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8XC51FX Datasheet(PDF) 5 Page - Intel Corporation

No. de Pieza. 8XC51FX
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Fabricante  INTEL [Intel Corporation]
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8XC51FX Datasheet(HTML) 5 Page - Intel Corporation

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Port 3 also serves the functions of various special
features of the MCS-51 Family as listed below
Port Pin
Alternate Function
RXD (serial input port)
TXD (serial output port)
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)
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-
tion when a minimum VIH1 voltage is applied wheth-
er the oscillator is running or not An internal pull-
down resistor permits a power-on reset with only a
capacitor connected to VCC
Address Latch Enable output pulse for latching
the low byte of the address during accesses to ex-
ternal memory This pin (ALEPROG) is also the
program pulse input during EPROM programming for
the 87C51FX
In normal operation ALE is emitted at a constant
rate of
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
Throughout the remainder of this data sheet ALE
will refer to the signal coming out of the ALEPROG
pin and the pin will be referred to as the ALEPROG
Program Store Enable is the read strobe to
external Program Memory
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-
EA VPP External Access enable EA must be
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
EA should be strapped to VCC for internal program
This pin also receives the programming supply volt-
age (VPP) during EPROM programming
Input to the inverting oscillator amplifier
Output from the inverting oscillator amplifi-
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-
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
capacitance Once the external signal meets the VIL
and VIH specifications the capacitance will not ex-
ceed 20 pF

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