BLE Latency between App and Sensor

[Anonymous]

Hi There,

   

I have a BLE keypad that measures the user's response speed (in milliseconds) from a START command that it receives from the App.

   

Obviously the BLE device is very accurate and we have no problem achieving 1 ms accuracy.

   

However, the START command that comes from the App via the BLE cause an unknown latency which we are trying to address somehow as we aim to build a system that is accurate down to the millisecond.

   

Any help will be greatly appreciated.

   

Thanks,

   

Moe 

[HeLi_263931 · Accepted solution]

You can try to measure round-trip times by having the BLE device answering that start commando, and measure the time it takes to do so (maybe doing it multiple times).

[Anonymous]

If we assume that the round time travel is equally divided between the app and the device, we should be fine but one side may cause higher latency than the other which I will never know and if altogether we have 20ms for a round trip that is split 70-30 between the devices, I won't be able to achieve 1ms accuracy.

[HeLi_263931]

Well BLE is not designed for low-latency applications, but for low-power.

   

You can quite reliably measure latency when you use multiple packets, send from both sides, with time-stamps in them (so each side knows when the other side did send a packet, and received another one). I think NTP does something similar since it can calculate the latency between the NTP server and the client.

   

But it gets complicated when the delay is not constant, though.