We're using a MinnowMax board (E3825 populated), and want to connect it to another processor/board/kit over an SPI connection. During our testing, we figured we'd take some power measurements to compare the power usage of USB versus the assumed lower-power SPI bus. We've got the board running a very stripped down Arch Linux distro, and are using the SPI port that is part of the Low-Speed-Serial block of the chip. We're running the SPI interface as fast as we can, ~12Mbps (there's another post out there about trying to achieve a higher data rate using DMA). When we run a simple program that continuously blasts data out of the SPI bus, we see an increased power usage of over 300mW. The SPI pins aren't connected to anything, so there should be no loading of the pins. Running "Top" shows that CPU utilization is very low, a couple percent. When we stop the program, the power consumption goes back down to normal.
We expected the power consumed by the SPI peripheral to be much lower, like 10's of mW. Anyone have any idea what's going on, what we might be doing wrong, or should this be expected?
One other interesting thing that's happening...if we don't touch the mouse or keyboard, and allow the graphics portion of the chip to go to sleep, and interact with the chip over the serial debug port, and start our SPI test in this configuration (graphics asleep), the power consumption of running SPI is even higher, over 600mW.
Thanks for any ideas!
Ted