Digital input to an IC - do I need a resistor?

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Pete Verdon

Posted: Fri Jan 12, 2007 9:51 am

Guest

Hi,

I'm currently designing a device based around a PIC microcontroller.
However, in getting to this point I have rather leapfrogged over more
basic electronics - I'm a software developer by trade. Any general
advice people care to give is welcome, but my specific question today is
whether I need to include resistors in digital input lines. That is, can
a pin which is configured on the chip as a digital input be connected
directly to a push-to-make button and thence to the power rail, or
should I include a resistor in series to limit the current that can
flow? (I'm aware that I will also need debounce, but intend to implement
this in firmware since the PIC is not working very hard).

The IC, if it matters, is Microchip's PIC16F914:
http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en020956.pdf

Thanks,

Pete

Peter Bennett

Posted: Fri Jan 12, 2007 1:49 pm

Guest

On Fri, 12 Jan 2007 13:51:14 +0000, Pete Verdon
<news@verdonet.organisation.unitedkingdom.invalid> wrote:

Quote:

If the switch is going to ground or the processor's Vcc, you don't
need a series resistor, but you will need a pull-up or pull-down
resistor to ensure that the input goes to the opposite state when the
switch is open.

From long experience with TTL logic, I would connect the switch from
the input pin to ground, and a 3 - 5K resistor from the input pin to
+Vcc.

Guest

Posted: Sat Jan 13, 2007 12:20 am

If im not mistaken, PICs are voltage controlled devices. Meaning that
if you are inputting TTL voltages, you will not need a series resistor
to limit current flow (assuming your logic voltages are being generated
by other such devices). If your logic is somehow directly connected to
your Vcc (ie, a switch, or a transistor switch) then you will need a
series resistor because you will short your entire Vcc source to
ground..and that quite frankly...is bad..

eP

On Jan 12, 8:51 am, Pete Verdon
<n...@verdonet.organisation.unitedkingdom.invalid> wrote:

Quote:

Hi,

I'm currently designing a device based around a PIC microcontroller.
However, in getting to this point I have rather leapfrogged over more
basic electronics - I'm a software developer by trade. Any general
advice people care to give is welcome, but my specific question today is
whether I need to include resistors in digital input lines. That is, can
a pin which is configured on the chip as a digital input be connected
directly to a push-to-make button and thence to the power rail, or
should I include a resistor in series to limit the current that can
flow? (I'm aware that I will also need debounce, but intend to implement
this in firmware since the PIC is not working very hard).

The IC, if it matters, is Microchip's PIC16F914:http://www.microchip.com/stellent/groups/picmicro_sg/documents/device...

Thanks,

Pete

Gerard Bok

Posted: Sat Jan 13, 2007 11:49 am

Guest

On Fri, 12 Jan 2007 13:51:14 +0000, Pete Verdon
<news@verdonet.organisation.unitedkingdom.invalid> wrote:

Quote:

I'm currently designing a device based around a PIC microcontroller.
However, in getting to this point I have rather leapfrogged over more
basic electronics - I'm a software developer by trade. Any general
advice people care to give is welcome, but my specific question today is
whether I need to include resistors in digital input lines. That is, can
a pin which is configured on the chip as a digital input be connected
directly to a push-to-make button and thence to the power rail, or
should I include a resistor in series to limit the current that can
flow?

No. You don't need a series-resistor to limit the current.
As others noted: you may need a pull-up or pull-down resistor.

But if you realy want to connect a push button, you may need
something else: a debounce circuit.

If you push the button near your frontdoor, your bell will
(simply) ring.
If you connect a push button to an electronic circuit it will
'see' a train of pulses, for upto 1 or 2 mSec at least.
Normally, you have to cope with that, either in software or by a
hardware circuit.

--
Kind regards,
Gerard Bok

Pete Verdon

Posted: Mon Jan 15, 2007 6:03 am

Guest

Peter Bennett wrote:

Quote:

Pete Verdon wrote:

my specific question today is whether I need to include resistors
in digital input lines. That is, can a pin which is configured on
the chip as a digital input be connected directly to a push-to-make
button and thence to the power rail, or should I include a resistor
in series to limit the current that can flow?

If the switch is going to ground or the processor's Vcc, you don't
need a series resistor, but you will need a pull-up or pull-down
resistor to ensure that the input goes to the opposite state when the
switch is open.

From long experience with TTL logic, I would connect the switch from
the input pin to ground, and a 3 - 5K resistor from the input pin to
+Vcc.

Thanks very much, that's really helpful.

How crucial is the resistor value, though? I ask because it seems my
colleagues are planning to use a lot of 10K resistors elsewhere; would
these be OK or is your 5K pretty much the limit?

Thanks,

Pete

Peter Bennett

Posted: Mon Jan 15, 2007 4:19 pm

Guest

On Mon, 15 Jan 2007 10:03:09 +0000, Pete Verdon
<news@verdonet.organisation.unitedkingdom.invalid> wrote:

Quote:

Peter Bennett wrote:
Pete Verdon wrote:

From long experience with TTL logic, I would connect the switch from
the input pin to ground, and a 3 - 5K resistor from the input pin to
+Vcc.

Thanks very much, that's really helpful.

How crucial is the resistor value, though? I ask because it seems my
colleagues are planning to use a lot of 10K resistors elsewhere; would
these be OK or is your 5K pretty much the limit?

10K should be fine - I'm probably stuck in an older, higher current,
era...

You might also put an 0.01 uF capacitor across the switch to reduce
the contact bounce problem.

Pete Verdon

Posted: Tue Jan 16, 2007 8:01 am

Guest

Peter Bennett wrote:

Quote:

On Mon, 15 Jan 2007 10:03:09 +0000, Pete Verdon wrote:

How crucial is the resistor value, though? I ask because it seems my
colleagues are planning to use a lot of 10K resistors elsewhere; would
these be OK or is your 5K pretty much the limit?

10K should be fine - I'm probably stuck in an older, higher current,
era...

Thanks. The order is going in very shortly!

Pete

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