Cypress AN2034 manual Application Note Abstract, Introduction, Rows and Columns

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.

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).

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

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

C0 C1 C2 C3

R0
[0,0]	[1,0]	[2,0]	[3,0]
R1
[0,1]	[1,1]	[2,1]	[3,1]

R2

[0,2] [1,2] [2,2] [3,2]

R3

[0,3] [1,3] [2,3] [3,3]

C0 C1 C2 C3

R0

[0,0] [1,0] [2,0] [3,0]

R1

[0,1] [1,1] [2,1] [3,1]

R2

[0,2] [1,2] [2,2] [3,2]

R3

[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 1

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