MPC555
/
MPC556
BURST BUFFER
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
4-15
4.4.1 Normal Operation
During normal operation, the burst buffer module transfers fetch accesses from the
CPU to the U-bus. When a new access is issued by the CPU, it is transferred in parallel
to both the IMPU and the BBC. The IMPU compares the address of the access to its
region programming. The BBC determines whether the access can be immediately
transferred to the U-bus. If not, it requests the U-bus for the next clock.
Each new BBC U-bus access is accompanying by the burst request attribute. If burst-
able access is enabled, the BBC performs a burst access; otherwise, it performs a sin-
gle access.
If the IMPU detects an access violation, it does the following:
• Cancels the request that was forwarded to the BBC
• Informs the RCPU core that the requested address generated an exception
If the required address contains show cycle or program trace attributes, the BBC de-
livers the access to the U-bus even if the request is cancelled (due to the exception it
caused).
The BBC forwards show cycle, program trace and debug port access attributes ac-
companying the CPU access along with the U-bus access.
4.4.2 Slave Operation.
The burst buffer module is operating as a U-bus slave module when the instruction
memory protection unit (IMPU) registers are accessed by the user in order to be pro-
grammed. This programming is done using the
mtspr /mfspr
instructions.
4.4.3 Reset Operation
On reset the BBC goes to an idle state, and all pending U-bus accesses are ignored.
The IMPU goes to a disabled state in which all memory space is accessible to both
user and supervisor.
4.4.4 Debug Mode Operation
When the CPU is in debug mode, fetch accesses are attached with a special attribute.
If this attribute is asserted, the BBC must initiate not-burstable accesses to the debug
port.
4.4.5 Standby Mode Operation
In this low-power mode the CPU stops issuing further accesses. The BBC clocks are
turned off, and the BBC enters a power-save state. When the low-power mode is ex-
ited, clocks are activated and a new access from the CPU will activate the BBC.
4.4.6 Burst Operation
The BBC can run burst accesses on the U-bus. Such burst cycles, if forwarded to ex-
ternal memory, are then exported to the EBI as burst cycles (if bursts are enabled by
the USIU).
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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