Sampling a BMI160 sensor at 1600hz

[Ranajyoti Chakraborti]

Hi
I am using a nrf52832 board with a BMI160 sensor. I am trying to sample Accel and Gyro at 1600hz using a timer. The timer timeout is set at 20 ticks (32768/1600) @32768 hz (which is the timer resolution of NRF52832. I am not getting anywhere near 1600hz. Is there a better way of doing this?

[Jørgen Holmefjord]

Hi,

• How you are interfacing the device, and what you are doing in the timer handler?
• Are you using SPI or TWI bus?
• What rate are you able to sample the sensor at?

Best regards,
Jørgen

[Jørgen Holmefjord]

How do you measure the sample rate? Sending notifications on BLE at a rate of 1600 Hz might not work, and will heavily depend on your BLE configurations.

Please try removing the part where you send the data over BLE, and toggle a GPIO to measure the rate you are able to read data over SPI. 

[Ranajyoti Chakraborti]

It would be very kind of you if you can elaborate on the BLE configuration you mentioned. One of the things I was planning on trying is to store the data in the program memory or in a flash memory and then transmit from there. But that too seems to be tough as the limit of BLE transmission seems to be 20 bytes and from whatever I could gather from my logging that the Softdevice internal buffer is holding around 400 bytes of data. To answer you question about measurement currently I have a .Net service running on a Windows 10 machine which receives the data and logs them. The BMI160 data includes a tick count of the BMI160 internal clock. The differential of consecutive tick count and the number of data received per second is what I am relying on to calculate the transmission speed.

[Jørgen Holmefjord]

Are you checking error codes when calling sd_ble_gatts_hvx? Could be that the buffer fills up and when you try to send new data you will get an error code. If you do not handle this, you might lose samples.

You should definitely buffer samples on the MCU and transmitt a larger packet over BLE. Our softdevices since s132 v3.0.0 support Data Length Extension (DLE), with packets up to 251 bytes. Since s132 v5.0.0, there is also support for 2 Mbit PHY in BLE for faster transfers. Please have a look at the ATT_MTU Throughput Example in the SDK.

[awneil]

Jørgen Holmefjord said:

You should definitely buffer samples on the MCU

Note that the sensor has a FIFO - are you making use of that?

[Ranajyoti Chakraborti]

Jorgen, Thanks for your your response. I am getting an error code of BLE_ERROR_NO_TX_PACKETS from time to time. That is actually causing the delay which I was able to debug today. I will try out both ouf your suggestions and let you know.