Analog Devices AN-660 Application Notes

AN-660

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • Fax: [phone] • www.analog.com

XY-Matrix Keypad Interface to MicroConverter

By Brian Moss

INTRODUCTION

Using a keypad input in a MicroConverter application is
sometimes required for specic operations. It may be
used to input data following a request from the Micro ­Converter or simply to trigger the MicroConverter to
initiate a particular operation. The MicroConverter can
be congured to poll the keypad continuously for an input,
or the keypad interface can be congured in an interrupt
driven system, depending on the system requirements.

KEYPAD

The keypa d use d for this app lic ation is a 16- way
XY- Matrix hexadecimal keypad (www.eaoswitch.com,
Mfgr. No. ECO 16250 06, Farnell order code: 467-212).
This keypad has eight connections at the rear—four
columns (X1–X4) and four rows (Y1–Y4). Figure 1 and
Figure 2 show the front and rear of the keypad.

OPERATION

There are two methods of operation: continuous polling,
where the MicroConverter spends most or all of its time
checking the keypad for a key press; and interrupt driven,
where the MicroConverter is free to perform whatever
other tasks are required and checks the keypad only
when a key is pressed.

®

Figure 1. Keypad (Front)

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Figure 2. Keypad (Rear)

AN-660

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AN-660

ADuC8xx

Y1

1

4 5 6 E

7

8 9

D

A 0 B C

2

3

F

Y2

Y3

Y4

X1

X2

X3

X4

10k�

10k�

10k�

10k�

P2.1

P2.0

P2.2

P2.3

P2.4

P2.5

P2.6

P2.7

RxD
TxD

RST

KEYPAD

RS-232

V

DD

V

DD

V

DD

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Figure 3. Continuous Polling Circuit

Continuous Polling Operations

In this mode of operation, the MicroConverter continu­ously polls the keypad for a key press. This operation
is used where the MicroConverter has completed a
task and is now waiting for input before proceeding. In
this mode, the keypad is connected to one port of the
MicroConverter, Port 2 in this example. Figure 3 shows
the connectivity. The output from the MicroConverter,
following a key press, is viewed using HyperTerminal
running on a PC. The MicroConverter is connected to
the PC via the COM1 port. This is the reason for showing
the RS-232 connection.

As can be seen in Figure 3, the four columns (X1 to X4)
are pulled up to VDD and are also connected to four of
the MicroConver ter port pins (P2.4 to P2.7). The four

rows (Y1 to Y2) are connected to the other four port pins
(P2.0 to P2.3). The MicroConverter outputs 0 or drives
low the keypad rows (P2.0 to P2.3) one at a time and
checks the columns (P2.4 to P2.7) for a low condition.

For example, the following is the sequence of events up
to a switch press detection (Switch 5 in this case). The
MicroConverter outputs a low on P2.0 (Y1) and checks
for a low on P2.4 to P2.7. In this case, no low is found and
so it returns P2.0 (Y1) to high and moves on to P2.1 (Y2).
The MicroConverter now drives P2.1 (Y2) low and again
checks P2.4 to P2.7 for a low condition. This time it nds
that P2.5 (X2) is low, due to Switch 5 being pressed.
The MicroConverter now knows that the interconnect
of Y2 and X2 has been shor ted, therefore, this is 5. The
code to perform this operation is given in the accom ­panying KEYSPOLL.ASM le.

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