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