AUTOMOTIVE 80C51FA/83C51FA
addresses (MOVX
@
Ri), Port 2 emits the contents of
the P2 Special Function Register.
Port 3
:
Port 3 is an 8-bit bidirectional I/O port with
internal pullups. The Port 3 output buffers can drive
LS TTL inputs. Port 3 pins that have 1's written to
them are pulled high by the internal pullups, and in
that state can be used as inputs. As inputs, Port 3
pins that are externally pulled low will source current
(I
IL
, on the datasheet) because of the pullups.
Port 3 also serves the functions of various special
features of the MCS 51 microcontroller family, as
listed below:
Port Pin
Alternate Function
P3.0
RXD (serial input port)
P3.1
TXD (serial output port)
P3.2
INT0 (external interrupt 0)
P3.3
INT1 (external interrupt 1)
P3.4
T0 (Timer 0 external input)
P3.5
T1 (Timer 1 external input)
P3.6
WR (external data memory write strobe)
P3.7
RD (external data memory read strobe)
RESET
:
Reset input. A high on this pin for two ma-
chine cycles while the oscillator is running resets the
device. An internal pulldown resistor permits a pow-
er-on reset with only a capacitor connected to V
CC
.
ALE/PROG
:
Address Latch Enable output pulse for
latching the low byte of the address during accesses
to external memory.
In normal operation ALE is emitted at a constant
rate of
(/6
the oscillator frequency, and may be used
for external timing or clocking purposes. Note, how-
ever, that one ALE pulse is skipped during each ac-
cess to external Data Memory.
Throughout the remainder of this datasheet, ALE will
refer to the signal coming out of the ALE/PROG pin,
and the pin will be referred to as the ALE/PROG pin.
PSEN
:
Program Store Enable is the read strobe to
external Program Memory.
When the 83C51FA is executing code from external
Program Memory, PSEN is activated twice each ma-
chine cycle, except that two PSEN activations are
skipped during each access to external Data Memo-
ry.
EA/V
PP
:
External Access enable. EA must be
strapped to V
SS
in order to enable the device to
fetch code from external Program Memory locations
0000H to 0FFFFH. Note, however, that if either of
the Program Lock bits are programmed, EA will be
internally latched on reset.
EA should be strapped to V
CC
for internal program
executions.
XTAL1:
Input to the inverting oscillator amplifier.
XTAL2
:
Output from the inverting oscillator amplifi-
er.
OSCILLATOR CHARACTERISTICS
XTAL1 and XTAL2 are the input and output, respec-
tively, of a inverting amplifier which can be config-
ured for use as an on-chip oscillator, as shown in
Figure 4. Either a quartz crystal or ceramic resonator
may be used. More detailed information concerning
the use of the on-chip oscillator is available in Appli-
cation Note AP-155, ``Oscillators for Microcontrol-
lers.''
To drive the device from an external clock source,
XTAL1 should be driven, while XTAL2 floats, as
shown in Figure 5. There are no requirements on the
duty cycle of the external clock signal, since the in-
put to the internal clocking circuitry is through a di-
vide-by-two flip-flop, but minimum and maximum
high and low times specified on the datasheet must
be observed.
An external oscillator may encounter as much as a
100 pF load at XTAL1 when it starts up. This is due
to interaction between the amplifier and its feedback
capacitance. Once the external signal meets the V
IL
and V
IH
specifications the capacitance will not ex-
ceed 20 pF.
270501±5
C1, C2
e
30 pF
g
10 pF for Crystals
For Ceramic Resonators, contact resonator manufacturer.
Figure 4. Oscillator Connections
5
