VC31B PPG sample rate

[espruino-discuss2]

Posted at 2025-02-02 by fishhf

I want to retrieve about 150hz of ppg data, but even changing the registers doesn't seem to help much for the vc31b sensor.

The interval setting on the JavaScript side kinda works when I'm just recording 15s (it changes the sampling rate, somewhere up to 80hz), once I try like 2 minutes, I get somewhere like 60hz.

Is there a manual for vc31b that includes info for the i2c registers stuff? I can try to fix it :)

[espruino-discuss2]

Posted at 2025-02-03 by @gfwilliams

I'm afraid the VC31B really isn't that well documented - pretty much all we have is what you see here: https://github.com/espruino/Espruino/blob/master/libs/misc/hrm_vc31.c#L48-L77

However as I recall the max sample rate is determined by the time it takes the sensor to read - it's like it's got a 'shutter speed'. You may have some luck if you set hrmGreenAdjust to false in Bangle.setOptions then manually set the registers to use a higher power for the green LED? Also maybe stopping using the env sensor.

There are some 'sensitivity' registers but I think those only apply to the env sensor, not the actual green PPG one.

Have you checked out https://github.com/espruino/BangleApps/blob/master/apps/flashcount/app.js which uses the PPG at a high speed? I can't remember how fast that was but I think it was over 60Hz

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Posted at 2025-02-04 by fishhf

Yeah, I looked through the firmware side and the javascript side and that flash count app.
I've sent an email to Vcare in Chinese, if I'm lucky I might get a more detailed document.

I didn't try the green adjust and power settings, let me try playing with it. Thanks!