94 www.xilinx.com PowerPC™ 405 Processor Block Reference Guide
1-800-255-7778 UG018 (v2.0) August 20, 2004
Chapter 2: Input/Output Interfaces
R
DSPLB Word Write/Line Read/Line Write
The timing diagram in Figure2-25 shows a sequence invo lvin g a word write, an eight-
word line read, and an eight-word line write. It demonstrates how read and write
operations can overlap due to the split read-data and write-data busses.
The word write (ww1) is requested by the DCU in cycle 2 and the BIU responds in the same
cycle. 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 line read (rl2) is address pipelined with the previous word write. The rl2 request is
made by the DCU in cycle 4 and the BIU responds in th e same cycle. Data is se nt from the
BIU to the DCU fill buffer in cycles 5 through 8. After all data associated with this line is
read, it is transferred by the DCU from the fill buffer to the data cache. This is represented
by the fill2 transaction in cycles 9 through 11.
The line write (wl3) is address pipelined with the previous line read. The wl3 request is
made by the DCU in cycle 6 in response to the cache flush in cycles 4 through 5 (flush3).
The BIU responds to the wl3 request in the same cycle it is asserted by the DCU. Data is
sent from the DCU to the BIU in cycles 6 through 9. Because of the split data bus, t he write
operations in cycles 6 through 8 overlap read operations from the previous read request
(rl2).

Figure 2-24: DSPLB Word Write/Word Rea d/Word Write/L ine Read

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_28_101701
PPC405 Outputs:
C405PLBDCUREQUEST
C405PLBDCURNW
C405PLBDCUABUS[0:31] adr1 adr2 adr3 adr4
val val val
d1 d3
C405PLBDCUBE[0:7]
C405PLBDCUWRDBUS[0:63]
C405PLBDCUSIZE2
DCU
rw2 rl4ww1 ww3
PLB/BIU Outputs:
PLBC405DCUADDRACK
PLBC405DCURDDBUS[0:63]
PLBC405DCURDWDADDR[1:3]
PLBC405DCURDDACK
PLBC405DCUWRDACK
PLBC405DCUBUSY
ww3ww1
rw2 rl4ww1 ww3
rw2 rl401 rl423 rl445 rl467
d2 d401 d423 d445 d467
0246
fill4