AVR Freaks

/* Two-Wire Interface */
typedef struct TWI_struct
{
    register8_t CTRLA;  /* Control A */
    register8_t reserved_0x01;
    register8_t DBGCTRL;  /* Debug Control Register */
    register8_t MCTRLA;  /* Master Control A */
    register8_t MCTRLB;  /* Master Control B */
    register8_t MSTATUS;  /* Master Status */
    register8_t MBAUD;  /* Master Baurd Rate Control */
    register8_t MADDR;  /* Master Address */
    register8_t MDATA;  /* Master Data */
    register8_t SCTRLA;  /* Slave Control A */
    register8_t SCTRLB;  /* Slave Control B */
    register8_t SSTATUS;  /* Slave Status */
    register8_t SADDR;  /* Slave Address */
    register8_t SDATA;  /* Slave Data */
    register8_t SADDRMASK;  /* Slave Address Mask */
    register8_t reserved_0x0F;
} TWI_t;

The avr-libc example code has:

C:\Program Files (x86)\Atmel\Studio\7.0\toolchain\avr8\avr8-gnu-toolchain\doc\avr-libc\examples\twitest>grep "TW.. " twitest.c
 * register to have valid contents while the TWINT bit in TWCR is set.
  /* initialize TWI clock: 100 kHz clock, TWPS = 0 => prescaler = 1 */
  TWSR = 0;
  TWBR = 10;                    /* smallest TWBR value, see note [5] */
  TWBR = (F_CPU / 100000UL - 16) / 2;
  TWCR = _BV(TWINT) | _BV(TWSTA) | _BV(TWEN); /* send start condition */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = sla | TW_WRITE;
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = (eeaddr >> 8);         /* 16-bit word address device, send high 8 bits of addr */
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = eeaddr;                /* low 8 bits of addr */
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWCR = _BV(TWINT) | _BV(TWSTA) | _BV(TWEN); /* send (rep.) start condition */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = sla | TW_READ;
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
      TWCR = twcr;              /* clear int to start transmission */
      while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWCR = _BV(TWINT) | _BV(TWSTO) | _BV(TWEN); /* send stop condition */
  TWCR = _BV(TWINT) | _BV(TWSTA) | _BV(TWEN); /* send start condition */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = sla | TW_WRITE;
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = (eeaddr>>8);           /* 16 bit word address device, send high 8 bits of addr */
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWDR = eeaddr;                /* low 8 bits of addr */
  TWCR = _BV(TWINT) | _BV(TWEN); /* clear interrupt to start transmission */
  while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
      TWDR = *buf++;
      TWCR = _BV(TWINT) | _BV(TWEN); /* start transmission */
      while ((TWCR & _BV(TWINT)) == 0) ; /* wait for transmission */
  TWCR = _BV(TWINT) | _BV(TWSTO) | _BV(TWEN); /* send stop condition */

To quote Sesame Street "one of these things is not like the other one" ;-)

EDIT working on the assumption that in an AVR-0/AVR-1 there's got to be a strong chance that TWIn.MCTRLA is going to be accessed at some stage then a google for:

avr twi "MCTRLA"

seems quite fruitful (mainly back here at Freaks in fact)

Serial comms issues asked here usually relates to improper baud rate due to not knowing what the cpu clock rate really is set to.

Where in your code above do you set the baud rate and what is the real cpu clock rate on your hardware.

Also one more tip, if you send the letter "U" repeatedly, it produces a square wave at the actual baud rate freq, easy to measure with scope or frequency counter.

Compare this value with what you expect to see and note any difference.

Jim

Why are you using 250kHz as the clock speed in one place and 16MHz in another?

The idea would be that you define one system wide symbol somewhere (often on the command line with a -D) and then use that symbol (traditionally most use "F_CPU") to make all other temporal calculations. That way is you one day switch from a 16MHz crystal to a 1.8432MHz crystal you just change that one define, in one place and all the calculations are re-done for the new speed.