MCS” 51
CONTROLLER
w
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 Memo-
ry
~/Vpp:
External Access enable ~
must be
strapped to VSS in order to enable any MCS 51 de-
vice to fetch code from external Program memory
locations starting at OOOOH to FFFFH. ~ must
up
be strapped to VCCfor internal program execution.
Note, however, that if the Security Bit in the EPROM
devices is programmed, the device will not fetch
code from any location in external Program Memory.
This pin also receives the programming supply volt-
age (VPP) during programming of the EPROM parts.
C2
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To drive the device from an external clock source,
XTAL1 should be grounded, while XTAL2 is driven,
as shown in Figure 4. 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 data sheet must
be observed.
EXTERNAL
OSCILLATOR
SIGNAL
XTAL2
XTAL1
Vss
272318-3
Cl, C2 = 30 PF +10 PF for Crystals
For Ceramic Resonators contact resonatormanufacturer.
El
XTAL2
n
XTAL1
cl
Vss
=
272318-4
Figure 4. External Drive Configuration
EXPRESS Version
The Intel EXPRESS system offers enhancements to
the operational specifications of the MCS 51 family
of microcontrollers. These EXPRESS products are
designed to meet the needs of those applications
whose operating requirements exceed commercial
standards.
The EXPRESS program includes the commercial
standard temperature range with burn-in, and an ex-
tended temperature range with or without burn-in.
With the commercial standard temperature range,
operational characteristics are guaranteed over the
temperature range of O“C to + 70”C. With the ex-
tended temperature range option, operational char-
acteristics are guaranteed over a range of –40”C to
+ 85”C.
The optional burn-in is dynamic, for a minimum time
of 160 hours at 125°C with VCC = 5.5V * 0.25V,
following guidelines in MIL-STD-883, Method 1015.
Package types and EXPRESSversions are identified
by a one- or two-letter prefix to the part number. The
prefixes are listed in Table 1.
For the extended temperature range option, this
data sheet specifies the parameters which deviate
from their commercial temperature range limits.
Figure 3. Oscillator Connections
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 an 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-
Iers,” Order No, 230659.
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INTEL [ INTEL CORPORATION ]
