> > If you are really after low power then you don't want any pull-ups.
>=20
> Conceptually, I concur, Alan, but actual implementation is another matter=
..
> Code becomes more complex and what if something happens and the PIC
> gets stuck and the pin is left to an Output?  Also, I don't think I am fo=
llowing
> your train of thought completely.  (Sorry.)  What are the long term
> implications of that 5us "short circuit" in terms of product life and rel=
iability?
> What if I am connecting to a DIP switch which is either open-circuit (flo=
ating)
> or GND?  How can you READ a floating state?  (You can't.)
>=20
> By the way, I need to poll DIP switches regularly, not just when the Igni=
tion is
> ON.  I don't know when someone will flip a switch.

There is very little extra code (2 instructions?) to implement this in the =
read loop.

Using Pseudo code ...
; arrive with port set to output, all pins at 0
Set port to all ones
Nop?
Read port ; value read from port (NOT LAT register) will reflect state of D=
IP switches, OFF =3D 1, ON =3D 0
Set port to all 0s
Use value read from port

The 'long term implications' of the short circuit if the PIC gets stuck wit=
h the outputs set to 1 is that the chip will draw the short circuit current=
 for each pin where the switch is on. This will be in the region of 25mA/pi=
n typically. Note that the chip you are using may have a maximum total curr=
ent you can draw through the power supply pins which may limit the number o=
f pins you can use this way.

There never is a 'floating state' as the port is always configured as an ou=
tput (how to confuse someone attempting to reverse engineer your code!). th=
e pins you call 'floating' (where the switch is OFF) will go to a one state=
 when you set the port to all 1s, and the pins where the switch is ON will =
stay at 0. You then read the port and the value is the 'inverse' of the swi=
tch settings.

If you do need to read the switches regularly then have a little subroutine=
 you call on whatever interval is desired that does the read. But realistic=
ally how regular do the switches need to be read? I would still think in te=
rms of requiring an ignition off/on cycle (or some other available function=
 such as an on board push button to reset the PIC) to read the configuratio=
n. Its surely not going to be done while the vehicle is mobile? If it is wh=
y the low power requirement?



--=20
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