how about beer?
i am going to be in spain this summer...


> Thanks a lot to all of yours, and specially Roland Andrag
for this message.
> And I had a bonus, the explanation of a 'open collector'
is (joke) a
> 'collector item' :)
>
> Well, if someone of the picsters come to Madrid during the
year or to
> Mallorca (only in summer..) has a paid coffee cup. General
prize for all.
> And this is no joke ;)
>
>
> Thanks again
> >Sebastian wrote:
> >
> >
> >>>> Could someone of yours explain me the basic uses and
advantadges of the
> >>pullups in a PIC?
> >
> ><snip>
> >
> >>Can I assume that if I set the portb internal pullups
via code, and I
> drive
> >>any portb pin with a 'below-5v-but-above-2v' signal, the
pullup will
> >'raise'
> >>the signal to 'full-5V', and this will improve my signal
adquisition? If
> >>this is correct, I understand PIC pullups. If this is
wrong.. I am
> confused
> >>yet.
> >
> >
> >No, that is not the use of a pullup, to put it bluntly.
A pullup is used
> to
> >give a floating pin a state - either high (pullup), or
low (pulldown).  In
> >pics it is normally used in one of the following two
cases:
> >
> >0 V ----- switch ----- pic port b input pin ------
internal pullup ---- 5 V
> >
> >In this case, when the switch (button of a keypad etc.)
is open, the pullup
> >raises the pin's level to 5 V (no current is flowing
through the switch,
> and
> >very small current is flowing into the pin (very high
input impedance), so
> >the pin is sitting at 5 V since only a very small voltage
is dropped across
> >the pullup).  Without the pullup the pin would have a
random state - it
> >would be floating, and often ends up following other pins
in its vicinity.
> >When the switch is closed, current flows from 5 V through
the pullup to 0
> V,
> >dropping 5 V across the pullup and leaving the pin at 0
V. The second case
> >is used with the open collector output on pin Ra4 (I
think its Ra4):
> >
> >some device ------ Ra4 ----- external pullup ---- 5 V
> >
> >In this case when Ra4 is low, 'some device' reads low
since current flows
> >from 5 V through pullup into Ra4, dropping 5 V across the
pullup.  When Ra4
> >is high, it is actually in a high impedance mode (not
connected to
> anything)
> >since it is an open collecter output.  An open collector
can only sink
> >current, it cannot source current (current can only flow
into the pin, not
> >out of the pin).  In this case the current is sourced
through the pullup,
> >and the input impedance of 'some device' has to be high
enough so that the
> >current flowing through the pullup doesn't cause a large
voltage to be
> >dropped across it. Next time the question 'Ra4 doing
funny things - tried 5
> >pics' comes up, you should know what the author forgot to
do.
> >
> >I hope thats clear
> >Roland
> >
>