Advanced Features
Table 1–15. altpll Megafunction Clock Switchover Signals
(Part 2 of 2)
Port
Description
Source
Destination
activeclock(1)
Signal to indicate which clock (0 =
PLL
Logic array
inclk0, 1= inclk1) is driving the PLL.
Note for Table 1–15:
(1) These ports are only available for enhanced PLLs and in auto mode and when using automatic switchover.
All the switchover ports shown in Table 1–15 are supported in the
altpllmegafunction in the Quartus II software. The altpll
megafunction supports two methods for clock switchover:
■
When selecting an enhanced PLL, you can enable both the automatic
and the manual switchover, making all the clock switchover ports
available.
■
When selecting a fast PLL, you can use only enable the manual clock
switchover option to select between inclk0or inclk1. The
clkloss, activeclockand the clkbad0, and clkbad1signals
are not available when manual switchover is selected.
If the primary and secondary clock frequencies are different, the
Quartus II software selects the proper parameters to keep the VCO within
the recommended frequency range.
f
For more information about PLL software support in the Quartus II
software, see the altpll Megafunction User Guide.
Guidelines
Use the following guidelines to design with clock switchover in PLLs.
■
When using automatic switchover, the clkswitchsignal has a
minimum pulse width based on the two reference clock periods. The
CLKSWITCHpulse width must be greater than or equal to the period
of the current reference clock (tfrom_clk) multiplied by two plus the
rounded-up version of the ratio of the two reference clock periods.
For example, if tto_clk is equal to tfrom_clk, then the CLKSWITCHpulse
width should be at least three times the period of the clock pulse.
t
CLKSWITCHCHmin tfrom_clk [2 + intround_up (tto_clk tfrom_clk)]
1–42
Stratix II Device Handbook, Volume 2
Altera Corporation
July 2009
ALTERA [ ALTERA CORPORATION ]
