Does anyone know if SPI typically draws more current than I2C? The reason I'm asking is that I'm playing with the LIS3DH accelerometer and driving it with an Arduino. When I2C is used, the current draw is about 30uA (300uV across a 10R shunt). This is achieved with SDO connected to VDD rather than GND. However, when SPI is used, the current draw is about 300uA (3mV across a 10R shunt).
Because I2C doesn't use SDO, it could be connected to VDD to reduce current draw. The same cannot be done when SPI is used since SDO is used – correct? I wonder if SDO somehow drains current.
Any help will be appreciated.
Thanks
Best Answer
First I want to consider I2C vs SPI in general terms.
I2C requires pull-up resistors to create the 3.3V or 5V idle state. When any device (either clock or data) pulls that line low, it will effectively put the pull-up resistor between VCC and GND. Depending on the value of pull-up you will see a much larger current draw than uA's.
E.g. 3.3V bus with 3.3k pull-ups is a 1mA current draw.
However it's not that simple: the clock won't be low for 100% of the time. Maybe 10% on a reasonably busy bus. In that case the RMS current is amplitude * sqrt(duty) [RMS formula for a pulse train] -> 1mA * sqrt(0.1) = 0.316mA. Also take into consideration there are 2 I2C lines. And worst of all; if you may be using a "lazy I2C software implementation"; watch out for bus collisions. E.g. a I2C slave that wants to use clock stretching, while the I2C master uses push-pull GPIO's to drive the bus.
SPI is much simpler because often there are no pull-ups. The only power consumption it has is dynamic consumption from switching transistors and I/O lines.
If you do use SPI with pull-ups you will also see a (slight) increase in power consumption. However also note that a SPI has different SPI modes, and may idle at a logic low level. In that case the pull-up will continuously consume extra current.
In your case, as we don't have a schematic on exactly which SDO pin you're pulling low, you are maybe pulling the I2C SDA pin low with the pull-ups still in place?