ECEN 3213 Spring 2005 Lab 8 March 23, 2005 page 1 of 3
ECEN 3213
Spring 2005
Lab 8
Due in Lab Section, Apr. 4, 6
Objective.
Keyboard and Display Interface.
Discussion.
Connecting a single switch to an input pin is not feasible when there are a large number of
switches, such as in a keyboard. For example, a typical typewriter keyboard has several
dozen keys, each implemented as a contact switch. We do not usually have enough input
port pins to connect to each key in the keyboard. Instead, we must use the I/O port pins to
control more than one key/switch.
One way to make the use of I/O pins more efficient is to put the keys/switches in a two
dimensional matrix as shown in Valvano, Fig. 8.25, p. 436. A low (0) signal is driven onto
each row line one at a time. If no key is pressed (no switch closed), then the column lines
will remain high (1). If a single key is pressed, then one of column lines will go low (0).
A key can be identified by its unique combination of row and column lows.
An assembly program to scan the keyboard is provided in Program 8.11, p. 437-8. It relies
on the CWOM feature for the 6811 microcontroller. This feature appears not to work cor-
rectly on the TExaS simulator. The following subroutine is recommended instead. It is a
combination of the 6811 and 6812 code that works on the 6811. The following uses
PORTC since bits 6 and 7 of PORTD are not functional on the 6811.
;Table for scanning 4 x 4 keyboard
;keys should be labeled as
; 1 2 3 4
; 5 6 7 8
; 9 A B C
; D E F 0
scantab:;most significant nibble has hex digit
db %01110000 PORTC for PC7 row
db %10000000 DDRC for PC7 row
db $10,$20,$30,$40
db %10110000 PORTC for PC6 row
db %01000000 DDRC for PC6 row
db $50,$60,$70,$80
db %11010000 PORTC for PC5 row
db %00100000 DDRC for PC5 row
db $90,$A0,$B0,$C0
db %11100000 PC4 row
db %00010000 DDRC for PC4 row
db $D0,$E0,$F0,$00
db $00 end of table marker
