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
>