Loading...
MOTOROLA M68040 USER’S MANUAL 7- 53
how the LOCKE signal can be used to end a locked sequence and to yield the bus one
bus cycle earlier than is normally possible. Figure 7-35 illustrates the state diagram of a
hypothetical external arbiter design.
BB Λ LOCK Λ LOCKE*
STATE A
STATE B
STATE C
STATE D
BR1* V BR1 Λ
LOCK Λ LOCKE*
BB Λ LOCK* V BB Λ
LOCK Λ LOCKE
BG1*, BG2 BG1, BG2*
BR2 Λ LOCK Λ LOCKE
V BR2 Λ LOCK*
BR2* V
BR2 Λ LOCK Λ LOCKE
BB*
BG1*, BG2 BG1, BG2*
BB Λ LOCK* V BB Λ
LOCK Λ LOCKE
BB*
BR1 Λ LOCK Λ LOCKE
V BR1 Λ LOCK*
Because this example uses two MC68040s, 1 and 2 refer to the processor and its signals.
*Indicates the signal is asserted for that device.
NOTES:
1.
2.
BB Λ LOCK Λ LOCKE*
Figure 7-35. Dual M68040 Fairness Arbitration State Diagram
Assuming that processor 1 currently owns the bus, the external arbiter is in state A. If
processor 2 asserts BR2, then processor 1 behaves in one of three ways:
1. If processor 1 is currently in the middle of a nonlocked bus access, then the external
arbiter proceeds to state B, in which BG1 is negated and BG2 is asserted. The
external arbiter then proceeds to state C only when BB is negated, signifying the end
of the bus cycle.
2. If processor 1 is currently in the middle of a locked bus access, then the external
arbiter stays in state A until LOCKE is asserted. Once LOCKE is asserted, the
external arbiter enters state B, in which BG1 is negated and BG2 is asserted. The
external arbiter proceeds to state C once BB is negated, signifying the end of the
bus cycle.
3. If processor 1 is in one of the three boundary conditions, then the external arbiter
proceeds to state B. During state B, the external arbiter checks for the possibility of a
newly initiated locked bus access. If it detects a locked bus cycle, it returns the bus
to processor 1 by entering state A. Note that even though processor 1 recognizes
BG1 is asserted, it does not take the bus because processor 1 asserts BB whenever
the boundary condition results in processor 1 performing another bus cycle. The
external arbiter stays in state A until LOCKE is asserted, then proceeds to state B to
Loading...
Terms of Use | Privacy Policy | DMCA Policy
2006-2020 Rsmanuals.com