Motorola MPC533 Reference Manual page 427

11.7 L-Bus Show Cycle Support
The L2U module provides support for L-bus show cycles. L-bus show cycles are external
visibility cycles that reflect activity on the L-bus that would otherwise not be visible 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 setting the
LSHOW[0:1] bits in the L2U_MCR. Table 11-3 shows the configurations of the
LSHOW[0:1] bits.
LSHOW
00
01
10
11
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 access being
processed at any time. To ensure that only one access at a time is processed, and not lose
an L-bus access that would have been show cycled, the L2U module will arbitrate for the
L-bus whenever it is processing any access. This L-bus arbitration will prevent any other
L-bus master from starting a cycle that might turn out to be a qualifiable L-bus show cycle.
For L-bus show cycles, the minimum performance impact on the L-bus will be three clocks.
This minimum impact assumes that the L-bus slave access is a 1-clock access, and the L2U
module acquires immediate bus grant on the U-bus. The L2U has to wait two clocks before
completing the show cycle on the U-Bus, thus using up five clocks for the complete
process.
A retried access on the L-bus (no address acknowledge) that qualifies to be show cycled,
will be accepted when it is actually acknowledged. This will cause a 1-clock delay before
an L-bus master can retry the access on the L-bus, because the L2U module will release
L-bus one clock later.
L2U asserts the internal bus request signal on the U-bus for a minimum of two clocks when
starting a show cycle on the U-bus.
MOTOROLA
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Table 11-3. L2U_MCR LSHOW Modes
Disable L-bus show cycles
Show address and data of all L-bus space write cycles
Reserved (Disable L-bus show cycles)
Show address and data of all L-bus space read and write cycles
Chapter 11. L-Bus to U-Bus Interface (L2U)
L-Bus Show Cycle Support
Action
11-11