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MOTOROLA
MC68349 USER'S MANUAL
3- 53
BR may be issued any time during a bus cycle or between cycles. BG is asserted in
response to BR. To guarantee operand coherency, BG is only asserted at the end of an
operand transfer. Additionally, BG is not asserted until the end of a read-modify-write
operation (when RMC is negated) in response to a BR signal. When the requesting device
receives BG and more than one external device can be bus master, the requesting device
should begin whatever arbitration is required. When the external device assumes bus
mastership, it asserts BGACK and maintains BGACK during the entire bus cycle (or
cycles) for which it is bus master. The following conditions must be met for an external
device to assume mastership of the bus through the normal bus arbitration procedure: 1) it
must have received BG through the arbitration process, and 2) BGACK must be inactive,
indicating that no other bus master has claimed ownership of the bus.
Figure 3-34 is a flowchart showing bus arbitration for a single device. This technique
allows processing of bus requests during data transfer cycles. Refer to Figures 3-35 and
3-36 for bus arbitration timing diagrams.
BR is negated at the time that BGACK is asserted. This type of operation applies to a
system consisting of the MC68349 and one device capable of bus mastership. In a system
having a number of devices capable of bus mastership, BR from each device can be wire -
ORed to the MC68349. In such a system, more than one bus request could be asserted
simultaneously. BG is negated a few clock cycles after the transition of BGACK. However,
if bus requests are still pending after the negation of BG , the MC68349 asserts another
BG within a few clock cycles after it was negated. This additional assertion of BG allows
external arbitration circuitry to select the next bus master before the current bus master
has finished using the bus. The following paragraphs provide additional information about
the three steps in the arbitration process. Bus arbitration requests are recognized during
normal processing, HALT assertion, and a CPU32+ halt caused by a double bus fault.