7-48
M68040 USER’S MANUAL
MOTOROLA
Table 7-6. M68040 Bus Arbitration States
BB
BG
State
Conditions
Negated
Negated
Idle
M68040 three-states
BB
; arbiter negates
BG
; bus is not driven.
Negated
Asserted
Implicit Ownership
M68040 three-states
BB
; arbiter asserts
BG
; bus is driven with undefined values.
Asserted
Negated
Active Bus Cycle
M68040 asserts
BB
; arbiter asserts
BG
;
bus is driven with defined values;
TIP
is asserted.
Asserted
Asserted
Park
M68040 asserts
BB
; arbiter asserts
BG
;
bus is driven with undefined values;
TIP
is
asserted.
Asserted
Asserted
Alternate Bus Master Ownership
and Snooped
M68040 three-states
BB
; arbiter asserts
BG
; M68040 does not drive the bus.
The M68040 can be in the active bus cycle, park, or implicit ownership states when
BG
is
negated. Depending on the state the processor is in when
BG
is negated, uncertain
conditions can occur. The only guaranteed time that the processor relinquishes the bus is
when
BG
is negated prior to the rising edge of BCLK in which the last
TA
or
TEA
is
asserted and the processor is in the active bus cycle state. However, if the processor is in
either the active bus cycle, park, or implicit ownership states and
BG
is negated at the
same time or after the last
TA
or
TEA
is asserted, then from the standpoint of the external
bus arbiter, the next action that the processor takes is undetermined because the
processor can internally decide to perform another active bus cycle (indeterminate
condition).
External bus arbiters must consider this indeterminate condition when negating
BG
and
must be designed to examine the state of
BB
immediately after negating
BG
to determine
whether or not the processor will run another bus cycle. A somewhat dangerous situation
exists when the processor begins a locked transfer after the bus has been granted to the
alternate bus master, causing the alternate bus master to perform a bus transfer during a
locked sequence. To correct this situation, the external bus arbiter must be able to
recognize the possible indeterminate condition and reassert
BG
to the processor when the
processor begins a locked sequence. The indeterminate condition is most significant when
dealing with systems that cannot allow locked transfers to be broken. Figure 7-31
illustrates an example of an error condition that is a consequence of the interaction
between the indeterminate condition and a locked transfer. External bus arbiters must be
designed so that all bus grants to all bus masters be nagated for at least one rising edge
of BCLK between bus tenures; preventing bus conflicts resulting from the above
conditions.