Bluetooth wireless connection data fluctuates

[xixicarry]

Hi John, when I use (HC-SR04), I found that the data obtained by HC-SR04 is stable with wired connection, when I use Bluetooth wireless connection, the data obtained by HC-SR04 fluctuates, which affects the results of the experiments

test video

[xixicarry]

I found that the data fluctuation is even more dramatic for devices with I2C added, such as the 4-way line sensor.

[John Maloney]

Thank you for reporting this.

It sounds as though Bluetooth is stealing enough cycles to make the time measurements of the HC-SR04 unreliable. That is an unexpected downside of using BLE!

We may need a low-level timing function to ensure timing accuracy for things like the HC-SR04 and IR remote features. It's a tricky problem but I will give it some thought.

[John Maloney]

I've investigated the problem. On the micro:bit, running BLE adds a tiny bit of timing jitter, up to about 60 microseconds. That does not introduce significant noise into the HC-SR04 distance measurements.

Unfortunately, things are much worse on the ESP32. I observed timing glitches of up to 2500 microseconds, which would translate to many cm of error in HC-SR04 measurements.

We'll need to either find some way to reduce the ESP32 timing jitter while using BLE or, as I said, use a low-level, interrupt driven timer to get an accurate time measurement of the HC-SR04 echo return pulse. I will work on that.

Meanwhile, your best bet to disconnect from BLE when using the HC-SR04 sensor.