Compaq EV68A specifications Bcache Transactions, Data-Sample/Drive Cbox CSRs

Bcache Port

Table 4–40 Program Values to Set the Cache Clock Period (Dual-Data Rate) (Continued)

Bcache	BC_CLK_LD_VECTOR1	BC_BPHASE_LD_VECTOR1	BC_FDBK_EN1
Transfer
3.5X-DD	C387	A	04
4.0X-DD	0F0F	0	01
5.0X-DD	7C1F	0	40
6.0X-DD	F03F	0	10
7.0X-DD	C07F	0	04
8.0X-DD	00FF	0	01

1These are hexadecimal values.

In addition to programming the clock CSRs, the data-sample/drive Cbox CSRs, at the pads, must be set appropriately. Table 4–41lists these CSRs and provides their pro- grammed value.

Table 4–41 Data-Sample/Drive Cbox CSRs

CBOX CSR Description

BC_DDM_FALL_EN[0]

BC_TAG_DDM_FALL_EN[0]

BC_DDM_RISE_EN[0]

BC_TAG_DDM_RISE_EN[0]

BC_DDMF_ENABLE[0] BC_DDMR_ENABLE[0] BC_FRM_CLK[0]

BC_CLKFWD_ENABLE[0]

Enables the update of the 21264/EV68A’s Bcache outputs referenced to the falling edge of the Bcache forwarded clock. Dual-data RAMs assert this CSR.

Enables the update of the 21264/EV68A’s Bcache tag outputs referenced to the falling edge of the Bcache forwarded clock. Alway deasserted.

Enables the update of the 21264/EV68A’s Bcache outputs referenced to the rising edge of the Bcache forwarded clock. Always asserted.

Enables the update of the 21264/EV68A’s Bcache tag outputs referenced to the rising edge of the Bcache forwarded clock. Always asserted.

Enables the rising edge of the Bcache forwarded clock. Always asserted.

Enables the falling edge of the Bcache forwarded clock. Always asserted.

Forces the 21264/EV68A to only start Bcache transactions on the rising edge of Bcache clocks that also coincide with the rising edge of GCLK. Must be asserted for all dual-data parts and single-data parts at 2.5X and 3.5X.

Enables clock forward enable. Always asserted.

4.8.3 Bcache Transactions

The Cbox uses the programmed clock values to start data read, tag read, data write, and tag write transactions on the rising edge of a Bcache clock. The Cbox can also be con- figured to introduce a programmable number of bubbles when changing between write and read commands. The following three sections describe these Bcache transactions.

4.8.3.1 Bcache Data Read and Tag Read Transactions

The 21264/EV68A always reads four pieces of data (64 bytes) from the Bcache during a data read transaction, and always interrogates the tag array on the first cycle. Once started, data read transactions are never cancelled. Assuming that the appropriate values

21264/EV68A Hardware Reference Manual Cache and External Interfaces 4–47