Cypress AN2034 No bits are set if no key is pressed. A single bit in the upper nibble and a single bit in the , Any other condition is a multiple-key closure and is

AN2034

January 16, 2009 Document No. 001-40409 Rev. *A 3

Code 1: Subroutine for Reading Keypad

;-----------------------------------------

; Keypad.asm

;

; This routine reads a 4 column by 4 row

; keypad on port1. The status of key

; closures is returned in A.

;

; P1.4 P1.5 P1.6 P1.7

; C0 C1 C2 C3

; P1.0 R0 --+----+----+----+-

; | | | |

; P1.1 R1 --+----+----+----+-

; | | | |

; P1.2 R2 --+----+----+----+-

; | | | |

; P1.3 R3 --+----+----+----+-

;

;------------------------------------------

export bReadKeypad

export _bReadKeypad

include "m8c.inc"

bReadKeypad:

_bReadKeypad:

mov reg[PRT1DR], f0h ;drive columns

mov X,SP

mov A, reg[PRT1DR] ;read rows

mov reg[PRT1DR], 0fh ;drive rows

push A ;store row info on stack

mov A, reg[PRT1DR] ;Read Columns

and [X], A ;combine them

pop A

ret

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.

Code 2. C Header Example

// Create a pragma to support

// proper argument and return

// value passing

#pragma fastcall bReadKeypad

extern BYTE bReadKeypad(void);

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:

 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.

 Any other condition is a multiple-key closure and is

defined as not valid.

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.

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.

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.

Figure 5. Block Diagram for the Keypad Project

213

546

879

0*#

C0P1.4

P1.5

P1.3

P1.0

P1.1

P1.2

P1.6 PSoC

MCU

P0.6

P0.5

P0.4

P0.3

P0.2

P0.1

P0.0

Vdd(common anode)

a cathode

b cathode

c cathode

d cathode

e cathode

f cathode

g cathode

MAN71A

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.

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.

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