//********************* Accessing external I2C-EEPROM *********************
#include <stdbool.h>
#include <util/delay.h>
#include "sbit.h"

#define SSDA            PORT_B0
#define SSDA_in         PIN_B0
#define SSDA_oe         DDR_B0
#define SSCL            PORT_B1
#define SSCL_oe         DDR_B1


#define PAGE_SIZE       128

#define EEPROM_ADDR     0xA0                            // A2,A1,A0 = GND

#define i2c_delay()     _delay_us(2.5 / 2)              // 2.5us = 400kHz


#define ssda_lo()       SSDA_oe = 1
#define ssda_hi()       SSDA_oe = 0
#define sscl_lo()       SSCL = 0
#define sscl_hi()       SSCL = 1


static uint16_t si2c_rw(uint16_t val)                   // line state: SDA = x, SCL = 0
{
  for (uint8_t i = 9; i; i--)                           // data + ACK
  {
    if (val & 0x8000)
      ssda_hi();
    else
      ssda_lo();
    i2c_delay();
    sscl_hi();
    i2c_delay();
    val <<= 1;
    if (SSDA_in)
      val |= 0x80;
    sscl_lo();
  }
  return val;
}


static uint8_t si2c_r(uint8_t nack)                     // input NACK
{
  if (nack)
    nack = 0x80;                                        // set bit 7
  return si2c_rw(0xFF00 | nack) >> 8;                   // return data
}


static uint8_t si2c_w(uint8_t val)                      // input data
{
  return si2c_rw(0x00FF | (val << 8)) & 0xFF;           // return NACK
}


static uint8_t si2c_start_w(uint8_t val)                // line state: SDA = 1, SCL = x
{
  SSCL_oe = 1;
  SSDA = 0;
  SSDA_oe = 0;
  sscl_hi();
  i2c_delay();
  ssda_lo();
  i2c_delay();
  sscl_lo();
  return si2c_w( val );
}


static void si2c_stop(void)                             // line state: SDA = 1, SCL = 0
{
  ssda_lo();
  i2c_delay();
  sscl_hi();
  i2c_delay();
  ssda_hi();
}


static uint8_t set_addr(uint16_t addr)
{
  uint8_t i;

  for ( i = 255; i; i-- )
  {
    if ( !si2c_start_w( EEPROM_ADDR ))                  // 0 = ACK received
    {
      si2c_w( addr >> 8 );                              // address high byte
      si2c_w( addr );                                   // address low byte
      break;
    }
    _delay_us( 40 );                                    // timeout: 10.2ms
    si2c_stop();
  }
  return i;                                             // 0 = timeout
}


bool eeprom_wr( uint16_t eeaddr, uint8_t* sram, uint16_t len )
{
  do
  {
    if ( !set_addr( eeaddr ))
      return false;
    do
      si2c_w( *sram++ );
    while ( --len && (++eeaddr & (PAGE_SIZE - 1)));     // end or next page
    si2c_stop();
  }
  while ( len );                                        // end
  return true;
}


bool eeprom_rd( uint16_t eeaddr, uint8_t* sram, uint16_t len )
{
  if ( !set_addr( eeaddr ))
    return false;
  si2c_start_w( EEPROM_ADDR + 1 );                      // read mode
  do
    *sram++ = si2c_r( !--len );                         // NACK on last byte
  while ( len );
  si2c_stop();
  return true;
}