PowerPC™ 405 Processor Block Reference Guide www.xilinx.com 91
UG018 (v2.0) August 20, 2004 1-800-255-7778
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DSPLB Three Consecutive Word Writes
The timing diagram in Figure2-22 shows three consecutive word writes. It provides an
example of the fastest speed at which the DCU can request and send single words over the
PLB. The word writes could be in response to non-cacheable stores, cacheable stores to
write-through memory, or cacheable stores that do not allocate a cache line. Consecutive
writes cannot be address pipelined between the DCU and BIU.
The first word write (ww1) is requested by the DCU in cycle 2. The BIU responds in the
same cycle the request is made by the DCU. A single word is sent from the DCU to the BIU
in cycle 2. The BIU uses the byte enables to select the appropriate bytes from the write-data
bus.
The second word write (ww2) is requested after the first write is complete. The DCU
makes the request in cycle 4 and the BIU responds in the same c ycle. A sin gle word is s ent
from the DCU to the BIU in cycle 4. The BIU uses the byte enables to select the appropriate
bytes from the write-data bus.
The third word write (ww3) is requested after the second write is complete. The DCU
makes the request in cycle 6 and the BIU responds in the same c ycle. A sin gle word is s ent
from the DCU to the BIU in cycle 6. The BIU uses the byte enables to select the appropriate
bytes from the write-data bus.

Figure 2-21: DSPLB Line Write/Wor d Write/Line Wri te

Cycle 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
PLBCLK an d CPMC40 5CLK
UG018_25_101701
PPC405 Outputs:
C405PLBDCUREQUEST
C405PLBDCURNW
C405PLBDCUABUS[0:31] adr1 adr2 adr3
flush1 flush3
C405PLBDCUBE[0:7]
C405PLBDCUWRDBUS[0:63]
C405PLBDCUSIZE2
DCU
ww2 wl3wl1
d101 d123 d145 d167 d2 d301 d323 d345 d367
PLB/BIU Outputs:
PLBC405DCUADDRACK
PLBC405DCURDDBUS[0:63]
PLBC405DCURDWDADDR[1:3]
PLBC405DCURDDACK
PLBC405DCUWRDACK
PLBC405DCUBUSY
ww2 wl3wl1
wl101wl123wl145wl167 ww2 wl301wl323 wl345wl367
val