4-72

Operation

The CDP1871A is made up of two major sections: the

counter/scan-selection logic and the control logic (Figure 2).

The counter and scan-selection logic scans the keyboard

array using the drive lines (D1-D11) and the sense lines (S1-

S8). The outputs of the internal 5-stage scancounter are

conditionally encoded by the ALPHA, SHIFT, and CONTROL

inputs (Table 1, Table 3) and are used to drive the D1-D11

output lines high one at a time. Each D1-D11 output may

drive up to eight keys, which are sampled by the sense line

inputs (S1-S8). The S1-S8 inputs are enabled by the internal

3-stage scancounter.

The control logic interfaces with the CDP1800-series I/O and

timing signals to establish timing and status conditions for

the CDP1871A.

The TPB input clocks the scancounters and is also used to

reset the Data Available output (DA). When a valid keydown

condition is detected on a sense line, the control logic inhib-

its the clock to the scancounters on the next low-to-high tran-

sition of TPB and the DA output is set low. The scancounter

outputs (C1 - C8) represent the ASCII and HEX key codes

and are used to drive the BUS 0 - BUS 7 outputs, which

interface directly to the CDP1800-Series data bus. The BUS

0 - BUS 7 outputs, which are normally three-stated, are

enabled by decoding the CS inputs during a CPU input

instruction (Table 2). The low-to-high transition of TPB during

the input instruction resets the DA output high. Once the DA

output has been reset, it cannot go low again until the

present key is released and a new keydown condition is

detected. (This prevents unwanted repeated keycode out-

puts which may be caused by fast software routines).

After the depressed key is released and the debounce delay

(determined by RX, CX) has occurred, the scan clock inhibit

is removed, allowing the scancounters to advance on the fol-

lowing high-to-low transitions of TPB. This provides an N-key

lockout feature, which prevents the entry of erroneous codes

when two or more keys are pressed simultaneously. The first

key pressed in the scanning order is recognized, while all

other keys pressed are ignored until the first key is released

TABLE 4. HEXIDECIMAL VALUES OF ASCII CHARACTERS

BITS

MSD

b7

00001111

b6

00110011

b5

01010101

b4

b3

b2

b1

HEX

01234567

LSD

0000

0

NUL

DLE

SP

0

@

P

\

p

0001

1

SOH

DC1

!

1

A

Q

a

q

0010

2

STX

DC2

“

2

B

R

b

r

0011

3

ETX

DC3

#

3

C

S

c

s

0100

4

EOT

DC4

$

4

D

T

d

t

0101

5

ENQ

NAK

%

5

E

U

e

u

0110

6

ACK

SYN

&

6

F

V

f

v

0111

7

BEL

ETB

/

7

G

W

g

w

1000

8

BS

CAN

(

8

H

X

h

x

1001

9

HT

EM

)

9

I

Y

i

y

1010

A

LF

SUB

*

:

J

Z

j

z

1011

B

VT

ESC

+

;

K

[

k

{

1100

C

FF

FS

,

<

L

\

l

|

|

1101

D

CR

GS

-

=

M

]

m

}

1110

E

SO

RS

.

>

N

↑

n~

1111

F

SI

US

/

?

O

-

o

DEL

CDP1871A, CDP1871AC