MPC555
/
MPC556
L-BUS TO U-BUS INTERFACE (L2U)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
11-9
11.7 L-Bus Show Cycle Support
The L2U module provides support for L-bus show cycles. L-bus show cycles are ex-
ternal visibility cycles that reflect activity on the L-bus that would otherwise not be vis-
ible to the external bus. L-bus show cycles are software controlled.
11.7.1 Programming Show Cycles
L-bus show cycles are disabled during reset and must be configured by writing the ap-
propriate bits in the L2U_MCR control register. L-bus show cycles are programmed by
setting the LSHOW[0:1] bits in the L2U_MCR. The
shows the configura-
tions of the LSHOW[0:1] bits.
11.7.2 Performance Impact
When show cycles are enabled in the L2U module, there is a performance penalty on
the L-bus. This occurs because the L2U module does not support more than one ac-
cess being processed at any time. To ensure that only one access at a time can be
Table 11-2 Reservation Snoop Support
Reserved Location On
Intruding Alternate Master
Action Taken on stwcx cycle
L-bus L-Master
Request to cancel the reservation.
1
NOTES:
1. If the RCPU tries to modify (
stwcx
) that location, the L2U does not have enough time to stop the write
access from completing. In this case, the L2U will drive cancel-reservation signal back to the core as
soon as it comes to know that the alternate master on the U-bus has touched the reserved location.
U-Master
Request to cancel the reservation.
U-bus
L-Master
Block
stwcx
2
2. If the RCPU tries to modify (
stwcx
) that location, the L2U does not start the cycle on the U-bus and it
communicates to the core that the current write has been aborted by the slave with no side effects.
U-Master
Block
stwcx
External Bus
L-Master
Block
stwcx
U-Master
Block
stwcx
Ext-Master
Transfer Status
3
3. If the RCPU tries to modify (
stwcx
) that location, the L2U runs a write-cycle-with-reservation request
on the U-bus. The L2U samples the status of the reservation along with the U-bus cycle termination
signals and it communicates to the core if the current write has been aborted by the slave with no side
effects.
IMB3
L-Master
Block
stwcx
U-Master
Block
stwcx
IMB3-Master
Transfer Status
Table 11-3 L2U_MCR LSHOW Modes
LSHOW
Action
00
Disable L-bus show cycles
01
Show address and data of all L-bus space write cycles
10
Reserved (Disable L-bus show cycles)
11
Show address and data of all L-bus space read and write cycles
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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