Powering i2c chain off of Vin?

Hey guys,

I’m wiring up a set of three I2C devices to a single imp I2C output. I’ve got the addressing sorted out and everything, but power has become a problem.

I’m currently powering all three of these devices, as well as an accelerometer off the 3v3 output of the imp. The I2C devices each have 4 7-segment displays though, for a total of around 80 LEDs. This is obviously more than the 3v3 out can supply, so after I add a second I2C I get noticeable dimming. I haven’t tried powering all three simultaneously yet.

Question is, since these devices are actually designed for 5v anyway, could I just power them off the Vin and Gnd pins next to the power header (april board)? Will this have any negative consequences for the SCL and SDA logic pins? I currently have those pins bridged to 3v3 with 1 10k ohm resistor each. Will I be okay bridging them to 5vin?

Each I2C device already has a couple of 10k ohm resistors from VCC to SDA and SCL. I’ve also got a couple of capacitors on across 3v3 to Gnd for power smoothing, which I would plan to move over between Vin and Gnd.

Yay or nay? If nay, how would I go about splitting off 5v usb power for these devices.

Thanks guys!

Really you should use a level shifter here to allow the I2C bus to be 5v on the device side and 3.3v on the imp side.

eg: https://www.sparkfun.com/products/12009 should work fine.

Oh cool, thanks I didn’t see those.

I’m totally shooting from the hip here, but I do have a handful of low dropout voltage regulators (these). I’m guessing they won’t operate in the reverse direction (3.3v->5v), but since the I2C devices seem happy with a 3.3v logic signal, is there a way to use those with some diodes to step down the 5v to 3.3v in one direction? Or is that crazy and I should just buy those logic level shifters?

I2C is a bidirectional bus, an LDO isn’t going to cut it here :slight_smile:

If you have any FETs with a Vgs < 3v then you can just follow the schematic of the level shifter though and build it yourself. One transistor with pullups on both sides (and you’ll have one set there already as it’s an open-drain bus)