AMICUS18-COMPANION-SHIELD AMICUS, AMICUS18-COMPANION-SHIELD Datasheet - Page 22

• Manufacturer Part Number: AMICUS18-COMPANION-SHIELD
• Description: Amicus18 Companion Shield
• Manufacturer: AMICUS

Specifications of AMICUS18-COMPANION-SHIELD

• Silicon Manufacturer: Microchip
• Core Architecture: PIC
• Core Sub-architecture: PIC18
• Features: USB Interface, In Circuit Serial Programming Interface
• Kit Contents: Board
• Silicon Family Name: PIC
• Silicon Core Number: PIC18F25K20
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: PIC18F25K20 Microcontroller

Amicus18 Companion Shield
Sensing the Outside World
Interacting with the outside world is always desirable when using a microcontroller, whether it’s choos-
ing a drink in a vending machine or deciding which way a pacman will move. The easiest method of out-
side influence is through the use of a switch or button.
However, there are certain rules that must be observed when adding a switch to a microcontroller’s pin.
When the pin is configured as an input, it can be brought high to 3.3 Volts or pulled low to ground,
floating
however if neither of these states is performed, the pin is neither high or low and this is termed
.
Even if a switch was placed from the microcontroller’s input pin to ground, when the switch is not being
floating
operated the input pin can be high or low (
).
What’s required is a pull-up resistor or a pull-down resistor in order to force a single state when not in
use. A pull-up resistor is a weak resistance from the input pin to the 3.3 Volt line, while a pull-down re-
sistor is a weak resistance from the input pin to ground.
Switch Input (Pulled-Up)
The layout below shows a pull-up resistance:
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Crownhill AssociatesLimited 2009 - All Rights Reserved
Version 1.0
06-10-2009