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MCS’5’-51 ARCHITECTURAL OVERVIEW
Examples of how to drive the clock with an external
Machine Cycles
oscillator are shown in Figure 14. Note that in the
HMOS devices(S051, etc.) the signal at the XTAL2 pin
A machine cycle consists of a sequence of 6 statea,
actually drives the internal clock generator. In the
numbered S1 through S6. Each state time lasts for two
CHMOS devices (SOC5lBH, ete.) the signsl at the
XTAL1 pin drives the internal clock generator. If only
oscillator periods. Thus a machine cycle takes 12 Oscil-
lator periods or 1 ps if the oscillator frequency is
one pin is going to be driven with the external oscillator
12 MHz.
signal, make sure it is the right pin.
Each state is divided into a Phase 1 half and a Phase 2
The internal clock generator defmea the sequence of
half. Figure 15 shows the fetch/execute sequences in
states that make up the MCS-51 machine cycle.
L
51 52 as se as .% s 52 as S4.SE as 51
Plm Prps PIP2 PIPS PIPs
Pips PIPS Pips PIP2
mm
P2 PIPS
Pips
(%L)
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270251-15
Figure 15. Stete Sequences in MCS@’-5l Devices
1-18
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