User Interface - Keypad Scan, PSoC® Style
AN2034
Author: Dave Van Ess
Associated Project: Yes
Associated Part Family: CY8C20x34, CY8C21x23, CY8C21x34
CY8C23x33, CY8C24x23A, CY8C24x94
CY8C27x43, CY8C29x66
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Software Version: PSoC Designer™ 5.0
Associated Application Notes: AN2354
Application Note Abstract
X–Y matrix keypads are an inexpensive interface enabling interaction with microcontroller-based products. This application
note shows how the PSoC® microcontroller’s unique I/O structure can build a keypad scan routine that is fast, uses minimal
RAM resources, and operates in a polled or interrupt mode. A function callable by either ‘C’ or assembly language is also
presented.
Closure of switch [i, j] (column i, row j) enables current
Introduction
flow from row j to column i. This keypad requires only eight
An X-Y keypad enables use of N column lines and M row
connections to the MCU. The 16 diodes can detect
lines to detect switch closures for N * M switches. For this
multiple key closures. However, because PSoC
application note, a keypad is defined as an X-Y matrix
microcontroller reduces the cost of external components
where only one key is pressed at a time, as opposed to a
such as op-amps, filters, and DACs, using 16 diodes is not
keyboard where simultaneous key closures are the norm
a good idea.
([Ctrl] [Shift] [Delete]). This keypad definition is valid for
Well known techniques have been developed to detect
multiple key presses without diodes. Figure 2 shows the
keypad without diodes.
telephones, calculators, security entry kiosks, or other
products where only one key is pressed at a time.
This application relies on PSoC General Purpose Input
Output (GPIO).
Figure 2. The Keypad you can Afford
C0
C1
C2
C3
Rows and Columns
This application note uses C columns and R rows. Figure
1 shows an example of such a keypad:
R0
R1
R2
R3
[0,0] [1,0] [2,0] [3,0]
[0,1] [1,1] [2,1] [3,1]
Figure 1. 4-Column by 4-Row Keypad
C0
C1
C2
C3
[0,2] [1,2] [2,2]
[3,2]
R0
R1
R2
R3
[0,0]
[1,0] [2,0] [3,0]
[0,3] [1,3] [2,3] [3,3]
[0,1] [1,1] [2,1] [3,1]
[0,2] [1,2] [2,2] [3,2]
[0,3] [1,3] [2,3] [3,3]
The standard algorithm for reading a keypad is to
individually drive each row and sample the status of all
columns. Correctly combining all this information enables
detection of at least two simultaneous switch closures.
The hardware cost is less but there is software overhead
required to scan all four rows, read column status, and
condense this information into an answer.
To develop a keypad scan that is low in both hardware
and software resources, limit its operation to single-key
presses.
January 16, 2009
Document No. 001-40409 Rev. *A
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AN2034
Code 1: Subroutine for Reading Keypad
Using the Output
The output of the function bReadKeypad is a single byte
that shows the status of key closure of the keypad. It is
translated and decoded as follows:
;-----------------------------------------
; Keypad.asm
;
; This routine reads a 4 column by 4 row
; keypad on port1. The status of key
; closures is returned in A.
;
ƒ
ƒ
No bits are set if no key is pressed.
A single bit in the upper nibble and a single bit in the
lower nibble are set for a single-key press.
;
;
P1.4 P1.5 P1.6 P1.7
C0 C1 C2 C3
; P1.0 R0 --+----+----+----+-
ƒ
Any other condition is a multiple-key closure and is
defined as not valid.
;
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; P1.1 R1 --+----+----+----+-
These rules can be decoded with discrete conditional code
that breaks up the byte into two nibbles to determine row
and column information. Use this information to determine
which key, if any, was pressed. This results in a complex
set of rules and is tedious.
;
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; P1.2 R2 --+----+----+----+-
;
; P1.3 R3 --+----+----+----+-
;
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Another scheme is to use a lookup table to decode this
data. The advantage is that the table stores the formatted
data. Different programmers could be working on the
same project and each use their own table to decode the
keypad when they are required to read it.
;------------------------------------------
export bReadKeypad
export _bReadKeypad
include "m8c.inc"
bReadKeypad:
_bReadKeypad:
The project file associated with this application note uses
such a table to decode the key closures. A block diagram
of the project is shown in Figure 5.
mov reg[PRT1DR], f0h ;drive columns
mov X,SP
mov A, reg[PRT1DR]
;read rows
mov reg[PRT1DR], 0fh ;drive rows
Figure 5. Block Diagram for the Keypad Project
push A
;store row info on stack
C0
C1
C2
R0
R1
R2
R3
Vdd(common anode)
mov A, reg[PRT1DR]
and [X], A
pop A
;Read Columns
;combine them
P1.4
P1.5
P1.6
P1.0
P1.1
P1.2
P1.3
P0.6
P0.5
P0.4
P0.3
P0.2
P0.1
P0.0
a
a cathode
b cathode
c cathode
d cathode
e cathode
ret
b
f
g
PSoC
MCU
1
4
7
*
2
5
8
0
3
6
9
#
The C header shown in example Code 2 can be found in
“Keypad.h.” It makes the subroutine shown in example
Code 1 a ‘C’ callable function.
e
c
f cathode
g cathode
d
MAN71A
Code 2. C Header Example
// Create a pragma to support
// proper argument and return
// value passing
For this project, set the drives for the port 1 pins to the Pull
Down mode (default) and the port 0 pins to either the Pull
Down or Strong mode.
#pragma fastcall bReadKeypad
The keypad is scanned and the appropriate bits are set on
the output port to turn on the desired LED segments. This
display is a single digit 7-segment common anode LED
display. Any particular segment is lit when its cathode is
pulled low. As an example, all output pins low result in an
“8” being displayed. All output pins high result in a blank
display.
extern BYTE bReadKeypad(void);
January 16, 2009
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Example Code 3 is the main function that implements the
design in Figure 5 on page 3.
Debouncing
Debouncing solutions are specific to each system’s
program structure and switch selection. However,
debouncing is simple; it requires two sequential keypad
scans to agree for the scan to be valid.
Code 3. Keypad Project Implemented
;-----------------------------------------
; This program reads the keypad
; at port 1 and control the LEDs
; on port0.
Interrupt Driven Keypad Scans
Execute the following steps for an interrupt-instigated key
scan:
;
; Copyright (c)
; Cypress Semiconductor 2002-2008.
; All Rights Reserved.
;------------------------------------------
include "m8c.inc"
ƒ
Configure the lower four (row) pins to be I/O interrupts
on a rising edge.
export _main
ƒ
ƒ
ƒ
Set the upper four (column) pins high.
Write an interrupt handler that calls bReadKeypad.
Enable the GPIO interrupt.
_main:
loop:
call bReadKeypad
index KeyToLED
mov reg[PRT0DR], A
jmp loop
The row pins stay low until a key press connects a column
to a row causing an interrupt.
ret
One possible application is a security gate keypad reader
where the MCU is placed in the power-saving-sleep mode.
When someone presses a key, the processor wakes up
and decodes the user’s input. After finishing, the MCU can
put itself back to sleep. For more information about sleep
mode operation please refer to application note AN2354.
xxh: equ 30h ;illegal character “E”
.Literal
KeyToLED:
db 7fh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,4fh,4ch,xxh,0fh,xxh,xxh,xxh
db 78h,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,12h,24h,xxh,00h,xxh,xxh,xxh
db 01h,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,06h,20h,xxh,04h,xxh,xxh,xxh
db 18h,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
db xxh,xxh,xxh,xxh,xxh,xxh,xxh,xxh
.EndLiteral
Summary
The PSoC microcontroller GPIO structure is ideal for fast
keypad scans. The “index” instruction enables simple
translation key presses to a user’s desired format. The
GPIO to interrupts makes for an interrupt-driven keypad
scan.
The “.Literal” and “.EndLiteral” macros are used to disable
the compiler’s code compression algorithms so that the
literal information in the KeyToLED table is not
compressed.
This code can also be found in the project associated with
this application note. The table makes for a simple
translation from key press to displayed character. Develop
your own table for a particular application.
January 16, 2009
Document No. 001-40409 Rev. *A
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AN2034
Document History
Document Title: User Interface – Keypad Scan, PSoC® Style
Document Number: 001-40409
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
*A
1532004
2640952
OGNE
JVY
10/02/07
01/20/09
New publication of existing application note.
Updated content.
Added part numbers CY8C20x34, CY8C21x23, CY8C21x34,
CY8C23x33, CY8C24x23A, CY8C24x94, CY8C27x43, and CY8C29x66.
In March of 2007, Cypress recataloged all of its Application Notes using a new documentation number and revision code. This new documentation
number and revision code (001-xxxxx, beginning with rev. **), located in the footer of the document, will be used in all subsequent revisions.
PSoC is a registered trademark of Cypress Semiconductor Corp. "Programmable System-on-Chip," PSoC Designer, and PSoC Express are trademarks
of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are the property of their respective owners.
Cypress Semiconductor
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January 16, 2009
Document No. 001-40409 Rev. *A
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