P87C511 datasheet(5/21 Pages) INTEL

5 / 21 page

87C5180C51BH80C31BH

In normal operation ALE is emitted at a constant

rate of 16 the oscillator frequency and may be

used for external timing or clocking purposes Note

however that one ALE pulse is skipped during each

access to external Data Memory

PSEN

Program Store Enable is the Read strobe to

External Program Memory When the 87C51BH is

executing from Internal Program Memory PSEN is

inactive (high) When the device is executing code

from External Program Memory PSEN is activated

twice each machine cycle except that two PSEN

activations are skipped during each access to Exter-

nal Data Memory

EA VPP External Access enable EA must be

strapped to VSS in order to enable the 87C51BH to

fetch code from External Program Memory locations

starting at 0000H up to FFFFH Note however that

if either of the Lock Bits is programmed the logic

level at EA is internally latched during reset

EA must be strapped to VCC for internal program

execution

This pin also receives the programming supply volt-

age (VPP) during EPROM programming

XTAL1

Input to the inverting oscillator amplifier

XTAL2

Output from the inverting oscillator amplifi-

272335 – 5

Figure 3 Using the On-Chip Oscillator

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

To drive the device from an external clock source

XTAL1 should be driven while XTAL2 is left uncon-

nected as shown in Figure 4 There are no require-

ments 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

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

272335 – 6

Figure 4 External Clock Drive

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