Compaq EV68A specifications

21264/EV68A Hardware Reference Manual Cache and External Interfaces 4–49
Bcache Port
priateprogramming of the Bcache clock period andde layparameters to satisfy SSRAM
setup/holdrequirements of the Bcache address latch, a Bcache write transaction pro-
ceeds throughthe Cbox as follows:
1. TheCbox transmits the index and write control signals during an Int_Adr_BcClk
edge.
2. Thedata is placed on Bcache data, tag, and tag status pins on the appropriate
Int_Data_BcClkedge from 0 to 7 Bcache bit-times later, based on the Cbox CSR
BC_LATE_WRITE_NUM[2:0]. The BC_LATE_WRITE_NUM[2:0] supportsthe
late-writeSSRAM, which optimize Bcache data bus bandwidth by minimizing
bubbles betweenread and write transactions. For example, single-data late-write
SSRAMs would need this CSR programmed to a valueof one, anddua l-datalate-
writeS SRAMswould need this CSR programmed to a value of two.
3. Thedifference between the data delivery (Int_Data_BcClk)and forwarded clocks
out providesthe setup for the data at the Bcache data flip-flop.
4. ForBcache writes, the 21264/EV68A drivers are enabled on the GCLK BPHASE
precedingthe start of a write transfer, and disabled on the succeedingGCLK
BPHASE at the endof a write transfer. Thus, the write data is envelopedby the
21264/EV68Adrivers to guarantee that every data transfer has the same data-valid
window.
4.8.3.3 Bubbles on the Bcache Data Bus
When changing between read and write transactions on the bidirectional bus, it is often
necessaryto introduce NOP cycles (bubbles) to allow the bus to settle and to drain the
Bcache readpipeline. The Cbox provides two CSRS, BC_RD_WR_BUBBLES[5:0]
and BC_WR_RD_BUBBLES[3:0],to he lp controlthe bubbles between read and write
transactions.
The optimumparameters for these CSRs are determined by formulas that include the
followingterms:
Term Description
bcfrm Bcacheframe clock.
•In dual-datamode, bcfrm is twice the ratio.
•In single-datamode, the value for bcfrm is determined by whether
the ratio is even orodd:
– When the ratio is even, bcfrm is equal to the ratio.
– When the ratio is odd, bcfrm is twice the ratio.
For example, in single-data mode:
GCLK Theprocessor clock.
Ratio Bcfrm
22
2.5 5