Acutally it is the F_READ bit. From the reference manual:
5.2 8-bit or 14-bit Data
The measured acceleration data is stored in the OUT_X_MSB, OUT_X_LSB, OUT_Y_MSB, OUT_Y_LSB, OUT_Z_MSB, and
OUT_Z_LSB registers as 2’s complement 14-bit numbers. The most significant 8-bits of each axis are stored in OUT_X (Y,
Z)_MSB, so applications needing only 8-bit results can use these 3 registers and ignore OUT_X,Y, Z_LSB. To do this, the
F_READ bit in CTRL_REG1 must be set. When the F_READ bit is cleared, the fast read mode is disabled.
The F_READ bit is in CTRL_REG1:
So with the F_READ bit set, reading continuously from the memory map of the accelerometer will skip every second byte for the X, Y, Z values.
With that bit cleared, I can read the 16bit registers.
Below is my code how I write that register:
/*
** ===================================================================
** Method : MMA1_SetFastMode (component MMA8451Q)
** Description :
** Turns the F_READ (Fast Read Mode) on or off
** Parameters :
** NAME - DESCRIPTION
** on - if to turn the F_READ mode on or off
** Returns :
** --- - Error code
** ===================================================================
*/
byte MMA1_SetFastMode(bool on)
{
uint8_t val, res;
res = GI2C1_ReadByteAddress8(MMA8451_I2C_ADDR, MMA8451_CTRL_REG_1, &val);
if (res!=ERR_OK) {
return res;
}
if (on) {
val |= MMA8451_F_READ_BIT_MASK; /* enable F_READ: Fast read mode, data format limited to single byte (auto increment counter will skip LSB) */
} else {
val &= ~MMA8451_F_READ_BIT_MASK; /* disable F_READ: Fast read mode, data format limited to single byte (auto increment counter will skip LSB) */
}
return GI2C1_WriteByteAddress8(MMA8451_I2C_ADDR, MMA8451_CTRL_REG_1, val);
}
I hope this helps,
Erich
