User manual

LPCXpresso Experiment Kit - User’s Guide

Page 39

Keep the previously mounted LED. Get a push-button (representing SW2) and a 330 ohm resistor

(representing R31) from the components bag. Note that there are two types of push-buttons; for pcb

mounting and for breadboard mounting. It is the latter that shall be used now. Mount the push-button

and resistor on the breadboard and connect to the LPCXpresso LPC111x board, as illustrated in

Figure 13.

Figure 13 – Breadboard Connections for SW2 and LED

It is common that microcontroller input pins have built-in pull-up resistors. If the input is not driven the

input is high. Sometimes the behavior of the pins is very programmable, for example if pull-up or pull-

down resistors and input hysteresis shall be enabled. In this experiment a pull-up resistor must be

enabled on the input pin. When pressing the push-button it will actively pull the input pin to ground.

Else the internal pull-up resistor will pull the input high.

It is important to check the datasheet how strong the pull-up resistors are so that the external signal

can pull the pin low and vice versa that the built-in pull-up resistor can pull an inactive signal high.

The series resistor is for protection if the (supposedly) input pin is an output. If that output is pulled high

by the microcontroller and the push-button is pressed, the output could be damaged due to excessive

current flowing to ground if a series resistor does not limit the current. The situation is not an imaginary

situation. Suppose there already is an application running on the microcontroller from a previous

experiment. That application might very well use the pin as an output. Before the correct application

has been downloaded the damaged can happen. Therefore it is a good practice to add series resistors

to all signals that can drive a microcontroller pin - the key in this case, which can drive the signal low.

Pin PIO1_5 is controlled by register IOCON_PIO1_5 (check chapter 7 -

LPC1100/LPC1100C/LPC1100L series: I/O configuration in the LPC111x user’s manual). In the

description for this register we can see that there are three alternative pin functions:

- PIO1_5, a general purpose input/output, port #1, pin #5