ACT apricot - Keyswitch Operation

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KEYBOARD

avoid

confusion

all

time

and

date

information

will

be

in

the

range

FOH

to

F9H

where

the

lower

nibble

contaIns

the

data.

KEYSWITCH OPERATION

The

keyswitch

array

consists

of 90 capacitive

keyswitches

and

6

membrane

keys.

The

capacitive keys are

organised

in

a

matrix

with

twelve

rows

of eight keys,

with

some

omissions,

which

are accessed

through

the

two

Keytronic proprietary chips

connected

to

ports 1

and

3 of

the

6301.

The

membrane

key

positions

are read

through

bits

o

to

5 of

port

4

on

the

6301.

The

key

array is

scanned

by

latching

the

appropriate

row

address (in

the

range 0

to

11)

into

the

driver IC

and

then

reading

the

output

of

the

detector

IC

to

determine

which

keys are depressed.

The

detector

has

eight

inputs,

thus

knowing

which

of

the

rows

has

been

strobed

the

identity

of

the

depressed

keys

can

be

determined.

The

detector

has

two

sensitivities,

which

are

selected

by

latching

row

address 14

or

15

into

the

driver,

thus

providing hysteresis

eliminating

teasing of

the

keys.

If

a

key

state

has

changed

then

the

row

is

scanned

again 250

us

later

at

the

other

sensitivity

to

give

hysteresis

and

immunity

to

external

interference.

If

the

key

is

detected

as

being

in

the

same

state

by

both

scans

and

this

state

is different

from

the

previous

state

then

the

key

position

has

changed. '

When

a

key

is

detected

as

having

been

depressed

the

appropriate

character

is

transmitted

to

the

System

Unit

(one

of

the

make

codes)

and

the

same

character

with

80H added

is

transmitted

when

the

key

is released.

The

actual

key

scan

codes

used

by

the

Keyboard

handler

of

the

BIOS are

illustrated

on

Figure 2.

The

corresponding

make

codes

in

decimal are also detailed

in

a

table

("KeyboarCl

to

System

Unit

Codes")

at

the

end

of

the

section.

The,,-capacitive

keyswitch

array

and

the

membrane

keyboard are

scanned

for changes

in

key

position

every 10

ms. Keys

that

are

newly

down

have

their

make

code placed

in

the

UART

transmit

buffer

and

also

in

the

delay stack,

which

is

eight

bytes

long.

If

a

key

is

found

to

be

newly

up

then

its

code

is

compared

with

those

on

the

delay stack.

If

it

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