LSM6DSO not reading 1G at rest

[Diogo_Goto]

I  am writing my own library in C++ to operate the LSM6DSO and when the sensor is at rest it reads 0.5 G on the z-axis, I've tried the code with 2 ICs, one in an Adafruit breakout board, and another one in an old PCB that had the sensor soldered one and both got the same error, so I believe that the issue might be in the code, but I can't identify where is the problem. Btw, I am using the dev kit NUH7AZIQ$ATZ3A140012 (STM32H7A3ZIT6Q). Also, when i turn the sensor 90 deg the 0.5g moves to the other axis, and regardless of the range scale I choose, the readings adjust themselves so that it still gives the 0.5 G after applying the conversion factor .

The first line shows the reading from the CTRL1_XL register and breaks it down into the configurations in that register (which in this picture is set up to +-2G range and sampling of Hz)
the second lines show the reading from the OUTZ_L_A (2Ch) and OUTZ_H_A (2Dh) registers, then it combines that into the raw data and multiply by the sensitivity factor suggested in the data sheet for each range ( in this case 0.0061) 

Small programs that plots the lsm6dso outputs, as I turned the imu, the 0.5 g readings moved to the other axis

Here is the part of the code, in this section I its not exactly the library i am writing but it has follows the same procedures and gets the same numbers.

[Federica Bossi]

Hi @Diogo_Goto ,

Is it possible you are using a wrong sensitivity for your FS? Can you check it?

In addition, can you try to implement our official drivers and let me know if you still have the problem? 

Thanks.

[Diogo_Goto]

Hello Federica Bossi,

Thanks for the reply.

---

@Federica Bossi wrote:

Is it possible you are using a wrong sensitivity for your FS? Can you check it?

---

If you look at the first picture on the original post i am reading the data directly from the registers. I am reading CTRL1_XL, which contains the FS. On that picture is set to 0x0 so the conversion factor is 0.061 and if you multiply that number  by the value read by the 2 registers for the z axis you still get 0.5G. I also tried different FS ranges, the number changes but after multiplying everything I still get the 0.5 G.

---

In addition, can you try to implement our official drivers and let me know if you still have the problem? 

---

Yeah, I tried that as well the code is below, i think the setup is correct, at least i got no error in initialization and it was still reading 0.5 G.

 

 

[Andrea VITALI]

A quick look at the screenshot of your C code seems to point to a data conversion problem: you read two uint8 and then make up a uint16 value out of it. But the accelerometer output is a SIGNED int16.

Have a look at how lsm6dso_acceleration_raw_get() is implemented in the official C driver on GitHub: 

lsm6dso-pid/lsm6dso_reg.c at 7cd1aae6017ca386b09f1814994e085bf6519344 · STMicroelectronics/lsm6dso-pid · GitHub

int32_t lsm6dso_acceleration_raw_get(const stmdev_ctx_t *ctx, int16_t *val)
{
uint8_t buff[6];
int32_t ret;

ret = lsm6dso_read_reg(ctx, LSM6DSO_OUTX_L_A, buff, 6);
val[0] = (int16_t)buff[1];
val[0] = (val[0] * 256) + (int16_t)buff[0];
val[1] = (int16_t)buff[3];
val[1] = (val[1] * 256) + (int16_t)buff[2];
val[2] = (int16_t)buff[5];
val[2] = (val[2] * 256) + (int16_t)buff[4];

return ret;
}

One could also use a C union to make the conversion faster but the code would be less portable as one would have to take into account the endianness of the memory representation (big-endian or little-endian)