{"id":1520,"date":"2019-04-11T14:24:54","date_gmt":"2019-04-11T13:24:54","guid":{"rendered":"https:\/\/ofalcao.pt\/blog\/?p=1520"},"modified":"2019-04-12T11:46:09","modified_gmt":"2019-04-12T10:46:09","slug":"remote-ultrasonic-sensor-for-ev3-with-microbit","status":"publish","type":"post","link":"https:\/\/ofalcao.pt\/blog\/2019\/remote-ultrasonic-sensor-for-ev3-with-microbit","title":{"rendered":"Remote ultrasonic sensor for EV3 with micro:bit"},"content":{"rendered":"
This post is part 1 of 1 of \u00a0EV3 and micro:bit<\/a><\/div>\n

Been testing the micro:bit BLE Uart service. It’s a service that uses Nordic chipset features to deliver a simple TX\/RX mechanism so we talk with the micro:bit without having to create our own BLE services.<\/p>\n\n\n\n

Someone offered me a ultrasonic sensor this week. It’s a HC-SR04<\/a>, very common with Arduino projects but can also be used with the micro:bit<\/a>, there’s even a “sonar” extension available in the Makecode editor.<\/p>\n\n\n\n

So I added the BLE Uart service and started writing the distance values to the TX characteristic every second:<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

Nordic nRF Connect App reads them fine but gatttool doesn’t… except when in Interactive mode. Strange!<\/p>\n\n\n\n

After a while, I found out a note<\/a> that explains that the way the TX characteristic works forces us to keep the connection or reset the micro:bit regularly. That’s why Interactive mode works (it keeps the connection). So I have to forget gatttool for a while and use something better like python.<\/p>\n\n\n\n

On Ubuntu, my scripts worked fine. Latest version of pygatt (4.0.3) allow me to subscribe to the TX Characterist (in “Indication” mode only – almost the same as notifications but with an extra acknowledge step) and I was getting distance values on my laptop through BLE.<\/p>\n\n\n\n

But on the EV3 the scripts failed because “No characteristic found matching”. Why? I’m using same version of pygatt and don’t think that kernel version make much difference now that pybluez has settled.<\/p>\n\n\n\n

I was almost giving up when something worked: decided to use the python exception mechanism to retry several times the subscribing step. <\/p>\n\n\n\n

So instead of just<\/p>\n\n\n\n

device.subscribe(\"6e400002-b5a3-f393-e0a9-e50e24dcca9e\", callback=handle_data, indication=True)<\/code><\/pre>\n\n\n\n
I included the subscribe command in a loop that only ends when the command completes:<\/p>\n\n\n\n
while True:\n    try:\n        device.subscribe(\"6e400002-b5a3-f393-e0a9-e50e24dcca9e\", callback=handle_data, indication=True)\n        break\n    except pygatt.exceptions.BLEError:\n        print(\"unknown characteristic\")<\/code><\/pre>\n\n\n\n
And it works?! Usually just retrying once:<\/p>\n\n\n\n
Connecting\u2026
 Connected
 unknown characteristic
 Dist = 1.19 m
 Dist = 1.19 m
 Dist = 1.18 m<\/pre>\n\n\n\n
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