 	pinMode(MOSI, OUTPUT);
 	pinMode(SCK, OUTPUT);
 
-	// Initialize serial and SPI
+	// Initialize serial via USB
 	Serial.begin(19200);
-	SPCR = _BV(SPE) | _BV(MSTR) | _BV(SPR0); // SPI master, clock rate fck/16
+
+	// Configure SPI as master, clock rate fck/16, enable interrupts	
+	SPCR = _BV(SPIE) | _BV(SPE) | _BV(MSTR) | _BV(SPR0);
 	
 	// Timer magic
 	cli();

	// because it completes before the next timer interrupt.
	SPDR = buf_front[pwm_i][8*row_i+col_i];
}

volatile uint8_t may_flip = 0;
volatile uint16_t pwm_lengths[] = { 0x30, 0x10, 0x06 }; // PWM cycle lengths
uint8_t pwm_i = 0; // Current PWM cycle
volatile uint8_t spi_row_change = false;

// Hardware configuration
#define PIN_COL_BIT0 A2
