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'Pull-up in battery powered application'
1998\06\04@064329 by Stuart Allen

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Hello,

I have a battery powered design using 16C84. I need to be able to read a
keypad, this requires 3 inputs. The design will spend a lot of time asleep.
Long battery life isn't critical, but I would like to extend it as much as
possible.

These inputs must be pulled up to 5v. I could use the weak pull-ups, but the
other pins on port B are inputs, and I would rather not pull them up. Also,
I think that it will be more economical (in terms of power consumption) to
just pull those 3 pins.

However, there is no reference of the input current required to pull-up a
input pin in the PICs specification. It mentions that the port B pull-up
current is typically 250uA, so if I divide by 8 pins I get 31.25uA. So
should I pull up each port with 160K (5/31.25uA)? Or is the 250uA per pin,
not for the port? Maybe is it completely wrong to use this value?

Any advice gratefully received,

Regards,

Stuart.

1998\06\04@085313 by Pasi T Mustalahti

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On Thu, 4 Jun 1998, Stuart Allen wrote:

> Hello,
>
> I have a battery powered design using 16C84. I need to be able to read a
> keypad, this requires 3 inputs. The design will spend a lot of time asleep.
> Long battery life isn't critical, but I would like to extend it as much as
> possible.
>
> These inputs must be pulled up to 5v. I could use the weak pull-ups, but the
> other pins on port B are inputs, and I would rather not pull them up. Also,
> I think that it will be more economical (in terms of power consumption) to
> just pull those 3 pins.
>
> However, there is no reference of the input current required to pull-up a
> input pin in the PICs specification. It mentions that the port B pull-up
> current is typically 250uA, so if I divide by 8 pins I get 31.25uA. So
> should I pull up each port with 160K (5/31.25uA)? Or is the 250uA per pin,
> not for the port? Maybe is it completely wrong to use this value?
PTM: It depends..
If you have very capacitive keypad and you try to read it very fast, you
got to use small resistors. The same goes with long cables: they are very
'capacitive'. If you can give some time to the keyput input to stabilise,
you can use large resistors. PIC has inbuildt resistors of about 50k that
you can set active. I have used them to pull up a keyboard with rather
good speed.
Another way is to use these inbuildt resistors and cut them off and set
the port as input when you don't read the keys. It is enough to read keys
about 10..20 times / sec. That leaves enormously time to go and have
a cup of coffee between read periods.
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1998\06\05@121052 by Dennis Plunkett

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At 11:44 AM 4/06/98 +0200, you wrote:
{Quote hidden}

It would be economical to only pull up the three lines if the other port
pins on B spend long periods of time low. As for the current required? I do
think that this is in the Microchip data book, you will find this under
"Input current" As for the size of the resistors? Well it depends on the CR
time constant of the keypad, and the number of times per second that you
sample it. Note that key bounce can exist for periods of up to 50mS,
generally the user will not notice any problems if the keypad is sampled
around 10Hz, any slower and the user may have to think in brain dead mode.
Also if the keypad switches low, then the resistor can be quite small, as
the on time is generally short. Also ensure wetting currents of 50uA
(General rule of thumb type thing) are used to keep the keypad contacts clean.


Dennis


-=====================================================================-

Dennis Plunkett: Embedded Hardware, Software design
NEC Australia DRMASS
ph 03 9264-3867

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