51模拟IIC问题，能写不能读

/**<

 * \fn i2c_write_byte(c)

 * \brief Writes a byte to device.

 * \param c data

 */

void i2c_write_byte(uint8 c)

{

  uint8 i;



  for (i = 0; i < 8; i++) {

    set_sda(c & 0x80);

    set_scl_high();

    delay_4us();

    c <<= 1;

    set_scl_low();

    delay_4us();

  }

}



/**<

 * \fn i2c_read_byte()

 * \brief Reads a byte from device.

 * \return Returned data.

 */

uint8 i2c_read_byte(void)

{

  uint8 i, c = 0;



  for (i = 0; i < 8; i++) {

    set_scl_high();

    delay_4us();

    c <<= 1;

    c |= get_sda();

    set_scl_low();

    delay_4us();

  }



  return c;

}



uint8 pcf8574_write_port(uint8 slave, uint8 bits)

{

  uint8 ret = 1;



  /* START condition.

   */

  set_sda_high();

  set_scl_high();

  delay_4us();

  set_sda_low();

  delay_4us();

  set_scl_low();



  delay_xms(5);



  ///< SLAVE address.

  i2c_write_byte(0x40 | (slave & 0x07) << 1);



  /**< ACK condition.

   */

  set_sda_high();

  delay_4us();

  set_scl_high();

  delay_4us();

  ACC = get_sda();

  set_scl_low();

  delay_4us();

  if (ACC)

    goto __i2c_write_finish;



  delay_xms(5);



  i2c_write_byte(bits);

  /**< ACK condition.

   */

  set_sda_high();

  delay_4us();

  set_scl_high();

  delay_4us();

  ACC = get_sda();

  set_scl_low();

  delay_4us();

  if (ACC)

    goto __i2c_write_finish;



  ret = 0;



__i2c_write_finish:

  delay_xms(5);



  /**< STOP condition.

   */

  set_sda_low();

  delay_4us();

  set_scl_high();

  delay_4us();

  set_sda_high();



  delay_xms(30);



  return ret;

}



uint8 pcf8574_read_port(uint8 slave, uint8 *out)

{

  uint8 ret = 1;



  /**< START condition.

   */

  set_sda_high();

  set_scl_high();

  delay_4us();

  set_sda_low();

  delay_4us();

  set_scl_low();



  delay_xms(5);



  ///< SLAVE address.

  i2c_write_byte(0x41 | (slave & 0x07) << 1);



  /**< ACK condition.

   */

  set_sda_high();

  delay_4us();

  set_scl_high();

  delay_4us();

  ACC = get_sda();

  set_scl_low();

  delay_4us();

  if (ACC)

    goto __i2c_read_finish;



  delay_xms(5);



  *out = i2c_read_byte();



  /**< NAK condition.

   */

  set_sda_high();

  set_scl_high();

  delay_4us();

  set_scl_low();

  set_sda_low();

  delay_4us();



  ret = 0;



__i2c_read_finish:

  delay_xms(5);



  /**< STOP condition.

   */

  set_sda_low();

  delay_4us();

  set_scl_high();

  delay_4us();

  set_sda_high();



  delay_xms(30);



  return ret;

}