refactor(src/machine/nrf52xxx/spi): move buffer allocation to Transfer method, use pointer receiver

Signed-off-by: Paul Schroeder <milkpirate@users.noreply.github.com>

Author: milkpirate

src/machine/machine_nrf52xxx.go

@@ -203,16 +203,8 @@ func (a ADC) Get() uint16 {
 // SPI on the NRF.
 type SPI struct {
 	Bus *nrf.SPIM_Type
-	buf *[1]byte // 1-byte buffer for the Transfer method
 }
 
-// There are 3 SPI interfaces on the NRF528xx.
-var (
-	SPI0 = SPI{Bus: nrf.SPIM0, buf: new([1]byte)}
-	SPI1 = SPI{Bus: nrf.SPIM1, buf: new([1]byte)}
-	SPI2 = SPI{Bus: nrf.SPIM2, buf: new([1]byte)}
-)
-
 // SPIConfig is used to store config info for SPI.
 type SPIConfig struct {
 	Frequency uint32
@@ -224,7 +216,7 @@ type SPIConfig struct {
 }
 
 // Configure is intended to setup the SPI interface.
-func (spi SPI) Configure(config SPIConfig) error {
+func (spi *SPI) Configure(config SPIConfig) error {
 	// Disable bus to configure it
 	spi.Bus.ENABLE.Set(nrf.SPIM_ENABLE_ENABLE_Disabled)
 
@@ -276,10 +268,9 @@ func (spi SPI) Configure(config SPIConfig) error {
 }
 
 // Transfer writes/reads a single byte using the SPI interface.
-func (spi SPI) Transfer(w byte) (byte, error) {
-	buf := spi.buf[:]
-	buf[0] = w
-	err := spi.Tx(buf[:], buf[:])
+func (spi *SPI) Transfer(w byte) (byte, error) {
+	buf := []byte{w}
+	err := spi.Tx(buf, buf)
 	return buf[0], err
 }
 
@@ -288,7 +279,7 @@ func (spi SPI) Transfer(w byte) (byte, error) {
 // as bytes read. Therefore, if the number of bytes don't match it will be
 // padded until they fit: if len(w) > len(r) the extra bytes received will be
 // dropped and if len(w) < len(r) extra 0 bytes will be sent.
-func (spi SPI) Tx(w, r []byte) error {
+func (spi *SPI) Tx(w, r []byte) error {
 	// Unfortunately the hardware (on the nrf52832) only supports up to 255
 	// bytes in the buffers, so if either w or r is longer than that the
 	// transfer needs to be broken up in pieces.