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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
12-46
Freescale Semiconductor
Each bus master must have bus request, bus grant, and bus busy
signals. The signals are described in detail
in
Section 12.2.1, “Detailed Signal Descriptions
.” The device that needs the bus asserts the bus request
(BR) signal. The device then waits for the arbiter to assert the bus grant (BG) signal. In addition, the new
master must sample the bus busy (BB) signal to ensure that no other master is driving the bus before it can
assert bus busy to assume ownership of the bus. The new master must sample bus busy negated for two
cycles before asserting bus busy, to avoid any potential conflicts. Any time the arbiter has taken the bus
grant away from the master, and the master wants to execute a new cycle, the master must re-arbitrate
before a new cycle can begin. The EBI, however, whether the internal or external arbiter is used,
guarantees data coherency for access to a small port size and for decomposed bursts. This means that the
EBI does not release the bus before the completion of the transactions which are considered as atomic.
describes the basic protocol for bus arbitration.
Figure 12-32. Bus Arbitration Flow Chart
12.4.2.8.1
External (or Central) Bus Arbiter
The external arbiter can be either another MCU in a two master system, or a separate central arbiter device.
When an MCU is configured to use external arbitration, that MCU asserts BR when it needs ownership of
the external bus, and it waits for BG to be asserted from the external arbiter. For timing reasons, a latched
(1 cycle delayed) version of BG is used by the EBI in external arbitration mode. This is not a requirement
of the protocol. After BG assertion is received and BB is sampled negated for two cycles, the MCU asserts
BB and initiates the transaction. An MCU operating under external arbitration may run back-to-back
accesses without rearbitrating as long as it is still receiving BG asserted. If BG is negated during a
transaction, the MCU must rearbitrate for the bus before the next transaction. The determination of priority
between masters is determined entirely by the external arbiter in this mode.
shows example timing for the case of two masters connected to a central arbiter. In this case,
the BR0 and BR1 signals shown are inputs to the arbiter from the BR pin of each master. The BG0 and
BG1 signals are outputs from the arbiter that connect to the BG pin of each master.
1. Wait for
BB
to be negated for 2 cycles.
2. Assert
BB
to become next master.
3. Negate
BR
(may choose to keep it
asserted if another request is pending).
Acknowledge Bus Mastership
1. Assert
BR
.
Request the Bus
1. Perform Data Transfer.
Operate as Bus Master
1. Negate
BB
.
Release Bus Mastership
1. Negate
BG
(may choose to keep it
Terminate Arbitration
1. Assert
BG
.
Grant Bus Arbitration
asserted to park bus master).
Requesting Device
Arbiter
Содержание MPC5553
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