Compaq EV68A specifications Four Timing Examples, SysDataInValidL and SysDataOutValidL

System Port

Table 4–27lists information for the four timing examples. In Table 4–27,note the fol- lowing:

•SysDc write commands are not affected by the SYSDC_DELAY parameter.

•The SYS_RCV_MUX_PRESET adds delay at the rate of one INT_FWD_CLK at a time. For example, adding the delay of one bit time to system 1 adds 1.5 GCLK cycles to the delay and drives the SysDc write command-to-data relationship from one to two SYSCLKs.

•For write transfers, the 21264/EV68A drivers are enabled on the preceding GCLK BPHASE, before the start of a write transfer, and disabled on the succeeding GCLK BPHASE at the end of the write transfer. The write data is enveloped by the 21264/ EV68A drivers to guarantee that every data transfer has the same data valid win- dow.

Table 4–27 Four Timing Examples

System	Bit Rate	System Framing Clock Ratio1	Write Data
System 1	1.5X	4:1	2 SYSCLKs
System 2	2.0X	2:1	3 SYSCLKs
System 3	2.5X	2:1	2 SYSCLKs
System 4	4X	2:1	2 SYSCLKs

1The system framing clock ratio is the number of INT_FWD_CLK cycles per SYSCLK cycles.

The four examples described here assume no skew for the 2.0X and 4.0X cases and one bit time of skew for the 1.5X and 2.5X cases.

For system 1, the distance between SysDc and the first SYSCLK is nine GCLK cycles but the additional delay of one bit time (1.5 GCLKs) puts the actual delay after perceiv- ing the SysDc command to 7.5 GCLKS, which misses the 8.5 cycle constraint. There- fore, the 21264/EV68A drives data two SYSCLKs after receiving the SysDc write command.

For system two, the distance between SysDc and the second SYSCLK is eight GCLK cycles, which also misses the 8.5 cycle constraint, so the 21264/EV68A drives data three SYSCLK cycles after receiving the SysDc write command (12 cycles).

The other two cases are derived in a similar manner.

4.7.8.4 SysDataInValid_L and SysDataOutValid_L

The SysDataValid signals (SysDataInValid_L and SysDataOutValid_L) are driven by the system and control the rate of data delivery to and from the 21264/EV68A.

SysDataInValid_L

The SysDataInValid_L signal controls the flow of data into the 21264/EV68A, and may be used to introduce an arbitrary number of cycles between octaword transfers into the 21264/EV68A. The rules for using SysDataInValid_L follow:

4–34Cache and External Interfaces 21264/EV68A Hardware Reference Manual