Fast Transfer Timing Example; Sysclk Cycles Between Sysaddout And Sysdata - Compaq EV68A Hardware Reference Manual

System Port
Table 4–25 SYSCLK Cycles Between SysAddOut and SysData
System framing clock ratio
1
2
4
Figure 4–5 show a simple example of a fast transfer. The data rate ratio is 1.5X with a
4:1 SYSCLK to INT_FWD_CLK ratio.
Figure 4–5 Fast Transfer Timing Example
SysAddOut_L[14:0]
SysData_L[63:0]
SYSCLK
SysAddOutClk_L
INT_FWD_CLK
GCLK
In fast data mode, movement of data into the 21264/EV68A requires turning around the
SysData bus that is being actively driven by the 21264/EV68A. Given a SysDc fill
command (ReadDataError, ReadData, ReadDataShared, ReadDataShared/Dirty, Read-
DataDirty), the 21264/EV68A responds as follows:
1. Three GCLK cycles after perceiving the SysDc fill command, the 21264/EV68A
turns off its drivers, interrupting any ongoing fast data write transactions.
2. The 21264/EV68A drivers stay off until the last piece of fill data is received, or a
new SysDc write command overrides the current SysDc fill command. It is the
responsibility of the external system to schedule SysDc fill or write commands so
that there is no conflict on the SysData bus.
3. The 21264/EV68A samples fill data in the GCLK clock domain, 10 +
SYSDC_DELAY GCLK cycles after perceiving the SysDc fill command. The
Cbox CSR SYSDC_DELAY[4:0] provides GCLK granularity for precisely placing
fills into the processor pipeline discussed in Section 2.2.
Cache and External Interfaces
4–32
GCLK/INT_FWD_CLK (Data Rate Ratio)
1.5X 2.0X 2.5X
4 3 2
2 2 1
1 1 1
Probe
Response
D0
3.0X 3.5X 4.0X 5.0X
2 2 2 1
1 1 1 1
1 1 1 1
D1 D2 D3 D4 D5
FM05822B.AI4
21264/EV68A Hardware Reference Manual
6.0X 7.0X 8.0X
1 1 1
1 1 1
1 1 1
D6