(BLE or BT5) Measure latency between device (PI) and nrf52840

[Kapteni]

Due to the covid-19 virus, my school project changed a lot. The new project is the following: "Study the latency between sending and receiving data from a device generating random CanBus data (e.g. Android phone) and a PC (with the Nordic nRF52840 development board) receiving the data"
This should be send over bluetooth 5 or BLE

Now, I'm planning to use a raspberry PI for generating and sending the data. But what is the best way to measure the latency?

I was thinking of using the nRF to send a request for candata to the PI and save the timestamp when sending the request. The PI will answer with candata and when I receive the candata on the nordicchip, I save another timestamp. Then I divide the timestamp by 2, so I ave the latency of sending data. Is this a good way to measure the latency? Or is there a better way to do this?

Thanks in advance for your help!

[Marjeris Romero]

Hi,

For BLE the minimum connection interval is specified by the Bluetooth Core Specification to be 7.5 ms, so for BLE the latency would be minimum 7.5 ms if you send one packet per connection interval. Please take a look at this old Devzone case for more information: https://devzone.nordicsemi.com/f/nordic-q-a/24013/minimum-connection-interval?ReplySortBy=CreatedDate&ReplySortOrder=Ascending

The radio in the nRF5 series chips can achieve much lower latency than 7.5ms, but you will need to use the radio in a propiertary protocol.

For your question about measurement latency, I think it will depend on how you define latency. I.e. are you measuring to/from application layer, transport-layer, PHY layer, etc.
When using timestamps you are also adding the time it takes to process the packet into the latency calculation, and the timestamps itself could also add some overhead to the calculation.
Another way to measure latency (on the PHY layer) will be to use a sniffer to track when the send and response packets are sent.

Best regards,

Marjeris