APPLICATION NOTE
ST7 KEYPAD DECODING TECHNIQUES, IMPLEMENTING
WAKE-UP ON KEYSTROKE
by Microcontroller Division Application Team
INTRODUCTION
The goal of this application note is to present an example of the use of the HALT mode.
In this application, the MCU (here a ST72251) is waked up by an external interrupt caused by
someone pressed a key on the 4x4 matrixed keypad.
1 ST7 / KEYBOARD INTERFACE
Rows are connected to inputs with pull-up and interrupts (Port C). Columns are connected to
Port A configured as output. The result of the interrupt (the value of the pressed key) is sent on
LEDS (Port B) and stored into the X register. In our configuration, we have to add 4 pull-up resistors on Port A (from PA0 to PA3) to be able to apply a high level on the corresponding pads.
Figure 1. ST7 / keypad interface set-up
4x100k
8x560
VDD PC0
PC1
PC2
PC3
ST7 PA0
PA1
PA2
GND
PA3
AN980/1098
1/10
1
ST72251 CONFIGURATION
2 ST72251 CONFIGURATION
The application has been validated with a ST72251. Its configuration is described in this part.
Refer to your datasheet for more details.
2.1 I/O CONTROL
Rows are connected to pins configured as inputs (Port C as input with pull up and interrupts).
Columns are connected to pins configured as outputs (Port A).
External interrupts are caused by a low level applied to a pin of Port C (caused by a key
pressed), they wake up the MCU which was in HALT mode.
Port B is configured as outputs to send the value of the pressed key on LEDS.
Please, refer to the Data Book to configure pins properly.
2.2 MISCELLANEOUS REGISTER
Bits 7 and 6 have to be set to configure events correctly: the external interrupt (EI1) has here
to be caused by a falling edge only.
Please, refer to the datasheet for more details.
2.3 HALT MODE
The HALT instruction places the ST72251 in its lowest power consumption mode. The core
and all peripherals are frozen. In this mode, the internal oscillator is turned off, causing all internal processing to be halted. The data remain unchanged. During the HALT mode, external
interrupts are still enabled. The MCU stays in this state until an external interrupt or a reset occurs. Then the internal oscillator is restarted and the core waits for 4096 CPU clock cycles
(512 µs for a fCPU = 8MHz) before running the external interrupt subroutine. Then the MCU
comes back to the main program (in our application to the HALT state).
Please, refer to the datasheet for more details.
3 EXTERNAL INTERRUPTS
The MCU is in HALT mode. When a key is pressed, a low level is applied to the pin corresponding to the row the key belongs (pins configured as inputs with pull-up). It’s a falling edge
applied to a pin of Port C which creates an external interrupt (EI1) and wakes up the MCU. The
MCU executes then the external interrupt subroutine (decoding the pressed key) and comes
back to its previous state (HALT state in the main program).
2/10
KEYPAD
4 KEYPAD
The keypad used is a 4x4 matrixed keypad. Rows are connected to pins configured as inputs
with pull-up. So the initial state of these pins are a high level (1). When a key is pressed, a low
level is applied to the corresponding pin. For this reason, the keypad is coded as follows:
Table 1. Key values
row
column
KEY
row
column
value
value
KEY
value
value
1
0x0E
0x0E
7
0x0B
0x0E
2
0x0E
0x0D
8
0x0B
0x0D
3
0x0E
0x0B
9
0x0B
0x0B
F
0x0E
0x07
D
0x0B
0x07
4
0x0D
0x0E
A
0x07
0x0E
5
0x0D
0x0D
0
0x07
0x0D
6
0x0D
0x0B
B
0x07
0x0B
E
0x0D
0x07
C
0x07
0x07
1
2
3
F
PC0
4
5
6
E
PC1
7
8
9
D
PC2
A
0
B
C
PC3
PA1
PA2
PA3
PA0
You have to press the chosen key at least 0.5 to 1 second depending on which key you
choose (table read from keypad_top to keypad). The faster the key is read into the table, the
faster it will be decoded and the faster the result will be sent on LEDS.
3/10
KEYPAD
The keypad code is in the file constant.asm as follows:
.keypad
keypad_top
4/10
DC.B
$0E,$0E,$1
;PC0PA0
DC.B
$0E,$0D,$2
;PC0PA1
DC.B
$0E,$0B,$3
;PC0PA2
DC.B
$0E,$07,$F
;PC0PA3
DC.B
$0D,$0E,$4
;PC1PA0
DC.B
$0D,$0D,$5
;PC1PA1
DC.B
$0D,$0B,$6
;PC1PA2
DC.B
$0D,$07,$E
;PC1PA3
DC.B
$0B,$0E,$7
;PC2PA0
DC.B
$0B,$0D,$8
;PC2PA1
DC.B
$0B,$0B,$9
;PC2PA2
DC.B
$0B,$07,$D
;PC2PA3
DC.B
$07,$0E,$A
;PC3PA0
DC.B
$07,$0D,$0
;PC3PA1
DC.B
$07,$0B,$B
;PC3PA2
DC.B
$07,$07,$C
;PC3PA3
FLOWCHARTS
5 FLOWCHARTS
Figure 2. Flowchart: Main program
Initializations
infinite
loop
HALT
Figure 3. Flowchart: external interrupt (EI1)
X = 48 (3x16 coded values)
one column activated
(PA0...PA3)
Is there a low level on the
intersection of the stored
row and column?
yes
no
store into X
next column
value
corresponding value of the pressed key
stored into A, X and sent on LEDS
EXIT
5/10
SOFTWARE
6 SOFTWARE
The assembly code given below is guidance only. The complete software with all the files can
be found in the software library.
st7/
; the first line is reserved
; for specifying the instruction set
; of the target processor
;************************************************************************
; TITLE:
WAKE.ASM
; AUTHOR:
PPG Microcontroller Applications Team
; DESCRIPTION:
Main program (use of the Halt mode for decoding
;
a keypad).
;
;************************************************************************
TITLE "WAKE.ASM"
; this title will appear on each
; page of the listing file
MOTOROLA
; this directive forces the Motorola
; format for the assembly (default)
#INCLUDE “st72251.inc”; include st72251 registers and memory mapping file
#INCLUDE “constant.inc” ; include general constants file
;***********************************************************************
;
Variables, constants defined and referenced locally
;
You can define your own values for a local reference here
;***********************************************************************
;************************************************************************
;
Public routines (defined here)
;************************************************************************
WORDS
segment 'rom'
.Init
LD
A,#$80
; interrupts are caused by falling edge (on Port C).
LD
MISCR,A
LD
LD
LD
A,#$0F
PBDDR,A
PBOR,A
LD
PADDR,A
CLR PAOR
CLR
PADR
LD
A,#$0F
6/10
; PB0 to PB3 configures as outputs(LEDS).
; PA0 to PA3 configured as outputs.
SOFTWARE
LD
CLR
CLR
PCOR,A
PCDDR
PCDR
; PCO to PC3 configured as input with pull-up and interrupt.
RET
.delay_45
LD
A,#200
loop2 LD
X,#$FF
loop1 DEC
X
JRNE loop1
DEC
A
JRNE loop2
RET
;**********************************************************
;
Program code
;**********************************************************
.main
CALL Init
loop
HALT
JRA
loop
; MCU put in lowest power mode.
; Infinite loop, wait an interrupt occurs.
; *****************************************************************
; This set of instructions uses simple assembly mnemoniques.
; We can notice that the loop label is defined only locally (no dot
; in front of it) so it can not be seen by others modules linked
; with this file.
; *****************************************************************
;
********************************************
;
*
*
;
* INTERRUPT SUB-ROUTINES LIBRARY SECTION *
;
*
*
;
********************************************
.dummy iret
.sw_rt iret ; Empty subroutine. Go back to main (iret instruction)
.ext0_rt iret
.ext1_rt
LD
X,#48
; Size of the table stored in X.
; 3 x 16 coded values.
begin
7/10
SOFTWARE
LD
OR
AND
LD
PUSH
CALL
POP
LD
AND
CP
JREQ
DEC
DEC
DEC
JRPL
A,PADR
A,#$0F
A,({keypad+1},X)
PADR,A
X
delay_45
X
A,PCDR
A,#$0F
A,(keypad,X)
OK
X
X
X
begin
JRA
exit
LD
LD
LD
A,({keypad+2},X)
X,A
PBDR,X
; Outputs forced to 1 (initial value).
; one column of the tested row is activated.
; wait 45ms (debouncing procedure).
; value of Port C stored into A.
; search of the low level column after column.
; if key found -> OK.
; 3 times to have next value of the column.
; do it again to read the whole table(matrix).
OK
; Store the key value in A.
; Copy it into X.
; Output the result on LEDS.
exit
CLR
CLR
PADR
PCDR
iret
.spi_rt iret
.tima_rt iret
.timb_rt iret
.i2c_rt iret
;
;
;
;
;
;
;
;
segment 'vectit'
*******************************************************************
This last segment should always be there in your own programs.
It defines the interrupt vector adresses and the interrupt routines' labels
considering the microcontroller you are using.
Refer to the MCU's datasheet to see the number of interrupt vector
used and their addresses.
Remind that this example is made for a ST72251 based application.
*******************************************************************
8/10
SOFTWARE
; *******************************************************************
; Each interrupt vector uses two addresses in rom, that's what the directive
; DC.W means. It says "reserve a word location (.W) in rom (DC) and code
; the routine's label in those two addresses.
; Yet, when an interrupt occurs, for example from the timerB, timerb's routine
; address (timb_rt) will be loaded in the PC and the program will jump to this
; label if allowed. It will execute this routine and then will go back to the main
; program (see interrupt chapter in the datasheet for a more precise description
; of how to handle interrupts in ST72 micros).
; *******************************************************************
DC.W dummy
;FFE0-FFE1h location
DC.W dummy
;FFE2-FFE3h location
.i2c_it
DC.W i2c_rt ;FFE4-FFE5h location
DC.W dummy
;FFE6-FFE7h location
DC.W dummy
;FFE8-FFE9h location
DC.W dummy
;FFEA-FFEBh location
DC.W dummy
;FFEC-FFEDh location
.timb_it
DC.W timb_rt ;FFEE-FFEFh location
DC.W dummy
;FFF0-FFF1h location
.tima_it
DC.W tima_rt ;FFF2-FFF3h location
.spi_it
DC.W spi_rt ;FFF4-FFF5h location
DC.W dummy
;FFF6-FFF7h location
.ext1_it
DC.W ext1_rt ;FFF8-FFF9h location
.ext0_it
DC.W ext0_rt ;FFFA-FFFBh location
.softit
DC.W sw_rt
;FFFC-FFFDh location
.reset
DC.W main
;FFFE-FFFFh location
; This last line refers to the first line.
; It used by the compiler/linker to determine code zone
END ; Be aware of the fact that the END directive should not
; stand on the left of the page like the labels's names.
9/10
SOFTWARE
THE PRESENT NOTE WHICH IS FOR GUIDANCE ONLY AIMS AT PROVIDING CUSTOMERS WITH INFORMATION
REGARDING THEIR PRODUCTS IN ORDER FOR THEM TO SAVE TIME. AS A RESULT, STMICROELECTRONICS
SHALL NOT BE HELD LIABLE FOR ANY DIRECT, INDIRECT OR CONSEQUENTIAL DAMAGES WITH RESPECT TO
ANY CLAIMS ARISING FROM THE CONTENT OF SUCH A NOTE AND/OR THE USE MADE BY CUSTOMERS OF
THE INFORMATION CONTAINED HEREIN IN CONNEXION WITH THEIR PRODUCTS.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without the express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics
1998 STMicroelectronics - All Rights Reserved.
Purchase of I2C Components by STMicroelectronics conveys a license under the Philips I2C Patent. Rights to use these components in an
I2C system is granted provided that the system conforms to the I2C Standard Specification as defined by Philips.
STMicroelectronics Group of Companies
Australia - Brazil - Canada - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Mexico - Morocco - The Netherlands Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A.
http://www.st.com
10/10
This datasheet has been download from:
www.datasheetcatalog.com
Datasheets for electronics components.