machine/nrf: SPI master implementation

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

src/machine/spi.go

@@ -0,0 +1,70 @@
+// +build nrf
+
+package machine
+
+import "errors"
+
+var (
+	ErrTxSlicesRequired   = errors.New("SPI Tx requires a write or read slice, or both")
+	ErrTxInvalidSliceSize = errors.New("SPI write and read slices must be same size")
+)
+
+// Tx handles read/write operation for SPI interface. Since SPI is a syncronous write/read
+// interface, there must always be the same number of bytes written as bytes read.
+// The Tx method knows about this, and offers a few different ways of calling it.
+//
+// This form sends the bytes in tx buffer, putting the resulting bytes read into the rx buffer.
+// Note that the tx and rx buffers must be the same size:
+//
+// 		spi.Tx(tx, rx)
+//
+// This form sends the tx buffer, ignoring the result. Useful for sending "commands" that return zeros
+// until all the bytes in the command packet have been received:
+//
+// 		spi.Tx(tx, nil)
+//
+// This form sends zeros, putting the result into the rx buffer. Good for reading a "result packet":
+//
+// 		spi.Tx(nil, rx)
+//
+func (spi SPI) Tx(w, r []byte) error {
+	if w == nil && r == nil {
+		return ErrTxSlicesRequired
+	}
+
+	var err error
+
+	switch {
+	case w == nil:
+		// read only, so write zero and read a result.
+		for i := range r {
+			r[i], err = spi.Transfer(0)
+			if err != nil {
+				return err
+			}
+		}
+	case r == nil:
+		// write only
+		for _, b := range w {
+			_, err = spi.Transfer(b)
+			if err != nil {
+				return err
+			}
+		}
+
+	default:
+		// write/read
+		if len(w) != len(r) {
+			return ErrTxInvalidSliceSize
+		}
+
+		for i, b := range w {
+			r[i], err = spi.Transfer(b)
+			if err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}