Datasheet

Bus Operation
6-58 MCF5206e USER’S MANUAL MOTOROLA
When the bus has been removed from the MCF5206e, one of two situations can occur. In
the first case, the bus lock bit in the SIM Configuration Register (SIMR) is cleared and the
MCF5206e has implicit ownership of the bus. When the external bus master negates BG,
the MCF5206e negates BD and three-state the address, data, TS, R/W, and SIZ signals
after completing the current bus cycle. Figure 6-35 illustrates two-wire bus arbitration with
the bus lock bit cleared.
Figure 6-35. Two-Wire Bus Arbitration with Bus Lock Negated
In Figure 6-35 during clocks C1 and C2, the external master is the bus owner. During C3,
the external master relinquishes control of the bus by asserting BG to the MCF5206e. At
this point, the bus lock bit is cleared, but because there is an internal access pending, the
MCF5206e asserts BD during C4 and begins the access. Thus, the MCF5206e becomes
the explicit master of the external bus. This access is a burst-inhibited access. During C5,
the external master removes the grant from the MCF5206e by negating BG
. Because the
MCF5206e is performing a burst-inhibited access, it continues to assert BD
until the final
transfer of the access has completed. The MCF5206e negates BD during C8, returning
ownership of the external bus to the external master.
In the second case, the bus lock bit in the SIM Configuration Register (SIMR) is set to 1
and the MCF5206e has explicit ownership of the bus. In this case, when the external bus
master negates BG
, the MCF5206e continues to assert BD and continues to drive
address, attributes, and control signals. The MCF5206e retains mastership of the bus until
the bus lock bit in the SIM Configuration Register (SIMR) is cleared. By setting the bus
lock bit to 1, you can select the MCF5206e to be the highest priority master, even when
CLK
TS
TRANSFER
TA
D[31:0]
A[4:2]
AT TRIB UT ES
BG
BD
EXTERNAL
MASTER MCF5206e
EXTERNAL
MASTER
BUS LOCK BIT
C1 C2
C3
C4 C5 C6 C7 C8 C9
Fr
eescale S
emiconduct
or
, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
nc...