MC68HC000RC12 Datasheet, PDF (62/189 Pages) Freescale Semiconductor, Inc – 8-/16-/32-Bit Microprocessors User’s Manual

Freescale Semiconductor, Inc.
INTERNAL SIGNAL VALID
EXTERNAL SIGNAL SAMPLED
CLK
BR (EXTERNAL)
47
BR (iNTERNAL)
Figure 5-17. External Asynchronous Signal Synchronization
Bus arbitration control is implemented with a finite-state machine. State diagram (a) in
Figure 5-18 applies to all processors using 3-wire bus arbitration and state diagram (b)
applies to processors using 2-wire bus arbitration, in which BGACK is permanently
negated internally or externally. The same finite-state machine is used, but it is effectively
a two-state machine because BGACK is always negated.
In Figure 5-18, input signals R and A are the internally synchronized versions of BR and
BGACK. The BG output is shown as G, and the internal three-state control signal is shown
as T. If T is true, the address, data, and control buses are placed in the high-impedance
state when AS is negated. All signals are shown in positive logic (active high), regardless
of their true active voltage level. State changes (valid outputs) occur on the next rising
edge of the clock after the internal signal is valid.
A timing diagram of the bus arbitration sequence during a processor bus cycle is shown in
Figure 5-19. The bus arbitration timing while the bus is inactive (e.g., the processor is
performing internal operations for a multiply instruction) is shown in Figure 5-20.
When a bus request is made after the MPU has begun a bus cycle and before AS has
been asserted (S0), the special sequence shown in Figure 5-21 applies. Instead of being
asserted on the next rising edge of clock, BG is delayed until the second rising edge
following its internal assertion.
5-16
M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL
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