Define 3 SPI transfering functions, and other minor changes

spi.go

@@ -1,15 +1,12 @@
+// SPI functionality is implemented here
 package rpio
 
 import (
 	"errors"
 )
 
-var (
-	SpiMapError = errors.New("SPI registers not mapped correctly - are you root?")
-)
-
 const (
-	SPI0 = iota // only spi0 supported for now
+	SPI0 = iota // only SPI0 supported for now
 	SPI1        // aux
 	SPI2        // aux
 )
@@ -20,9 +17,13 @@ const (
 	clkDivReg = 2
 )
 
-// Sets SPI pins of given device to SIP mode
-// (CE0, CE1, [CE2], SCLK, MOSI, MISO)
-// also reset SPI control register
+var (
+	SpiMapError = errors.New("SPI registers not mapped correctly - are you root?")
+)
+
+// Sets SPI pins of given device to SPI mode
+// (CE0, CE1, [CE2], SCLK, MOSI, MISO).
+// It also resets SPI control register.
 func SpiBegin(dev int) error {
 	spiMem[csReg] = 0 // reset spi settings to default
 	if spiMem[csReg] == 0 {
@@ -39,7 +40,7 @@ func SpiBegin(dev int) error {
 	return nil
 }
 
-// Sets SPI pins of given device to default (Input) mode
+// Sets SPI pins of given device to default (Input) mode.
 func SpiEnd(dev int) {
 	var pins = getSpiPins(dev)
 	for _, pin := range pins {
@@ -47,7 +48,7 @@ func SpiEnd(dev int) {
 	}
 }
 
-// Set (maximal) speed [Hz] of SPI clock
+// Set (maximal) speed [Hz] of SPI clock.
 // Param speed may be as big as 125MHz in theory, but
 // only values up to 31.25MHz are considered relayable.
 func SpiSpeed(speed int) {
@@ -57,8 +58,8 @@ func SpiSpeed(speed int) {
 }
 
 // Select chip, one of 0, 1, 2
-// for selecting slave on CE0, CE1, or CE2
-func SpiChipSelect(chip int) { // control & status
+// for selecting slave on CE0, CE1, or CE2 pin
+func SpiChipSelect(chip int) {
 	const csMask = 3 // chip select has 2 bits
 
 	cs := uint32(chip & csMask)
@@ -66,32 +67,50 @@ func SpiChipSelect(chip int) { // control & status
 	spiMem[csReg] = spiMem[csReg]&^csMask | cs
 }
 
+// SpiTransmit takes one or more bytes and send them to slave.
+//
+// Data received from slave are ignored.
+// Use spread operator to send slice of bytes.
+func SpiTransmit(data ...byte) {
+	SpiExchange(append(data[:0:0], data...)) // clone data because it will be rewriten by received bytes
+}
+
+// SpiReceive receives n bytes from slave.
+//
+// Note that n zeroed bytes are send to slave as side effect.
+func SpiReceive(n int) []byte {
+	data := make([]byte, n, n)
+	SpiExchange(data)
+	return data
+}
+
 // Transmit all bytes in data to slave
-// and simultaneously receives bytes from slave to data
-func SpiTransfer(data []byte) { // control & status
+// and simultaneously receives bytes from slave to data.
+//
+// If you want to only send or only receive, use SpiTransmit/SpiReceive
+func SpiExchange(data []byte) {
 	const ta = 1 << 7   // transfer active
 	const txd = 1 << 18 // tx fifo can accept data
 	const rxd = 1 << 17 // rx fifo contains data
 	const done = 1 << 16
 
-	length := len(data)
-	i := 0 // data index
-
 	clearSpiTxRxFifo()
 
 	// set TA = 1
 	spiMem[csReg] |= ta
 
-	for i < length {
-		// Poll TXD writing bytes to SPI_FIFO
+	for i := range data {
+		// wait for TXD
 		for spiMem[csReg]&txd == 0 {
 		}
+		// write bytes to SPI_FIFO
 		spiMem[fifoReg] = uint32(data[i])
-		// Poll RXD reading bytes from SPI_FIFO
+
+		// wait for RXD
 		for spiMem[csReg]&rxd == 0 {
 		}
+		// read bytes from SPI_FIFO
 		data[i] = byte(spiMem[fifoReg])
-		i++
 	}
 
 	// wait for DONE
@@ -102,32 +121,27 @@ func SpiTransfer(data []byte) { // control & status
 	spiMem[csReg] &^= ta
 }
 
-func setSpiDiv(div uint32) {
-	const divMask = 1<<16 - 1 - 1 // cdiv have 16 bits and must be odd (for some reason)
+// set spi clock divider value
+func setSpiDiv(cdiv uint32) {
+	const cdivMask = 1<<16 - 1 - 1 // cdiv have 16 bits and must be odd (for some reason)
 	spiMem[clkDivReg] = div & divMask
 }
 
+// clear both FIFOs
 func clearSpiTxRxFifo() {
 	const clearTxRx = 1<<5 | 1<<4
-
 	spiMem[csReg] |= clearTxRx
 }
 
 func getSpiPins(dev int) []Pin {
 	switch dev {
 	case SPI0:
-		return []Pin{
-			7, 8, 9, 10, 11,
-			// 35, 36, 37, 38, 39, // only one set of SPI0 can be set in Spi mode at a time
-		}
+		return []Pin{7, 8, 9, 10, 11}
+		// ommit 35, 36, 37, 38, 39 - only one set of SPI0 can be set in Spi mode at a time
 	case SPI1:
-		return []Pin{
-			16, 17, 18, 19, 20, 21,
-		}
+		return []Pin{16, 17, 18, 19, 20, 21}
 	case SPI2:
-		return []Pin{
-			40, 41, 42, 43, 44, 45,
-		}
+		return []Pin{40, 41, 42, 43, 44, 45}
 	default:
 		return []Pin{}
 	}

spi_test.go

@@ -0,0 +1,16 @@
+package rpio
+
+import ()
+
+func Example_SPI() {
+	SpiBegin(SPI0) // BCM pins 7 to 11
+
+	SpiSpeed(144000) // 144kHz
+	SpiChipSelect(1) // CE1
+
+	SpiTransmit(0xFF)
+	SpiTransmit(0xDE, 0xAD)
+	SpiTransmit(data...)
+
+	SpiEnd(SPI0)
+}