
+	machine.SPI0.Configure(machine.SPIConfig{
+		Frequency: 4000000,
+		Mode:      3})
+
+	tx = make([]byte, 3)
+	rx = make([]byte, 3)
+
+	for {
+		val, _ = Read(0)
+		println(val)
+		time.Sleep(50 * time.Millisecond)
+	}
+}
+
+// Read analog data from channel
+func Read(channel int) (uint16, error) {
+	if channel < 0 || channel > 7 {
+		return 0, errors.New("Invalid channel for read")
+	}
+
+	tx[0] = 0x01
+	tx[1] = byte(8+channel) << 4
+	tx[2] = 0x00
+
+	cs.Low()
+	machine.SPI0.Tx(tx, rx)
+	result = uint16((rx[1]&0x3))<<8 + uint16(rx[2])
+	cs.High()
+
+	return result, nil
+}