CPE328 (2/2019) RTC

ds1307.c

#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>

void sendMax7219Command(uint8_t address, uint8_t data) 
{
    PORTB &= ~(1 << PORTB2);

    SPDR = address;
    while (!(SPSR & (1 << SPIF)));

    SPDR = data;
    while (!(SPSR & (1 << SPIF)));

    PORTB |= (1 << PORTB2);
}

uint8_t readDS1307(uint8_t address, uint8_t* data)
{
    // send start bit
    TWCR = (1 << TWINT) | (1 << TWSTA) | (1 << TWEN);
    while (!(TWCR & (1 << TWINT)));
    if ((TWSR & 0xF8) != 0x08)
        return 1;

    // send SLA + W
    TWDR = 0b11010000;
    TWCR = (1 << TWINT) | (1 << TWEN);
    while (!(TWCR & (1 << TWINT)));
    if ((TWSR & 0xF8) != 0x18)
        return 2;

    // send register adress
    TWDR = address;
    TWCR = (1 << TWINT) | (1 << TWEN);
    while (!(TWCR & (1 << TWINT)));
    if ((TWSR & 0xF8) != 0x28)
        return 3;
    
    // repeat start
    TWCR = (1 << TWINT) | (1 << TWSTA) | (1 << TWEN);
    while (!(TWCR & (1 << TWINT)));
    if ((TWSR & 0xF8) != 0x10)
        return 4;

    // send SLA + R
    TWDR = 0b11010001;
    TWCR = (1 << TWINT) | (1 << TWEN);
    while (!(TWCR & (1 << TWINT)));
    if ((TWSR & 0xF8) != 0x40)
        return 5;

    // get data and return NACK
    TWCR = (1 << TWINT) | (1 << TWEN);
    while (!(TWCR & (1 << TWINT)));
    *data = TWDR;
    if ((TWSR & 0xF8) != 0x58)
        return 6;

    // send stop 
    TWCR = (1 << TWINT) | (1 << TWSTO) | (1 << TWEN);
    return 0;
}

int main()
{   
    DDRB |= (1 << PORTB2) | (1 << PORTB3) | (1 << PORTB5);
    PORTB |= (1 << PORTB2);
    SPCR |= (1 << SPE) | (1 << MSTR);

    // initialize TWI module 
    TWBR = 8;               // SCL 100KHz
    TWSR |= (1 << TWPS0);   // prescalar / 4
    TWCR |= (1 << TWEN);

    // set decode mode
    sendMax7219Command(0x09, 0xFF);
    sendMax7219Command(0x0B, 0x01);
    sendMax7219Command(0x0A, 0x07);
    sendMax7219Command(0x0C, 0x01);

    while (1)
    {
        uint8_t second = 0;
        uint8_t error = readDS1307(0x00, &second);
        sendMax7219Command(0x01, (second & 0x70) >> 4);
        sendMax7219Command(0x02, second & 0x0F);
        _delay_ms(500);
    }
}