Intel MCS48 User Manual page 357

8278
INTERFACE CONSIDERATIONS
Scanned
Keyboard
Mode
With N-key
rollover
each key
depression
is
treated
independently from
all
others.
When
a
key
is
depressed
the
debounce
logic
waits for a
full
scan
of
128 keys
and
then
checks
to
see
if
the
key
is still
down.
If it
is,
the
key
is
entered
into the FIFO.
1
ti
r
x
?
r
x
n
n tl_ji
RL
SAMPLED
Figure
3.
Keyboard Timing
If
two key
closures
occur
during the
same
scan
the
ERROR
output
is
set,
the
KE
flag
is
set
in
the Status word,
the
TONE
output
is
activated
and IRQ
is
set,
and no
further
inputs are
accepted. This condition
is
cleared by a high
signal
on
the
CLEAR
input or by
a
system
RESET
input or
by
the
CLEAR ERROR
command.
In
the special function
mode
both
the
key closure and
the
key
release
cause an entry
to
the FIFO.
The
release
is
entered with the
MSB=1.
Any
key entry triggers the
TONE
output
for
10ms.
The HYS
and
KCL
outputs enable the analog
multiplexer
and
detector
to
be synchronized
for interface to
capacitive
coupled keyboards.
SCAN CYCLE
|
IRQ
KEY
1
DEPRESSED
KEY
1
KEY
1
ENTERED READ
BY
MASTER
KEY
2
KEY
3
DEPRESSED DEPRESSED
Figure
4.
Key
Entry
and
Error Timing
8-67
00227A