SH7021 datasheet(176/509 Pages) RENESAS

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Bus released

BREQ received

Bus acquisition

BREQ = low

BACK = low

acknowledge

BACK

Bus request

External device

SuperH

Bus release response

Address, data, strobe pin:

High impedance

Strobe pin:

High-level output

Figure 8.35 Bus Release Procedure

8.10.1

The Operation of Bus Arbitration

This LSI has the bus arbitration function which can give bus ownership to an external device when

the device requests the bus ownership. When

BREQ is input and the bus cycle being executed by

the CPU or DMAC is completed,

BACK becomes low and a bus is released for an external device.

At this time, the following operates when bus arbitration conflicts with refresh.

1. If DRAM refresh is requested in this LSI when a bus is released and

BACK is low, BACK

becomes high and the occurrence of the refresh request can be informed externally. At this

time, the external device may generate a bus cycle when

BREQ is low even if BACK is high.

Therefore, a bus remains released to the external device. Then, when

BREQ becomes high, this

LSI gets bus ownership, and executes refresh and the bus cycle of the CPU or DMAC. After

the external device gets bus ownership and

BACK is low, refresh is requested when BACK

becomes high even if the low level of

BREQ is input. Therefore, turn BREQ high immediately

to release a bus for this LSI to hold DRAM data (See figure 8.36).

2. When

BREQ changes from high to low and internal refresh is requested at the timing of the

bus release of this LSI,

BACK may remain high (do not become low). A bus is released to the

external device since the low level of

BREQ is input. This operation is based on the above

specification (1). To hold DRAM data, turn

BREQ high and release a bus to this LSI

immediately when the external device detects that

BACK does not change to low during a

fixed time this LSI (See figure 8.37). When a refresh request is generated and

BACK returns

to high, as shown in figure 8.37, a momentary narrow pulse-shaped spike may be output where

BACK was originally supposed to become low.

SH7021 Datasheet(PDF) 176 Page - Renesas Technology Corp

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