Setting correct batch size

[Mahdi-Hosseinali]

Hi,
I want to read 4 variables (acc, linAcc, gyro, and game rotation vector(GRV)) from BNO080 but having some issues.

The initialization process is

1. A soft reset
2. Request the product ID and reset info
3. Flush out channel 0 and read product ID on channel 2
4. Flush data on channel 2 by multiple readings
5. Setting up sensors using setFeatureCommand and following registers:
   Feature flags = 0, sensivity=0, report interval=batch interval=50ms, sensor specific=0
6. Enabling dynamic calibration

Every 50ms, the MCU reads the imu sensor reports over the i2c until the data length is zero. Based on the reference manual pp80, all the data are reported in batch format so the expectation is to receive a single sensor input report every that has a size of 4(header)+3x9(acc, linAcc, gyro)+11(GRV)=42 bits and any further call from MCU should receive 0 data length until next 50ms.

However, there is always three reports with a total of 243 to 267 data bits. Usually the first and the last reports are less than 42, and the second one is around 170 to 200 bits long. Looking at the data, the second report has multiple readings of each sensor which puts extra load on the MCU and often produces very unrealistic values (such as 50radians/s in gyro while it's sitting on the desk).

Things I have tried:

1. Setting the batch interval to zero increases the total size of bits to the range of 223 to 300 bits with up to 8 reports.
2. Increasing the batch interval to 100ms causes larger report bits (up to 307 bits).
3. Setting feature flag to 8 (keeps the IMU on) does not change anything.

I would appreciate it if you could help me figure out what is wrong with my setting.

[kay-hc]

Hi Mahdi,

The report rates are dependent on the discrete sample rates (please refer to the BNO08X datasheet Section 6.9). When you request reports at 50ms, acc runs at 31.25Hz, gyro, linAcc and GRV will run at 50Hz. Best way to read these reports is to use H_INT (report is available) and read the data until the length is zeros or polling at higher rates.

[Mahdi-Hosseinali]

Dear Kay,
Thanks for the quick reply.

1. Is there anyway to calculate the discrete sampling rate of the sensors to come up with the best request rate?
2. What is the correct way to calculate the batch delay?
3. Even with the rates you have mentioned, I should get 2 report for gyro, linAcc, and GRV and 1 to 2 reports for the acc every 50ms, so the total bits should be 80 including the header, and in one report. This doesn't explain the above 200 bits and multiple reports.

UPDATE
The reason I was getting very high number of bits was that I was reading it in debug mode so the MCU was running slower but not the IMU, hence the accumulation of data. After logging the counts it looks much more reasonable, however very occasionally I get no data for gyro, linAcc, or GRV which causes a problem as I'm reading multiple values and dividing them by the count. Also still interested in the answers of questions 1 and 2.