I believe the only problem with a floating input is increased current draw
by the chip. Otherwise, it should do no harm. Depending on how often the
chip selects for the other chips go true, the MISO line may be pulled high
or low often enough to not have time to float to some mid-voltage. If the
chips are selected rarely, a high resistance pull-up or pull-down on MISO
would slightly reduce current draw when the chips are deselected.

Harold

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> I have a circuit that uses software implemented SPI to control two
> devices. Both use CS pins to select between the devices. The circuit
> (my light recorder artwork) is supposed to be low power, IE uA, so
> 99.99% of the time CS is high and the chips turn off and go into
> high-impedance mode.
>
> This leaving my SPI input pin floating isn't it? I already installed a
> simple 100k pull down resistor to prevent this on two of the devices,
> before that my multimeter definetely seemed to show the pin
> floating as you would expect.
>
> However... I've already made one, and it's running and well embedded
> behind a good half inch of hermetically sealed acrylic and is definetely
> not coming out. The pcb was well cleaned and the atmosphere is dry, so
> anything floating should stay floating.
>
> So what can I expect from this mistake? The circuit has been working
> perfectly otherwise for 3 weeks now, but what about increased power
> drain? Any rule-of-thumb for what it might be?
>
> - --
> http://petertodd.org
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