Moteino current supply to components on board

[malik]

Hi Guys,


We are powering a multiplexer and 2 temperature and humidity sensors (Si7021) from an Moteino pin. Multiplex is needed because Si7021 have fixed I2C addresses.

Reason for powering these from a Moteino pin is to reduce power consumptions, since we can easily put the pin to low when not needed.  On the rest of the board we have a transistor which is driving a servo, both moteino and servo are powered from a single 5v power rail. Finally we have couple of leds and 2 limit switches in our design

Its all working fine, but is there a chance of a brown-out ? Since Moteino can only provide limited amount of current.

[perky]

Generally speaking powering devices from pins is only good if you're supplying very low currents. The pin may be specified at few 10s of mA, but that's to logic levels, the top FET in the I/O buffer has resistance that will cause a voltage drop. I'd always recommend a high side load switch instead if you want to maintain VCC within tolerance on your powered parts.

Mark.

[Uncle Buzz]

Shutting down power pin on some I2C component is useless since SDA and SCL are still powered by the microcontroller, so those chips still get power from I2C bus, it's better to put the chip in standby mode (if exists) via I2C command.