added keyword argument to UARTDevice to set power on one of the power…

[ste7anste7an]

The UARTDevice iodevice is a nice generic way to communicate with external devices using plain uart communciation. When external devices have more power needs than the 3v3 line can deliver (e.g. when driving NeoPixels or Servo motors), it would be nice when such a device can use 8V power coming from one of the power lines P1 or P2.

The PUPDevice iodevice allows for setting power on either P1 or P2 depending how that is negotiatied in the PUP protocol. For I2CDevices, there is a keyword argument powered that allows for powering P1 (not P2). Unfortunately, I2Cdevice is not present for prime hub or technic hub, only for EV3 hub (and there the powered does work, but is not effective, as the P1 pin is connected through a 330Ohm resistor, and the voltage drops sharply when connecting a device that draws some current.)

Proposed solution

I propose to add a keyword argument power_pin to the UARTDevice init method where the argument can be 0 (no power), 1 (P1 powered) or 2 (P2 powered).

In a pybricks program that would look like:

uart = UARTDevice(Port.A, power_pin = 1)

resulting in P1 powered and

uart = UARTDevice(Port.A, power_pin = 0)

resulting in P1 nor P2 powered.

[laurensvalk]

The tricky part about allowing this is that we won't have a way to automatically turn off the power when a custom device is unplugged.

We could turn power off at the end of the program for safety reasons, but I imagine you'd want to keep it on between runs just like the Ultrasonic Sensor.

A few ideas:

• We could require some kind of keep-alive message to keep the power on. But this does require a background process on the hub too and some kind of task on the sensor side.
• We could add a safety prompt the first time the user applies this. Something like: Using custom electronics may damage your LEGO hub. Press 'y' to proceed or 'n' to cancel. We could make the user choice persist so we only ask the first time. This is simpler for both the hub and the custom sensor.

[laurensvalk]

I have pushed an alternative branch here to implement the second option discussed above.

The ready made firmware files can be download here.

The UARTDevice now takes a power_in argument.

I do not currently have the equipment to test this. @ste7anste7an, could you please test the following on all hubs?

Please confirm the polarity for 1 and 2. Please also test that it returns to off when you choose 0. It should ask for permission only the first time after you update the firmware. On EV3, only 1 should do something, 2 turns it off.

I think we can include this in the next release after full testing.

[ste7anste7an]

Thank you for looking a this pull request. The warning is a good option. I have a Spike prime, a Lego technic hub and an EV3 hub. I will try it for all these hubs and do some tests with polarity and switching external power on and off (will do within a few days).

[dlech]

• We could add a safety prompt the first time the user applies this. Something like: Using custom electronics may damage your LEGO hub. Press 'y' to proceed or 'n' to cancel. We could make the user choice persist so we only ask the first time. This is simpler for both the hub and the custom sensor.

People don't always have an interactive terminal, e.g. when running with pybricksdev, so I'm not sure implementing this on the hub itself would be the most robust option.

Maybe we could just add another argument for controlling if power stays on after the program exits/device is disconnected?

[laurensvalk]

It's a one-off safety confirmation that persists, so Pybricksdev users won't have to deal with this at runtime.

It's not really a convenience/technical issue, more of an awareness issue.

I think that the stock sensors are all protected against this, but other third-party sensors might not be.