UART(1) and current leakage observation

[JimTal001]

I'm using the AdaFruit USB to TTL Serial Cable connected from PC to pyboard (Tx and Rx lines, no power line).

I've noted that when using UART(1) the red LED of the pyboard glows when the pyboard is completely un-powered (current leakage). This is not the case when I use UART(4). I assume this has something to do with the fact that UART(1) also provides I2C capability.

Is this a possible issue or can I ignore it?

[dhylands]

It's quite common for there to be leakage current like what you described.

The reason is that the Tx pin from the USB-to-serial converter idles at a logic 1 (3.3v or 5v depending on the exact converter you're using) and this voltage can be enough to power portions of the pyboard.

Preventing it could be done by using a MOSFET on the Tx line and have the MOSFET powered by the pyboard supply voltage.

[JimTal001]

Thanks Dave,

I assume there is no reason to be concerned, i.e. no potential damage can occur due to this leakage correct?

[Damien]

The current will be flowing from the PC through the tx line, through the 4k7 I2C pull up resistor, and then into the 3v3 tracks on the pyboard. This is probably weakly turning on the MCU which then powers the red led. The MCU won't actually be running because there is not enough current (voltage drop is too high from the 4k7).

I think it's safe to do this but not 100% sure...

Also, I assume you are connecting ground along with tx and rx.

[JimTal001]

right, I'm connecting ground also.