Add SPI driver

drivers/spi.c

@@ -0,0 +1,23 @@
+#include "spi.h"
+#include <avr/io.h>
+
+uint8_t spi_init(uint8_t spcr) {
+  PORTB |= (1 << 5); // set SCK hi
+  DDRB |= (1 << 5); // set SCK as output
+  DDRB &= ~(1 << 4); // set MISO as input
+  DDRB |= (1 << 3); // set MOSI as output
+  DDRB |= (1 << 2); // SS must be output for Master mode to work
+
+  SPCR = spcr | SPE;
+  return(SPSR);
+}
+
+uint8_t spi_transfer(uint8_t in) {
+  uint8_t v;
+  SPDR = in;
+  // wait for transfer to complete
+  while(! (SPSR & (1<<SPIF)) ) {};
+  v = SPDR;
+  return(v);
+}
+

drivers/spi.h

@@ -0,0 +1,4 @@
+#include <stdint.h>
+
+uint8_t spi_init(uint8_t spcr);
+uint8_t spi_transfer(uint8_t in);