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User Interface - Keypad Scan, PSoC

Style

January 16, 2009 Document No. 001-40409 Rev. *A 1

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.

Introduction

An X-Y keypad enables use of N column lines and M row

lines to detect switch closures for N * M switches. For this

application note, a keypad is defined as an X-Y matrix

where only one key is pressed at a time, as opposed to a

keyboard where simultaneous key closures are the norm

([Ctrl] [Shift] [Delete]). This keypad definition is valid for

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).

Rows and Columns

This application note uses C columns and R rows. Figure

1 shows an example of such a keypad:

Figure 1. 4-Column by 4-Row Keypad

[0,0]

[0,1]

[0,2]

[0,3]

[1,1]

[1,0]

[1,2]

[1,3]

[2,0]

[2,1]

[2,2]

[2,3]

[3,0]

[3,1]

[3,2]

[3,3]

Closure of switch [i, j] (column i, row j) enables current

flow from row j to column i. This keypad requires only eight

connections to the MCU. The 16 diodes can detect

multiple key closures. However, because PSoC

microcontroller reduces the cost of external components

such as op-amps, filters, and DACs, using 16 diodes is not

a good idea.

Well known techniques have been developed to detect

multiple key presses without diodes. Figure 2 shows the

keypad without diodes.

Figure 2. The Keypad you can Afford

[0,0]

[0,1]

[0,2]

[0,3]

[1,1]

[1,0]

[1,2]

[1,3]

[2,0]

[2,1]

[2,2]

[2,3]

[3,0]

[3,1]

[3,2]

[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.

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Indice de contenidos

Pagina 1 - Rows and Columns

User Interface - Keypad Scan, PSoC® Style January 16, 2009 Document No. 001-40409 Rev. *A 1 AN2034Author: Dave Van EssAssociated Project: Yes Ass

Pagina 2 - PSOC General Purpose IO

AN2034 January 16, 2009 Document No. 001-40409 Rev. *A 2 PSOC General Purpose IO To detect single-switch closures, use the following algorithm:  D

Pagina 3 - Using the Output

AN2034 January 16, 2009 Document No. 001-40409 Rev. *A 3 Code 1: Subroutine for Reading Keypad ;----------------------------------------- ; Keypad.

Pagina 4 - Summary

AN2034 January 16, 2009 Document No. 001-40409 Rev. *A 4 Example Code 3 is the main function that implements the design in Figure 5 on page 3. Code

Pagina 5 - Document History

AN2034 January 16, 2009 Document No. 001-40409 Rev. *A 5 Document History Document Title: User Interface – Keypad Scan, PSoC® Style Document Number

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