SPI / Pull-Down Resistors?

Other Hardware General Electronics
1

I have a project that involves putting the microcontroller to sleep while an SPI slave will still have power. It's a long story to explain why but it's essentially to save the space/cost/weight required to completely cut power to the slave because of the way the entire thing is powered. The slave draws an acceptably low amount of current in this configuration so everything is OK in that regard.

However, the sleep mode I need to put the micro in will cause the CLK/MOSI pins to float. I'm pretty sure this will end up causing the slave to read occasional noise as data which I can't allow to happen. My question is this: Can I just put a high value (10k?) pull-down resistor on SCK/MOSI to keep the lines from floating when the micro is asleep? I'm running SPI at 21MHz in Mode 0 so low is idle and signal shape is pretty important at that speed. I just don't know if this is commonly done or if I'm doing something that would be considered a crappy hack.

Any input would be appreciated! Thanks in advance.

EDIT: If this is bad and there's a better approach you can think of I'd love to hear it. This is just the best thing I've thought of so far. -?-

2

I see no problem with using 10k pull-down resistors in your case.

3

Pull up chip select - then SCK/MOSI/MISO won't matter.

I'm running SPI at 21MHz

You have a 42 MHz clock? Only way you'll get 21 MHz.

4

Thanks, I'm glad to hear my idea wasn't crazy as I don't have a lot of experience stepping outside of basic usages shown in datasheets.

Crossroads, the micro I'm using is a 168MHz STM32 ARM so the 21MHz SPI wasn't an error. I'm just more comfortable with this forum and figured my question was general enough for here. Sorry to disappoint if you were hoping I had some spectacular 8bit AVR over clocking skills. :slight_smile:

5

PS - the slave doesn't have a CS line (LPD8806) so it's a bit like the American NSA, always listening!