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4–50 Cache and External Interfaces
21264/EV68A Hardware Reference Manual
Bcache Port
The Relationship Between Write-to-Read — BC_WR_RD_BUBBLES and wr_rd
The following formulas calculate the relationship between the Cbox CSR
BC_WR_RD_BUBBLES and wr_rd:
wr_rd = (BC_WR_RD_BUBBLES – 1) * bcfrm
or
BC_WR_RD_BUBBLES = ((wr_rd + bcfrm – 1) / bcfrm) + 1
There is never a need to use a value of 0 or 1 for BC_WR_RD_BUBBLES.
If wr_rd = 4*ratio, then value 3 would be the minimum
BC_WR_RD_BUBBLES value when bcfrm = 2*ratio, and value 5 would be the
minimum BC_WR_RD_BUBBLES value when bcfrm = ratio.
There is a special case for ratio = 2.0 in single-data mode. In this case, the for-
mula is:
wr_rd = (BC_WR_RD_BUBBLES – 2) * bcfrm
The Relationship Between Read-to-Write — BC_RD_WR_BUBBLES and rd_wr
Use the following formula to calculate the value for the Cbox CSR
BC_RD_WR_BUBBLES that produces the minimum rd_wr restriction:
BC_RD_WR_BUBBLES = rd_wr – 6
Note that a value for BC_RD_WR_BUBBLES of zero really means 64 GCLK cycles.
In that case, amend the formula. For example, it is impossible to have rd_wr = 6 in
the 1.5x dual-data rate mode case.
4.8.4 Pin Descriptions
This section describes the characteristics of the Bcache interface pins.
Ratio The number of GCLK cycles per peak Bcache bandwidth transfer. For example, a
ratio of 2.5 means the peak Bcache bandwidth is 16 bytes for every 2.5 GCLK
cycles.
rd_wr The minimum spacing required between the read and write indices at the data/tag
pins, expressed as GCLK cycles.
wr_rd The minimum spacing required between the write and read indices at the data/tag
pins, expressed as GCLK cycles.
Term Description