Inputs left floating at the very start

In college, I was always told not to leave input pins floating on a
device such as a 74HCT chip. I was told that the device might behave
erratically if any of its inputs are floating.

I have the PIC16F684. One of its inputs goes to the data input of a
shift register, and the other goes to the clock input of the same
shift register.

On the shift regsiter, there's an RC circuit on the master reset with
a charge time of about a microsecond (which is plenty slow enough to
have it set to all zeroes initially).

When the PIC boots up, all of its IO pins are set as inputs initially.
What I'm concerned about though is that shift register's input pins
must be either high or low, so I'm wondering if I'll get erratic
behaviour at the very start because the pins are effectively floating?

On May 3, 8:37=A0am, Tom=E1s =D3 h=C9ilidhe <t...@lavabit.com> wrote:
> In college, I was always told not to leave input pins floating on a
> device such as a 74HCT chip. I was told that the device might behave
> erratically if any of its inputs are floating.
>
> I have the PIC16F684. One of its inputs goes to the data input of a
> shift register, and the other goes to the clock input of the same
> shift register.
>
> On the shift regsiter, there's an RC circuit on the master reset with
> a charge time of about a microsecond (which is plenty slow enough to
> have it set to all zeroes initially).
>
> When the PIC boots up, all of its IO pins are set as inputs initially.
> What I'm concerned about though is that shift register's input pins
> must be either high or low, so I'm wondering if I'll get erratic
> behaviour at the very start because the pins are effectively floating?

Unless the shift register has a reset pin which you're asserting in
the power-up sequence, it will probably be in an indeterminate state
due to both power-up and to the floating inputs.  Even if it has a
reset pin connected to the PIC reset, once that reset goes inactive
the chip could end up having a bit shifted into it because of the
floating inputs.  Once your code has set the PIC pints to outputs you
can shift the proper startup data into the SR as part of your I/O
initialization code.  If this brief indeterminate state is
unacceptable to your hardware (like it turns on the Giant Laser of
Doom) then you have to add additional hardware to either keep the
shift register in reset or to go between the shift register and the
GLoD to keep the outputs in a safe startup state, all until the PIC is
up and running and in control of things.

Mike


Mike

On May 3, 2:05=A0pm, Mike Silva <snarflem...@yahoo.com> wrote:

> Unless the shift register has a reset pin which you're asserting in
> the power-up sequence, it will probably be in an indeterminate state
> due to both power-up and to the floating inputs. =A0Even if it has a
> reset pin connected to the PIC reset, once that reset goes inactive
> the chip could end up having a bit shifted into it because of the
> floating inputs. =A0Once your code has set the PIC pints to outputs you
> can shift the proper startup data into the SR as part of your I/O
> initialization code.

In my college project this year, what I did was give the shift-
register's master reset a much longer RC charge time, somewhere like
50 milliseconds. This gave me enough time to boot up the PIC and set
the pins to outputs.

No to play devil's advocate here, but who's to say that master reset
will work properly if all the other inputs are floating? I've been
told you can't rely on a chip's behaviour if any of its inputs are
floating.

I still haven't figured out how long it takes the PIC16F684 to boot up
so I guessed it to be about 20 milliseconds. If anyone has exact
figures I'd be appreciative :-D

On May 3, 9:19=A0am, Tom=E1s =D3 h=C9ilidhe <t...@lavabit.com> wrote:
> On May 3, 2:05=A0pm, Mike Silva <snarflem...@yahoo.com> wrote:
>
> > Unless the shift register has a reset pin which you're asserting in
> > the power-up sequence, it will probably be in an indeterminate state
> > due to both power-up and to the floating inputs. =A0Even if it has a
> > reset pin connected to the PIC reset, once that reset goes inactive
> > the chip could end up having a bit shifted into it because of the
> > floating inputs. =A0Once your code has set the PIC pints to outputs you
> > can shift the proper startup data into the SR as part of your I/O
> > initialization code.
>
> In my college project this year, what I did was give the shift-
> register's master reset a much longer RC charge time, somewhere like
> 50 milliseconds. This gave me enough time to boot up the PIC and set
> the pins to outputs.

Yes, that would work.  Just make sure you're not trying to set the SR
to some startup state while the reset is still asserted.  You'll
either have to wait long enough to guarantee worst case that the SR
reset has been de-asserted, or you'll have to read the reset signal
and wait for it to de-assert.

> No to play devil's advocate here, but who's to say that master reset
> will work properly if all the other inputs are floating? I've been
> told you can't rely on a chip's behaviour if any of its inputs are
> floating.

I would be astonished if a reset pin didn't actually reset the chip,
assuming only proper power to the chip.  But if we're talking GLoD
territory here, then you may have to prove that to your own
satisfaction.

Mike

On Sat, 3 May 2008 05:37:16 -0700 (PDT), the renowned Tom�s � h�ilidhe
<toe@lavabit.com> wrote:

>
>In college, I was always told not to leave input pins floating on a
>device such as a 74HCT chip. I was told that the device might behave
>erratically if any of its inputs are floating.
>
>I have the PIC16F684. One of its inputs goes to the data input of a
>shift register, and the other goes to the clock input of the same
>shift register.
>
>On the shift regsiter, there's an RC circuit on the master reset with
>a charge time of about a microsecond (which is plenty slow enough to
>have it set to all zeroes initially).
>
>When the PIC boots up, all of its IO pins are set as inputs initially.
>What I'm concerned about though is that shift register's input pins
>must be either high or low, so I'm wondering if I'll get erratic
>behaviour at the very start because the pins are effectively floating?

Another thing to consider is that there is a period of time at
power-on when the power has not yet stabilized to a level within the
guaranteed region of operation. Parts can get up to mischief during
this time (and there is an often much longer duration period at power
down that novice designers sometimes fail to consider). As far as the
few tens of msec on power-up, it's usually not a concern (for one
exception, it could greatly increase power consumption for a brief
moment and cause a very low power battery powered device being
supplied by a nearly dead battery to fail to start- perhaps some times
and not others). 

If you're concerned about it you can use pull-up or pull-down
resistors. If you have a supervisory/reset circuit that is guaranteed
to hold the proper state at the LOWEST possible voltage where
'trouble' can occur, then the combination of that, hardware reset on
your SR and pull-up/pull-down resistors should be enough. 

To use Mike's example, suppose your SR drives a relay which controls
your home-built giant rooftop chemical laser focused on a nearby
orphanage. The relay is guaranteed not to pull in below 3V. Your SR
and supervisor is guaranteed to hold the SR outputs low above 3V, all
the way up to 5V. After a couple hundred msec delay from your
supervisor circuit your PIC clock will be up and running in a stable
fashion, the clock and data pins will still be pulled low (say) by the
resistors (and driven high-Z by the low /MCLR input controlled by the
supervisory circuit, and the PIC will start up and take over driving
the pins low after the program starts running and sets the ports &
data direction registers (TRIS). On the way down, the supervisory
circuit will immediately drive /MCLR low and drive the SR outputs low
as soon as the voltage drops below its threshold (which should be
higher than the minimum voltage the parts will operate at), and keeps
it there at least until there's not enough voltage to pull in the
laser control contactor. Of course you'd probably want to add a bit
more circuitry and more safeties in an actual safety-critical system,
or even if it was 'only' data that could be corrupted. 

In general, simple RC circuits are not a good idea unless you don't
really care if the thing works every time. 


Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

On May 3, 9:19 am, Tom=E1s =D3 h=C9ilidhe <t...@lavabit.com> wrote:
> On May 3, 2:05 pm, Mike Silva <snarflem...@yahoo.com> wrote:
>
> > Unless the shift register has a reset pin which you're asserting in
> > the power-up sequence, it will probably be in an indeterminate state
> > due to both power-up and to the floating inputs.  Even if it has a
> > reset pin connected to the PIC reset, once that reset goes inactive
> > the chip could end up having a bit shifted into it because of the
> > floating inputs.  Once your code has set the PIC pints to outputs you
> > can shift the proper startup data into the SR as part of your I/O
> > initialization code.
>
> In my college project this year, what I did was give the shift-
> register's master reset a much longer RC charge time, somewhere like
> 50 milliseconds. This gave me enough time to boot up the PIC and set
> the pins to outputs.
>
> No to play devil's advocate here, but who's to say that master reset
> will work properly if all the other inputs are floating? I've been
> told you can't rely on a chip's behaviour if any of its inputs are
> floating.
>
> I still haven't figured out how long it takes the PIC16F684 to boot up
> so I guessed it to be about 20 milliseconds. If anyone has exact
> figures I'd be appreciative :-D

Why can't you just measure it?  As soon as the CPU start executing
code, toggle a pin from low to high to low.  No matter what state the
pin was before initialization, you will get an edge from this
sequence.

Do you use an external reset of any sort?  Many CPUs do not generate a
reliable internal power on reset, even if the data sheet claims it
does.

Rick

In article <da6e9bf6-4659-4fc5-8d47-8f09f33f7cd7
@l42g2000hsc.googlegroups.com>, toe@lavabit.com says...
> 
> In college, I was always told not to leave input pins floating on a
> device such as a 74HCT chip. I was told that the device might behave
> erratically if any of its inputs are floating.
> 
> I have the PIC16F684. One of its inputs goes to the data input of a
> shift register, and the other goes to the clock input of the same
> shift register.
> 
> On the shift regsiter, there's an RC circuit on the master reset with
> a charge time of about a microsecond (which is plenty slow enough to
> have it set to all zeroes initially).
> 
> When the PIC boots up, all of its IO pins are set as inputs initially.
> What I'm concerned about though is that shift register's input pins
> must be either high or low, so I'm wondering if I'll get erratic
> behaviour at the very start because the pins are effectively floating?
> 
A couple of 100K pullups or pulldowns should eliminate your doubts.
The 100K value is subject to change if the PIC pins have internal
pullups of their own.

Mark Borgerson

On May 3, 5:37=A0am, Tom=E1s =D3 h=C9ilidhe <t...@lavabit.com> wrote:
> In college, I was always told not to leave input pins floating on a
> device such as a 74HCT chip. I was told that the device might behave
> erratically if any of its inputs are floating.
>
> I have the PIC16F684. One of its inputs goes to the data input of a
> shift register, and the other goes to the clock input of the same
> shift register.
>
> On the shift regsiter, there's an RC circuit on the master reset with
> a charge time of about a microsecond (which is plenty slow enough to
> have it set to all zeroes initially).
>
> When the PIC boots up, all of its IO pins are set as inputs initially.
> What I'm concerned about though is that shift register's input pins
> must be either high or low, so I'm wondering if I'll get erratic
> behaviour at the very start because the pins are effectively floating?

If you are driving LEDs with the Shift Registers, what difference does
it make to have random outputs for a few milliseconds?  Noboby can
really catch it anyway.

On May 3, 3:55=A0pm, Mark Borgerson <mborger...@comcast.net> wrote:

> A couple of 100K pullups or pulldowns should eliminate your doubts.
> The 100K value is subject to change if the PIC pins have internal
> pullups of their own.


Brilliant solution, thanks! I never really understood how pull-up and
pull-down resistors work but I put thought into it just a second ago
and realised how they work!

Tom�s � h�ilidhe wrote:
> On May 3, 2:05 pm, Mike Silva <snarflem...@yahoo.com> wrote:

> No to play devil's advocate here, but who's to say that master reset
> will work properly if all the other inputs are floating? I've been
> told you can't rely on a chip's behaviour if any of its inputs are
> floating.

The data sheet, that's who.  Get in the habit of rsolving questions with 
the data sheets.

> I still haven't figured out how long it takes the PIC16F684 to boot up
> so I guessed it to be about 20 milliseconds. If anyone has exact
> figures I'd be appreciative :-D

See above.

-- 
Thad