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6.7
Bus Release
6.7.1
Overview
The chip can release the external bus in response to a bus request from an external device. In the
external bus released state, the internal bus master continues to operate as long as there is no
external access.
If an internal bus master wants to make an external access in the external bus released state, it can
issue a bus request off-chip.
6.7.2
Operation
In external expansion mode, the bus can be released to an external device by setting the BRLE bit
in BCRL to 1. Driving the BREQ pin low issues an external bus request to the chip. When the
BREQ pin is sampled, at the prescribed timing the BACK pin is driven low, and the address bus,
data bus, and bus control signals are placed in the high-impedance state, establishing the external
bus released state.
In the external bus released state, an internal bus master can perform accesses using the internal
bus. When an internal bus master wants to make an external access, it temporarily defers
activation of the bus cycle, and waits for the bus request from the external bus master to be
dropped.
If the BREQOE bit in BCRL is set to 1, when an internal bus master wants to make an external
access in the external bus released state, the BREQO pin is driven low and a request can be made
off-chip to drop the bus request.
When the BREQ pin is driven high, the BACK pin is driven high at the prescribed timing and the
external bus released state is terminated.
If an external bus release request and external access occur simultaneously, the order of priority is
as follows:
(High) External bus release > Internal bus master external access (Low)
Rev. 5.00, 12/03, page 177 of 1088
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