SH7721 datasheet(466/1524 Pages) RENESAS

466 / 1524 page

Section 9

Bus State Controller (BSC)

Rev. 3.00 Jan. 18, 2008 Page 404 of 1458

REJ09B0033-0300

9.6

Usage Notes

(1)

Reset

The bus state controller (BSC) can be initialized completely only at power-on reset. At power-on

reset, all signals are negated and output buffers are turned off regardless of the bus cycle state. All

control registers are initialized. In standby, sleep, and manual reset, control registers of the bus

state controller are not initialized. At manual reset, the current bus cycle being executed is

completed and then the access wait state is entered. If a 16-byte transfer is performed by a cache

or if another LSI on-chip bus master module is executed when a manual reset occurs, the current

access is cancelled in longword units because the access request is cancelled by the bus master at

manual reset. If a manual reset is requested during cache fill operations, the contents of the cache

cannot be guaranteed. Since the RTCNT continues counting up during manual reset signal

assertion, a refresh request occurs to initiate the refresh cycle. In addition, a bus arbitration request

by the

BREQ signal can be accepted during manual reset signal assertion.

Some flash memories may specify a minimum time from reset release to the first access. To

ensure this minimum time, the bus state controller supports a 5-bit counter (RWTCNT). At power-

on reset, the RWTCNT is cleared to 0. After power-on reset, RWTCNT is counted up

synchronously together with CKIO and an external access will not be generated until RWTCNT is

counted up to H

′001F. At manual reset, RWTCNT is not cleared.

(2)

Access from the Site of the LSI Internal Bus Master

There are three types of LSI internal buses: a cache bus, internal bus, and peripheral bus. The CPU

and cache memory are connected to the cache bus. Internal bus masters other than the CPU and

bus state controller are connected to the internal bus. Low-speed peripheral modules are connected

to the peripheral bus. Internal memories other than the cache memory and debugging modules

such as a UBC and AUD are connected bidirectionally to the cache bus and internal bus. Access

from the cache bus to the internal bus is enabled but access from the internal bus to the cache bus

is disabled. This gives rise to the following problems.

Internal bus masters such as DMAC other than the CPU can access on-chip memory other than the

cache memory but cannot access the cache memory. If an on-chip bus master other than the CPU

writes data to an external memory other than the cache, the contents of the external memory may

differ from that of the cache memory. To prevent this problem, if the external memory whose

contents is cached is written by an on-chip bus master other than the CPU, the corresponding

cache memory should be purged by software.

If the CPU initiates read access for the cache, the cache is searched. If the cache stores data, the

CPU latches the data and completes the read access. If the cache does not store data, the CPU

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