Intel MCS48 User Manual page 52

SINGLE
COMPONENT
SYSTEM
Designation
RESET
WR
ALE
PSEN
SS
EA
Pin
Number
XTAL1
XTAL2
10
11
Function
Input
which
is
used
to
initialize
the processor.
Also
used
during
PROM
programming and
verification.
(Active low)
(Internal
pullup
* 200KO)
Output strobe during a
BUS
write.
(Active low)
Used
as
write
strobe
to
external data
memory.
Address
Latch Enable. This
signal occurs
once
during
each
cycle
and
is
useful
as a
clock
output.
The
negative
edge
of
ALE
strobes address
into
external
data
and program memory.
Program
Store Enable. This output occurs only during
a
fetch
to
external
program memory.
(Active
Low)
Single step
input
can be used
in
conjunction
with
ALE
to "single step" the processor through each
instruction.
(Active
Low)
(Internal
pullup
* 300KO)
External
Access
input
which
forces
all
program
memory
fetches
to
reference
external
memory.
Useful
for
emula-
tion
and debug, and
essential
for testing
and program
verification.
(Active High)
(Internal
pullup
*
10MO
on
8048/8049, 8035L,
8039
only)
One
side of crystal input for internal
oscillator.
Also
input
for external source.
Other
side of crystal/external source
input.
Unless otherwise
stated inputs
do
not
have
internal pullup
resistors.
2.3
Programming,
Verifying
and
Erasing
EPROM
The
internal
Program
Memory
of
the
8748
may
be
erased
and reprogrammed
by
the user
as explained
in
the following sections. See
also the 8748
data
sheet.
2.3.1
Programming/Verification
In brief,
the
programming
process
consists
of:
activating the
program mode,
applying
an
address,
latching the address, applying
data,
and
applying
a
programming
pulse.
Each
word
is
programmed
completely before moving
on
to
the next
and
is
followed by a
verifica-
tion step.
The
following
is
a
list
of
the pins
used
for
programming and
a
description
of
their
functions:
Pin
XTAL1
Reset
TestO
EA
BUS
P20-1
Vdd
PROG
Function
Clock
Input
(1
to
6MHz)
Initialization
and Address
Latching
Selection
of
Program
(0V)
or
Verify
(5V)
Mode
Activation
of
Program/Verify
Modes
Address and Data
Input
Data Output
During
Verify
Address
Input
Programming Power
Supply
Program
Pulse
Input
2-16