#include <avr/io.h>
#include <util/delay.h>

#ifndef BAUD
#define BAUD 9600
#endif

#include <util/setbaud.h>

#define DS1302_CLK        PD6 // arduino pin 6
#define DS1302_CLK_DDR    DDRD
#define DS1302_CLK_PORT   PORTD

#define DS1302_DIO        PD5 // arduino pin 5
#define DS1302_DIO_DDR    DDRD
#define DS1302_DIO_PORT   PORTD
#define DS1302_DIO_PIN    PIND

#define DS1302_CE         PD4 // arduino pin 4
#define DS1302_CE_DDR     DDRD
#define DS1302_CE_PORT    PORTD

#define DS1302_DELAY_USEC 3

void UART_Init ( void ) {
UBRR0H = UBRRH_VALUE ;
UBRR0L = UBRRL_VALUE ;
#if USE_2X
UCSR0A |= ( 1 << U2X0 ) ;
#else
UCSR0A &= ~ ( 1 << U2X0 ) ;
#endif

/* Enable UART transmitter/receiver */
UCSR0B = ( 1 << TXEN0 ) | ( 1 << RXEN0 ) ;
/* 8 data bits, 1 stop bit */
UCSR0C = ( 1 << UCSZ01 ) | ( 1 << UCSZ00 ) ;
}

void UART_TransmitByte ( uint8_t data ) {
/* Wait for empty transmit buffer */
loop_until_bit_is_set ( UCSR0A , UDRE0 ) ;
/* Send data */
UDR0 = data ;
}

uint8_t UART_ReceiveByte ( void ) {
/* Wait for incoming data */
loop_until_bit_is_set ( UCSR0A , RXC0 ) ;
/* Return register value */
return UDR0 ;
}

void UART_TransmitString ( const char * str ) {
while ( * str ) {
UART_TransmitByte ( ( uint8_t ) * str ) ;
str ++;
}
}

/* Converts 4 bits into hexadecimal */
char nibbleToHexCharacter ( uint8_t nibble ) {
if ( nibble < 10 ) {
return ( ‘0’ + nibble ) ;
} else {
return ( ‘A’ + nibble — 10 ) ;
}
}

/* Prints a byte as its hexadecimal equivalent */
void UART_TransmitByteHex ( uint8_t byte ) {
uint8_t nibble ;
nibble = ( byte & 0xF0 ) >> 4 ;
UART_TransmitByte ( nibbleToHexCharacter ( nibble ) ) ;
nibble = byte & 0x0F ;
UART_TransmitByte ( nibbleToHexCharacter ( nibble ) ) ;
}

void DS1302_Init ( void ) {
// CE — output, set low
DS1302_CE_DDR |= ( 1 << DS1302_CE ) ;
DS1302_CE_PORT &= ~ ( 1 << DS1302_CE ) ;

// CLK — output, set low
DS1302_CLK_DDR |= ( 1 << DS1302_CLK ) ;
DS1302_CLK_PORT &= ~ ( 1 << DS1302_CLK ) ;

// DIO — output, set low (for now)
DS1302_DIO_DDR |= ( 1 << DS1302_DIO ) ;
DS1302_DIO_PORT &= ~ ( 1 << DS1302_DIO ) ;
}

void DS1302_Select ( void ) {
// set CE high
DS1302_CE_PORT |= ( 1 << DS1302_CE ) ;
}

void DS1302_Deselect ( void ) {
// set CE low
DS1302_CE_PORT &= ~ ( 1 << DS1302_CE ) ;
}

void DS1302_TransmitByte ( uint8_t byte ) {
// DIO — output, set low
DS1302_DIO_DDR |= ( 1 << DS1302_DIO ) ;
DS1302_DIO_PORT &= ~ ( 1 << DS1302_DIO ) ;

// transmit byte, lsb-first
for ( uint8_t i = 0 ; i < 8 ; i ++ ) {
if ( ( byte >> i ) & 0x01 ) {
// set high
DS1302_DIO_PORT |= ( 1 << DS1302_DIO ) ;
} else {
// set low
DS1302_DIO_PORT &= ~ ( 1 << DS1302_DIO ) ;
}

// send CLK signal
DS1302_CLK_PORT |= ( 1 << DS1302_CLK ) ;
_delay_us ( DS1302_DELAY_USEC ) ;
DS1302_CLK_PORT &= ~ ( 1 << DS1302_CLK ) ;
}
}

uint8_t DS1302_ReceiveByte ( void ) {
// DIO — input
DS1302_DIO_DDR &= ~ ( 1 << DS1302_DIO ) ;

// NB: receive is always done after transmit, thus
// falling edge of CLK signal was already sent
// see «Figure 4. Data Transfer Summary» for more details

// receive byte, lsb-first
uint8_t byte = 0 ;
for ( uint8_t i = 0 ; i < 8 ; i ++ ) {
if ( DS1302_DIO_PIN & ( 1 << DS1302_DIO ) ) {
byte |= ( 1 << i ) ;
}

// send CLK signal
DS1302_CLK_PORT |= ( 1 << DS1302_CLK ) ;
_delay_us ( DS1302_DELAY_USEC ) ;
DS1302_CLK_PORT &= ~ ( 1 << DS1302_CLK ) ;
}

return byte ;
}

void set_time ( ) {
// see «Table 3. Register Address/Definition»
const uint8_t bytes [ 8 ] =
// sec   min  hour   day   mon  dow(1-7) year  wp (in BCD!)
{ 0x00 , 0x11 , 0x21 , 0x26 , 0x04 , 0x04 , 0x18 0x00 } ;

DS1302_Select ( ) ;
// 0xBE = clock burst write
DS1302_TransmitByte ( 0xBE ) ;
for ( uint8_t i = 0 ; i < sizeof ( bytes ) ; i ++ ) {
DS1302_TransmitByte ( bytes [ i ] ) ;
}
DS1302_Deselect ( ) ;
}

int main ( int argc , char * argv [ ] ) {
UART_Init ( ) ;
DS1302_Init ( ) ;

// set current time
// set_time();

for ( ;; ) {
_delay_ms ( 1000 ) ;

// read current time
uint8_t bytes [ 8 ] ;

DS1302_Select ( ) ;
// 0xBF = clock burst read
DS1302_TransmitByte ( 0xBF ) ;
for ( uint8_t i = 0 ; i < sizeof ( bytes ) ; i ++ ) {
bytes [ i ] = DS1302_ReceiveByte ( ) ;
}
DS1302_Deselect ( ) ;

// send the time over UART
UART_TransmitString ( «Time = » ) ;
for ( uint8_t i = 0 ; i < sizeof ( bytes ) ; i ++ ) {
UART_TransmitByteHex ( bytes [ i ] ) ;
}
UART_TransmitString ( » r n » ) ;
}
}