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APPLICATION NOTE
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XY-Matrix Keypad Interface to MicroConverter®
By Brian Moss
INTRODUCTION
Using a keypad input in a MicroConverter application is
sometimes required for specific operations. It may be
used to input data following a request from the MicroConverter or simply to trigger the MicroConverter to
initiate a particular operation. The MicroConverter can
be configured to poll the keypad continuously for an input,
or the keypad interface can be configured in an interrupt
driven system, depending on the system requirements.
KEYPAD
The keypad used for this application 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)
Figure 2. Keypad (Rear)
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1
10k�
10k�
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2
3
F
VDD
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P2.0
Y1
4
5
6
E
Y2
P2.1
7
8
9
D
A
0
B
C
Y3
RxD
TxD
P2.2
RS-232
VDD
RST
Y4
P2.3
P2.4
X1
P2.5
X2
P2.6
X3
KEYPAD
P2.7
X4
Figure 3. Continuous Polling Circuit
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.
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.
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 finds
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 shorted, therefore, this is 5. The
code to perform this operation is given in the accompanying KEYSPOLL.ASM file.
As can be seen in Figure 3, the four columns (X1 to X4)
are pulled up to V DD and are also connected to four of
the MicroConverter port pins (P2.4 to P2.7). The four
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10k�
10k�
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1
2
3
F
ADuC8xx
4
5
6
E
10k�
Y1
P2.0
Y2
RxD
TxD
P2.1
7
8
9
D
A
0
B
C
Y3
RS-232
P2.2
VDD
RST
Y4
P2.3
P2.4
X1
P2.5
X2
P2.6
X3
INT1
P2.7
X4
������
OUT2
OUT3
OUT1
INT1
GAL 16V8
Figure 4. Interrupt Driven Circuit
NOTE: In both circuit diagrams, inherent circuitry critical to the operation of the MicroConverter is omitted for clarity, i.e., EA, PSEN, decoupling, and detailed RST.
This circuitry should not be omitted in practice.
This negative edge triggers the INT1 interrupt service
routine, which in turn runs the keypad software. Once
the keypad software routine has run and no additional
key presses have been detected, the MicroConverter
resumes executing whatever other code it was running
before the key was pressed. The code for this application is given in the accompanying KEYSINT.ASM file.
Interrupt Driven Operations
The basic operation of an interrupt driven scheme is
similar to the polling scheme except the MicroConverter
does not continuously poll the keypad. Instead, it polls
the keypad only when a key is pressed. This operation
requires some external circuitry, as shown in Figure 4.
The objective of this logic is to produce a negative-going
edge for each key press (see the truth table below).
X1
X2
OUT1
X3
X4
OUT2
OUT1
OUT2
OUT3
INT1
0
0
1
1
0
1
0
1
1
1
1
0
0
0
1
1
0
1
0
1
1
1
1
0
0
0
1
1
0
1
0
1
0
1
1
0
1
0
0
1
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I2C Patent Rights to use these components in an I2C system, provided that the system conforms to the I2C Standard Specification as defined by Philips.
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