AVR Freaks

So have the gyro read/write all working.  The chip boots in SLEEP mode by default (i2c comm only).  Once I programmed registers for that and tuned, I started getting output.

data from Xhi, Xlo, Yhi, Ylo, Zhi, Zlo gyro (started moving 1/2 way through this sequence:

 FF 43 FF EA 00 1F                                                              
 FF 43 FF ED 00 1F                                                              
 FF 43 FF EF 00 1F                                                              
 FF 41 FF F0 00 1E                                                              
 FF 45 FF EE 00 1D                                                              
 FF 4C FF C4 00 23                                                              
 FA B4 2C 96 07 78                                                              
 FA 50 D3 A3 F7 D7                                                              
 11 6D 1D 96 FC 2E                                                              
 F8 2C C4 04 FF D2                                                              
 E7 E3 F4 3B 07 56                                                              
 17 F0 0A A5 F2 E3                                                              
 F0 66 D9 E5 07 9A                                                              
                                                        

The final program:

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

#include "../uart.c"

void dump_data6(uint16_t addr) {
  uint8_t *p = (uint8_t*)addr;
  char buf[4];

  for (uint8_t i = 0; i < 6; i++) {
    cnvt_u8(p[i], buf);
    uart_putchar(' ');
    uart_putchar(buf[0]);
    uart_putchar(buf[1]);
  }
  uart_putchar('\r');
  uart_putchar('\n');
}

void dump_data(uint16_t addr) {
  dump_data16(addr + 0x00);
  uart_putchar('\r');
  uart_putchar('\n');
}

uint8_t stat[16];			/* status codes */
uint8_t rega[16];			/* register address */
uint8_t regv[16];			/* register value */

#define _TWINT _BV(TWINT)
#define _TWEA _BV(TWEA)
#define _TWSTA _BV(TWSTA)
#define _TWSTO _BV(TWSTO)
#define _TWEN _BV(TWEN)

uint8_t i2c_read_reg(uint8_t sla_, uint8_t reg) {
  uint8_t val;
  uint8_t i = 8;

  /* send START and wait */
  TWCR = _TWINT | _TWSTA | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send device address */
  TWDR = sla_ | 0;
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send register address */
  TWDR = reg;
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send START */
  TWCR = _TWINT | _TWSTA | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send device address again */
  TWDR = sla_ | 1;
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* recv register contents, send ACK  */
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  val = TWDR;
  stat[i++] = TWSR;
  
  /* send STOP */
  TWCR = _TWINT | _TWSTO | _TWEN;
  loop_until_bit_is_set(TWCR, TWSTO);

  return val;
}

void i2c_write_reg(uint8_t sla_, uint8_t reg, uint8_t val) {
  uint8_t i = 0;

  /* send START and wait */
  TWCR = _TWINT | _TWSTA | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send device address */
  TWDR = sla_ | 0;
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send register address */
  TWDR = reg;
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send register value */
  TWDR = val;
  TWCR = _TWINT | _TWEN;
  loop_until_bit_is_set(TWCR, TWINT);
  stat[i++] = TWSR;

  /* send STOP */
  TWCR = _TWINT | _TWSTO | _TWEN;
  loop_until_bit_is_set(TWCR, TWSTO);
}

void twi_init(void) {
  TWSR = 0;				/* prescalar of 1 */
  TWBR = 12;				/* to get 400 kHz bit rate */
}

#define SLA_ (0x68 << 1)		/* 7-bit IMU address */

int main(void) {
  uint8_t i, imu[12];
  
  uart_init();
  twi_init();

  DDRB |= _BV(DDB5);
  PORTB &= ~_BV(PORTB5);		/* LED off */
  _delay_ms(500);

  for (uint8_t i = 0; i < 16; i++) stat[i] = 0xff;
  for (uint8_t i = 0; i < 16; i++) rega[i] = 0xff;
  for (uint8_t i = 0; i < 16; i++) regv[i] = 0xff;

  rega[0] = 0x75;			/* whoami */
  rega[1] = 0x6B;			/* device control */
  rega[2] = 0x19;			/* sample rate divider */
  rega[3] = 0x1A;			/* config */
  rega[4] = 0x1B;			/* gyro config */
  
  PORTB |= _BV(PORTB5);			/* LED on */

  i2c_write_reg(SLA_, 0x6B, 0x00);	/* device control: wake it up */
  i2c_write_reg(SLA_, 0x19, 80);	/* sample rate => 100 Hz */
  i2c_write_reg(SLA_, 0x1A, 0x03);	/* FSYNC=0, 42Hz gyro, 44Hz accel */
  i2c_write_reg(SLA_, 0x1B, 0x08);	/* 27: gyro config */

  for (i = 0; i < 16; i++) {
    if (rega[i] != 0xff) regv[i] = i2c_read_reg(SLA_, rega[i]);
  }
  
  dump_data16((uint16_t) stat);
  dump_data16((uint16_t) rega);
  dump_data16((uint16_t) regv);

  for (;;) {
    PINB = _BV(PINB5);
    imu[0] = i2c_read_reg(SLA_, 0x43);
    imu[1] = i2c_read_reg(SLA_, 0x44);
    imu[2] = i2c_read_reg(SLA_, 0x45);
    imu[3] = i2c_read_reg(SLA_, 0x46);
    imu[4] = i2c_read_reg(SLA_, 0x47);
    imu[5] = i2c_read_reg(SLA_, 0x48);
    dump_data6((uint16_t)imu);
    _delay_ms(500);
  } /* LED blink */

  return 0;
}