Datasheet

Bus Operation
6-54 MCF5206e USER’S MANUAL MOTOROLA
NOTE
If TA is asserted when transfer error-acknowledge (TEA) is
asserted, the transfer is terminated with a bus error.
NOTE
For the MCF5206e to accept the transfer as successful with
an ATA, TEA must be negated until the internal asynchronous
transfer acknowledge is asserted or the transfer is completed
with a bus error.
6.9 BUS ARBITRATION
The MCF5206e bus protocol provides for one bus master at a time: either the MCF5206e
or an external device. If more than one external bus master is connected to the bus, an
external arbiter can prioritize requests and determine which device is granted access to
the bus. Bus arbitration is the protocol by which the MCF5206e or an external device
becomes the bus master. When the MCF5206e is the bus master, it uses the bus to read
instructions and transfer data not contained in its internal cache or memory to and from
external memory. When an external bus master owns the bus, the MCF5206e can monitor
the external bus master’s transfers and assert chip select and DRAM control, and transfer
termination signals. This capability is discussed in more detail in Section 6.10 External
Bus Master Operation.
The MCF5206e bus arbitration can be used in two modes. A two-wire mode is provided
for systems where the MCF5206e and a single external bus master are the only two
masters arbitrating for use of the external bus. This arbitration mode uses the bus grant
(BG
) and bus driven (BD) signals. The bus request (BR) signal can be ignored by the
external bus master.
The second mode is provided for systems where multiple external bus masters are
arbitrating for use of the external bus. This arbitration mode requires an external bus
arbiter and uses the bus grant (BG), bus driven (BD) and bus request (BR) signals to
control usage of the external bus.
In either arbitration mode, the bus arbitration unit in the MCF5206e operates
synchronously and transitions between states on the rising edge of CLK.
For systems where the MCF5206e is the only possible bus master, the bus can be
continuously granted to the MCF5206e by tying bus grant (BG) to GND. An arbiter is not
required.
6.9.1 Two Master Bus Arbitration Protocol (Two-Wire Mode)
The two-wire mode of bus arbitration allows the MCF5206e to share the external bus with
a single external bus master without requiring an external bus arbiter. Figure 6-33 is a
block diagram showing the MCF5206e connecting to an external bus master using the
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