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Bus Operation
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MCF5206 USERS MANUAL Rev 1.0
MOTOROLA
6.9.2 Multiple External Bus Master Arbitration Protocol (Three-Wire
Mode)
The three-wire mode of bus arbitration allows the MCF5206 to share the external bus with
any number of external bus masters. In this mode, an external arbiter must be provided
to assign priorities to each of the possible bus masters and determine which master
should be allowed use of the external bus. The bus arbitration signals of the MCF5206,
BR, BD, and BG connect to the bus arbiter, allowing the bus arbiter to control use of the
external bus by the MCF5206.
The MCF5206 requests the bus from the external bus arbiter by asserting bus request
(BR) whenever an internal bus request is pending (the ColdFire core requests an access).
The MCF5206 continues to assert BR until after the start of the external bus transfer. The
MCF5206 can negate BR at any time regardless of the bus grant (BG) status. If the bus
is granted to the MCF5206 when an internal bus request is generated, the MCF5206
asserts bus driven (BD) simultaneously with transfer start, allowing the access to begin
immediately. The MCF5206 always drives BR and BD. They cannot be directly wire-ORed
with other devices.
The external arbiter asserts BG to indicate to the MCF5206 that it has been granted the
bus and may begin a bus cycle after the rising edge of the next CLK. If BG is negated while
a bus cycle is in progress, the MCF5206 relinquishes the bus at the completion of the bus
cycle, except if the bus lock (BL) bit in the SIMR is set. To guarantee that the bus is
relinquished, BL must be cleared and BG must be negated prior to the rising edge of the
CLK in which the last TA, TEA or internal asynchronous transfer acknowledge is asserted.
Note that the MCF5206 considers any series of bus transfers of a burst or a burst-inhibited
transfer to be a single bus cycle and does not relinquish the bus until completion of the
last transfer of the series.
When the bus has been granted to the MCF5206 in response to the assertion of BR, one
of two situations can occur. In the first case, the MCF5206 has an internal bus request
pending, the MCF5206 asserts BD to indicate explicit bus ownership and begins the
pending bus cycle by asserting TS. The MCF5206 continues to assert BD until the
external bus master negates BG, after which BD is negated at the completion of the bus
cycle. As long as BG is asserted, BD remains asserted to indicate the bus is owned by
the MCF5206 and the MCF5206 continuously drives the address bus, attributes and
control signals.
In the second situation, the bus is granted to the MCF5206, but the MCF5206 does not
have an internal bus request pending and the bus lock bit in the SIMR is cleared. In this
case, the MCF5206 takes implicit ownership of the bus. Implicit ownership of the bus
occurs when the MCF5206 is granted the bus, but there are no pending bus cycles. The
MCF5206 does not drive the bus and does not assert BD if the bus is implicitly owned. If
an internal bus request is generated or the bus lock bit in the SIMR is set to 1, the
MCF5206 assumes explicit ownership of the bus. If explicit ownership was assumed due
to an internal request being generated, the MCF5206 immediately begins an access and
simultaneously asserts BD and TS. If explicit ownership was assumed due to the bus lock
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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