8XC51FX
Port 3 also serves the functions of various special
features of the MCS-51 Family, as listed below:
When the 8XC51FX 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.
Port Pin
Alternate Function
RXD (serial input port)
TXD (serial output port)
P3.0
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
EA/V
: External Access enable. EA must be
PP
strapped to VSS in order to enable the device to
fetch code from external Program Memory locations
0000H to 0FFFH. Note, however, that if either of the
Program Lock bits are programmed, EA will be inter-
nally latched on reset.
INT0 (external interrupt 0)
INT1 (external interrupt 1)
T0 (Timer 0 external input)
T1 (Timer 1 external input)
WR (external data memory write strobe)
RD (external data memory read strobe)
EA should be strapped to V
executions.
for internal program
CC
RST: Reset input. A high on this pin for two machine
cycles while the oscillator is running resets the de-
vice. The port pins will be driven to their reset condi-
This pin also receives the programming supply volt-
age (V ) during EPROM programming.
PP
tion when a minimum V
voltage is applied wheth-
IH1
XTAL1: Input to the inverting oscillator amplifier.
er the oscillator is running or not. An internal pull-
down resistor permits a power-on reset with only a
XTAL2: Output from the inverting oscillator amplifi-
er.
capacitor connected to V
.
CC
ALE: Address Latch Enable output pulse for latching
the low byte of the address during accesses to ex-
ternal memory. This pin (ALE/PROG) is also the
program pulse input during EPROM programming for
the 87C51FX.
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 3. 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.’’
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.
If desired, ALE operation can be disabled by setting
bit 0 of SFR location 8EH. With this bit set, the pin is
weakly pulled high. However, the ALE disable fea-
ture will be suspended during a MOVX or MOVC in-
struction, idle mode, power down mode and ICE
mode. The ALE disable feature will be terminated by
reset. When the ALE disable feature is suspended or
terminated, the ALE pin will no longer be pulled up
weakly. Setting the ALE-disable bit has no affect if
the microcontroller is in external execution mode.
To drive the device from an external clock source,
XTAL1 should be driven, while XTAL2 floats, as
shown in Figure 4. 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
divide-by-two flip-flop, but minimum and maximum
high and low times specified on the data sheet 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
Throughout the remainder of this data sheet, 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.
capacitance. Once the external signal meets the V
IL
and V specifications the capacitance will not ex-
IH
ceed 20 pF.
PSEN: Program Store Enable is the read strobe to
external Program Memory.
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