I'm trying to develop a board that includes I2C and SPI devices that are unpowered for the majority of the time to save power. I found that for these particular devices (Honeywell HSC pressure sensors) you definitely can't have the I2C ones unpowered because you end up sort-of powering them through the I2C lines and getting a weird voltage of around 1V.
However, I have unpowered SPI microsd cards on the same bus as some other things (a RTC etc.) and it seems to work fine.
Both devices say that the peak voltage on any line is something like Vdd+0.3V, which if Vdd is 0V is 0.3V (e.g. for microsd). So my questions are:
- Do they really mean Vdd+0.3V even if Vdd is 0V?
- Is there any nice way to do this? I've found an I2C buffer chip (PCA9515) but it draws 3 mA all the time which isn't ideal - I'd have to turn that chip off too, which I'm not sure it will like. For SPI I found this 74LCX125 chip, which seems to sip power as long as the inputs are not floating, so very weak pull-downs might help there. It all seems a bit complicated though.