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'[EE] Floating SPI input?'
2007\06\30@122838 by Peter Todd

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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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2007\06\30@131812 by Gerhard Fiedler

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Peter Todd wrote:

> 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?

I don't know of any rule of thumb. AFAIK this is pretty arbitrary -- it may
work, or it may work most of the time, or it may work but with increased
power drain, or it just may not work.

Given that it worked for 3 weeks, it seems that it may continue to work or
not :)

Gerhard

2007\06\30@135828 by Harold Hallikainen

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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

{Quote hidden}

> -


'[EE] Floating SPI input?'
2007\07\02@082729 by Peter Todd
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On Sat, Jun 30, 2007 at 02:17:46PM -0300, Gerhard Fiedler wrote:
{Quote hidden}

Oh well, as Harold pointed out the chips may be selected just often
enough to keep the lines in a known state due to capacitance, hopefully
the 42 minute interval will be enough.

I'll tell you if it all worked out, in, oh... 6 years or so? :)

- --
http://petertodd.org
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2007\07\02@140509 by Gerhard Fiedler

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Peter Todd wrote:

> Oh well, as Harold pointed out the chips may be selected just often
> enough to keep the lines in a known state due to capacitance, hopefully
> the 42 minute interval will be enough.

According to my experience, yes. I've never had a problem due to a floating
PIC input. (I don't have them that often, but it happens occasionally.) But
people have said that simply because of a floating input a PIC just didn't
work at all. So that's where the "arbitrary" part comes from :)

Gerhard

2007\07\02@141504 by Dario Greggio

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Gerhard Fiedler wrote:

> According to my experience, yes. I've never had a problem due to a floating
> PIC input. (I don't have them that often, but it happens occasionally.) But
> people have said that simply because of a floating input a PIC just didn't
> work at all. So that's where the "arbitrary" part comes from :)

I can say that I've just reduced power consumption from 500uA to 35uA,
just by setting all unused pins to GND on PIC, and an unused pin on the
RF transceiver to VCC...

--
Ciao, Dario
--
ADPM Synthesis sas - Torino
--
http://www.adpm.tk

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