Add SPISettings (Matthijs Kooijman)

SPI.h

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2010 by Cristian Maglie <c.maglie@bug.st>
  * Copyright (c) 2014 by Paul Stoffregen <paul@pjrc.com> (Transaction API)
+ * Copyright (c) 2014 by Matthijs Kooijman <matthijs@stdin.nl> (SPISettings)
  * SPI Master library for arduino.
  *
  * This file is free software; you can redistribute it and/or modify
@@ -38,6 +39,89 @@
 #define SPI_CLOCK_MASK 0x03  // SPR1 = bit 1, SPR0 = bit 0 on SPCR
 #define SPI_2XCLOCK_MASK 0x01  // SPI2X = bit 0 on SPSR
 
+
+class SPISettings {
+public:
+	SPISettings(uint32_t clock, uint8_t bitOrder, uint8_t dataMode) {
+		if (__builtin_constant_p(clock)) {
+			init_AlwaysInline(clock, bitOrder, dataMode);
+		} else {
+			init_MightInline(clock, bitOrder, dataMode);
+		}
+	}
+private:
+	void init_MightInline(uint32_t clock, uint8_t bitOrder, uint8_t dataMode) {
+		init_AlwaysInline(clock, bitOrder, dataMode);
+	}
+	void init_AlwaysInline(uint32_t clock, uint8_t bitOrder, uint8_t dataMode)
+	  __attribute__((__always_inline__)) {
+		// Clock settings are defined as follows. Note that this shows SPI2X
+		// inverted, so the bits form increasing numbers. Also note that
+		// fosc/64 appears twice
+		// SPR1 SPR0 ~SPI2X Freq
+		//   0    0     0   fosc/2
+		//   0    0     1   fosc/4
+		//   0    1     0   fosc/8
+		//   0    1     1   fosc/16
+		//   1    0     0   fosc/32
+		//   1    0     1   fosc/64
+		//   1    1     0   fosc/64
+		//   1    1     1   fosc/128
+
+		// We find the fastest clock that is less than or equal to the
+		// given clock rate. The clock divider that results in clock_setting
+		// is 2 ^^ (clock_div + 1). If nothing is slow enough, we'll use the
+		// slowest (128 == 2 ^^ 7, so clock_div = 6).
+		uint8_t clockDiv;
+
+		// When the clock is known at compiletime, use this if-then-else
+		// cascade, which the compiler knows how to completely optimize
+		// away. When clock is not know, use a loop instead, which generates
+		// shorter code.
+		if (__builtin_constant_p(clock)) {
+			if (clock >= F_CPU / 2) {
+				clockDiv = 0;
+			} else if (clock >= F_CPU / 4) {
+				clockDiv = 1;
+			} else if (clock >= F_CPU / 8) {
+				clockDiv = 2;
+			} else if (clock >= F_CPU / 16) {
+				clockDiv = 3;
+			} else if (clock >= F_CPU / 32) {
+				clockDiv = 4;
+			} else if (clock >= F_CPU / 64) {
+				clockDiv = 5;
+			} else {
+				clockDiv = 6;
+			}
+		} else {
+			uint32_t clockSetting = F_CPU / 2;
+			clockDiv = 0;
+			while (clockDiv < 6 && clock < clockSetting) {
+				clockSetting /= 2;
+				clockDiv++;
+			}
+		}
+
+		// Compensate for the duplicate fosc/64
+		if (clockDiv == 6)
+		clockDiv = 7;
+
+		// Invert the SPI2X bit
+		clockDiv ^= 0x1;
+
+		// Pack into the SPISettings class
+		spcr = _BV(SPE) | _BV(MSTR) | ((bitOrder == LSBFIRST) ? _BV(DORD) : 0) |
+			(dataMode & SPI_MODE_MASK) | ((clockDiv >> 1) & SPI_CLOCK_MASK);
+		spsr = clockDiv & SPI_2XCLOCK_MASK;
+	}
+	uint8_t spcr;
+	uint8_t spsr;
+	friend class SPIClass;
+};
+
+
+
 class SPIClass {
 public:
 	// Initialize the SPI library
@@ -56,7 +140,7 @@ public:
 	// Before using SPI.transfer() or asserting chip select pins,
 	// this function is used to gain exclusive access to the SPI bus
 	// and configure the correct settings.
-	inline static void beginTransaction(uint8_t clockDiv, uint8_t bitOrder, uint8_t dataMode) {
+	inline static void beginTransaction(SPISettings settings) {
 		if (interruptMode > 0) {
 			#ifdef SPI_AVR_EIMSK
 			if (interruptMode == 1) {
@@ -69,10 +153,8 @@ public:
 				cli();
 			}
 		}
-		// these long expressions each compile to only 2 instructions
-		SPCR = _BV(SPE) | _BV(MSTR) | ((bitOrder == LSBFIRST) ? _BV(DORD) : 0) |
-			(dataMode & SPI_MODE_MASK) | (clockDiv & SPI_CLOCK_MASK);
-		SPSR = (clockDiv >> 2) & SPI_2XCLOCK_MASK;
+		SPCR = settings.spcr;
+		SPSR = settings.spsr;
 	}
 
 	// Write to the SPI bus (MOSI pin) and also receive (MISO pin)

keywords.txt

@@ -23,6 +23,7 @@ setSCK	KEYWORD2
 usingInterrupt	KEYWORD2
 beginTransaction	KEYWORD2
 endTransaction	KEYWORD2
+SPISettings	KEYWORD2
 
 #######################################
 # Constants (LITERAL1)