7seg_dual.c

#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>
#include "lcd.h"

// Shift register pin info
#define SRCLK_PIN 2
#define DATA_PIN  1
#define RCLK_PIN  0

#define SR_PORT PORTB

// Shift register timing info
// Data sheet says the shift register can run faster than
// the attiny84, but let's make it adjustable anyway.

#define SETUP_TIME  0
#define PULSE_TIME  0
#define HOLD_TIME   0

// Because the attiny84 apparently takes twice as long
// to << 2 as it does to << 1.  WTF?

static uint16_t bitmask[] = {
  0x0001,
  0x0002,
  0x0004,
  0x0008,
  0x0010,
  0x0020,
  0x0040,
  0x0080,
  0x0100,
  0x0200,
  0x0400,
  0x0800,
  0x1000,
  0x2000,
  0x4000,
  0x8000
};

// Send a pair of bytes to the shift registers
void streamBytes(uint16_t byte) {
  uint8_t i, j;
  // Need to send in reverse; this could be optimized
  // Can't just have i count down because all ints are unsigned
  for (i=0, j=15; i < 16; i++, j--) {
    // Write the bit to the serial pin
    if (byte & bitmask[j]) {
      SR_PORT |= _BV(DATA_PIN);
    } else {
      SR_PORT &= ~_BV(DATA_PIN);
    }
    // Pulse the clock
    _delay_us(SETUP_TIME);
    SR_PORT |= _BV(SRCLK_PIN);
    _delay_us(PULSE_TIME);
    SR_PORT &= ~_BV(SRCLK_PIN);
    _delay_us(PULSE_TIME);
  }
  // Turn off data pin
  SR_PORT &= ~_BV(DATA_PIN);
  // Move the shift register data to the storage register
  _delay_us(SETUP_TIME);
  SR_PORT |= _BV(RCLK_PIN);
  _delay_us(HOLD_TIME);
  SR_PORT &= ~_BV(RCLK_PIN);
}

// Use the pin mapping in lcd.h to prepare a bitstream for transmission
// to the shift registers
void displayByte(byte) {
  // Build pin info
  uint16_t bitstream;
  uint8_t msb, lsb;
  msb = byte >> 4;
  lsb = byte & 0x0f;
  bitstream = (characters[msb] << 8) | characters[lsb];

  streamBytes(bitstream);
}

int main(void) {
  uint8_t value = 0x00;

  // Enable output on shift register pins
  DDRB |= _BV(SRCLK_PIN) | _BV(DATA_PIN) | _BV(RCLK_PIN);

  while(1) {
    displayByte(value);
    _delay_ms(250);
    value++;
  }
  return (0);
}