msp430: Add preprocessor macro for CPU speed

src/arch/msp430fr5969lp/Makefile.inc

@@ -5,6 +5,7 @@ MCU = msp430fr5969
 
 INCLUDES += -I/opt/msp430/ti/gcc/include
 COMMON_FLAGS += -mcpu=${CPU} -mmcu=${MCU} -DMULTIPASS_ARCH_msp430fr5969lp
+COMMON_FLAGS += -DF_CPU=16000000UL
 COMMON_FLAGS += -DMULTIPASS_ARCH_HAS_I2C
 
 CC = /opt/msp430/ti/gcc/bin/msp430-elf-gcc

src/arch/msp430fr5969lp/driver/spi_b.cc

 #include "driver/spi_b.h"
 #include <msp430.h>
 
+#ifndef F_I2C
+#define F_I2C 1000000UL
+#endif
+
 void SPI::setup()
 {
 	UCB0CTLW0 |= UCSWRST;
@@ -22,7 +26,8 @@ void SPI::setup()
 	//P1REN |= BIT6;
 
 	UCB0CTLW0 = UCCKPH | UCMSB | UCMST | UCSYNC | UCMODE_0 | UCSSEL__SMCLK | UCSWRST;
-	UCB0BRW = 15; // /16 -> 1MHz
+	UCB0BRW = (F_CPU/F_I2C)-1; // /16 -> 1MHz
+	// UCB0BRW = (F_CPU / F_I2C) - 1
 	UCB0CTLW0 &= ~UCSWRST;
 }