AUTOMOTIVE MICROCONTROLLER MCS 51
RESET
Reset input A high on this pin for two machine cy-
cles while the oscillator is running resets the device
XTAL1
Input to the inverting oscillator amplifier
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
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
XTAL2
Output from the inverting oscillator amplifier
OSCILLATOR CHARACTERISTICS
XTAL1 and XTAL2 are the input and output respec-
tively of an 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 grounded while XTAL2 is driven
as shown in Figure 5 There are no requirements on
the duty cycle of the external clock signal since the
input to the internal clocking circuitry is through a
divide-by-two flip-flop but minimum and maximum
high and low times specified on the datasheet must
be observed
PSEN
Program Store Enable is the read strobe to external
Program Memory
When the device 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
Memory
EA V
PP
External Access enable EA must be strapped to V
SS
in order to enable any MCS 51 microcontroller de-
vice to fetch code from external Program memory
locations 0 to 0FFFH (0 to 1FFFH in the 8032AH
and 8052AH)
270499 –6
Figure 5 External Drive Configuration
270499 –5
C1 C2
e
30 pF
g
10 pF for Crystals
For Ceramic Resonators contact resonator manufacturer
Figure 4 Oscillator Connections
5
INTEL [ INTEL CORPORATION ]
