PIC I/O pins

[Christophe]

Hi,

I have some questions on I/O pins of my PIC16F877A

1. What is input impedance of a digital input?
1.a What is the maximum pull-up value that you can use on an input?
1.b Is there a drawback on using high value Pull-up resistors?

2. I/O that you do not want high because of the current that can leak into a device; what do you do with them for the least current consumption?
* Set as an output LOW
* Set as an input

tks

[Ttelmah]

1) Typically in the order of 5 Meg ohm, paralleled with with about 5pF.
1a) Several hundred K ohm.
1b) Inputs often draw more power, when they are transitioning from 'high' to 'low' (and vice versa). Larger resistors, mean more time spent in this region, and can result in extra current being drawn.

2) This is completely dependant on what is attached. With normal PCB layouts, leakage current for a 'high', should be microscopic. The signal should always be drawn either high or low by _something_. Leaving it as an input, with nothing attached, can result in it floating in the transition region, and extra current being drawn. However if there is something attached, and this ensures the signal is high/low, then setting as an input is acceptable. Otherwise drive it high or low, depending on which setting draws the least current on anything else that is attached.

Best Wishes

[Christophe]

When 2 inputs are connected to each other; are then both floating?