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© 2007 Microchip Technology Inc. DS01081A-page 1
AN1081
INTRODUCTION
This application note discusses interfacing a 4x4 matrix
keypad with MCP23X08 8-Bit GPIO Expander. This
application note references the MCP23X08/17 GPIO
Expander Keypad/LCD Demo Board (GPIODM-
KPLCD).
GPIO Expanders provide easy I/O expansion using
standard serial interfaces such as I
2
C™ and SPI. They
are especially useful in applications where pin count is
limited on the microcontroller unit (MCU) or if remote
inputs / outputs (I/O’s) are needed. It is best to think of
an 8-bit GPIO Expander like adding another 8-bit wide
digital port to the MCU.
The MCP23X08 8-Bit GPIO Expanders family consists
of two (2) devices which are similar, except for their
serial interfaces:
• MCP23008 - I
2
C
• MCP23S08 - SPI
This application note does not detail all of the features
of the MCP23X08. Refer to the MCP23008/MCP23S08
Data Sheet, “8-Bit I/O Expander with Serial Interface”
(DS21919) for more information.
FEATURE DESCRIPTIONS
The MCP23X08 devices have several features that
make them ideal for controlling a 4x4 matrix keypad.
These features have been broken down into two main
groups:
1. The ports input and output characteristics.
2. The interrupt-on-change feature, which is an
important aspect of the key scan method used.
Input and Output Characteristics
There are three (3) registers that control the port pins
input and output characteristics that need to be
manipulated:
• GPIO Pull-Up Resistor Register (GPPU): The
GPPU register controls the individually selectable
internal 100 kΩ pull-up resistors. When set, the
pull-up resistor is enabled.
• I/O Direction Register (IODIR): The IODIR
register controls the direction of the port pins. A
high on the corresponding bit configures the pin
as an input and a low configures it as an output.
• General Purpose I/O Port Register (GPIO): The
GPIO register is the data port of the MCP23X08.
Reading from this register reads the state of the
port pins. Writing to the GPIO register will write
that value to the output latches for each pin. Also,
reading of GPIO unlocks the Interrupt Capture
Register (INTCAP) and deactivates the interrupt
pin (INT), which will be explained in detail later.
Interrupt-On-Change Feature
The MCP23X08 devices have one interrupt output pin
(INT), which, if enabled, will activate when an input pin
(GP7-GP0) changes state. This is known as Interrupt-
on-change and can be configured to function with one
of the following options:
1. If the state of the pin changes from the current
state. This option is useful for monitoring a pin
where anytime the state of the pin is changed,
an action needs to be taken. For example, if the
state of the pin switches from high to low, an
interrupt will occur. Assuming the interrupt is
serviced (cleared) while the pin is low, when the
pin changes back to a high state another
interrupt will occur. This is a useful option for
many applications, but is not desired for this
one.
2. If the pin changes state as compared to a pre-
configured default value in the Default Value
Register (DEFVAL). This option is used for the
key scan which this application note discusses.
An interrupt will occur if the pin is in an opposite
state as compared to a pre-configured default
input value. For example, if the default value
(configured in DEFVAL) is set high, and assum-
ing the pin’s idle state is high, once the corre-
sponding pin changes state from high to low an
interrupt will occur. The interrupt condition will
stay active as long as the input pin remains in
the low state. Once the input pin returns to a
high state and the INTCAP or GPIO register is
read the interrupt will clear.
Author: Mike Curran
Microchip Technology Inc.
Interfacing a 4x4 Matrix Keypad with an
8-Bit GPIO Expander
