Arduino fails to start fromUSB if 5V is applied to data pin beforehand - normal?

Other Hardware General Electronics
1

Hi,
if Arduino has +5V one one of it's data pins that is set to become input, Arduino fails to be recognized as USB device, yet everything is fine if +5V is sent after connecting to USB. Is this normal or should I look for mistakes?

Background:
I'm trying to control a servo driver, it has a separate power supply connected. As a bit of surprise (no manual) came that it has "inverted logic", having +5V on input, needed to be pulled down to generate a step.
Arduino (Nano) needs to communicate with computer, so it's powered from USB. Arduino and stepper driver PSU grounds are connected. There's 150ohm resistor between Arduino data pin and servo driver clock pulse pin.

  • If driver psu is powered first, and then Arduino is plugged into USB, win says - "USB device not recognized".
  • If Arduino is connected first then the driver is powered - everything seems to work fine.
2

First off , you should not be applying voltage on I/O pins without the Vcc pins having power applied - that violates the Electrical Spec for the '328 of applied voltage being no more than Vcc + 0.5V.
Second, with power applied to an IO pin like that, voltage is likely to 'leak' around internally and may leave the '328 in an undefined state - and perhaps an inability to start up from a power off state as expected.

3

Sounds logical :slight_smile:
Thanks!

4

This is known technically as "latch up" and happens with a lot of devices. It is a problem when you are trying to make a device have a low power standby mode.

5

hm, useful to know. Thanks.