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Alpha 21264/EV67 Hardware Reference Manual
Cache and External Interfaces 4–33
System Port
Table 4–26 shows four example configurations and shows their use of the
SYSDC_DELAY[4:0].
System 1 has six GCLKs to every SYSCLK and only sends 4-cycle commands to the
21264/EV67. Thus, a period of three SYSCLKs between the SysDc command and data
leaves a period of 15 GCLKs between SysDc and data (SysDc is in the middle of the 4-
cycle command). A SYSDC_DELAY[4:0] of five would align sampling and receipt of
SysData.
System 2 has four GCLKs in every SYSCLK, so leading data by three SYSCLK cycles,
and programming the SYSDC_DELAY[4:0] to two, aligns sampling and receiving.
Timing for systems 3 and 4 is derived in a similar manner.
Note: The maximum valid value for SYSDC_DELAY must be less than the min-
imum number of GCLK cycles between two consecutive SYSDC com-
mands to the 21264/EV67.
If a fast data transfer is interrupted and fails to complete, the system must use the con-
ventional protocol to send a SysDc WriteData command to the 21264/EV67, removing
the desired data buffer. Section 4.7.8.3 describes the timing events for transferring data
from the 21264/EV67 to the system.
4.7.8.3 Fast Data Disable Mode
The system controls all data movement to and from the 21264/EV67. Movement of data
into and out of the 21264/EV67 is preceded by a SysDc command. The 21264/EV67
drivers are only enabled for the duration of an 8-cycle transfer of data from the 21264/
EV67 to the system. Systems must ensure that there is no overlap of enabled drivers
and that there is adequate settle time on the SysData bus.
Given a SysDc fill command, the 21264/EV67 samples data 10 + SYSDC_DELAY
GCLK cycles after the command is perceived within the 21264/EV67 clock domain.
Because there is no linkage with the output driver, fills into the 21264/EV67 are not
affected by the SYS_RCV_MUX_PRESET[1:0] value.
In both modes, given a SysDc write command, the 21264/EV67 looks for the next
SYSCLK edge 8.5 cycles after perceiving the SysDc write command in its clock
domain. Because the SysDc write command must be perceived before its use, SysDc
write commands are dependent upon the amount of delay introduced by Cbox CSR
SYS_RCV_MUX_CNT_PRESET[1:0].
Table 4–26 Cbox CSR SYSDC_DELAY[4:0] Examples
System Bit Rate System Framing Clock Ratio
1
1
The system framing clock ratio is the number of INT_FWD_CLK cycles per
SYSCLK cycles.
SYSDC_DELAY
System 1 1.5X 4:1 5 (3 SYSCLK cycles)
System 2 2.0X 2:1 2 (3 SYSCLK cycles)
System 3 2.5X 2:1 0 (2 SYSCLK cycles)
System 4 4X 2:1 6 (2 SYSCLK cycles)