Compensated clock in Stratix

[banesh]

hi,

I want to know what is meant by clock compensation and which
clock in the design should be compensated or used as compensated clock
and advantages
of doing so in an enhanced pll.

 

[Vaughn Betz]

bana8@rediffmail.com (banesh) wrote in message news:<98d2c8bc.0407051155.4343fe8b@posting.google.com>...

hi,

I want to know what is meant by clock compensation and which
clock in the design should be compensated or used as compensated clock
and advantages
of doing so in an enhanced pll.

Clock compensation refers to using the PLL to align the clock at some
point in your design with a reference clock edge. Typically the
reference clock comes from an input IO in your design.

The most common compensation is to align the clock edge that the FPGA
registers receive with the clock edge of the reference clock. That
means the PLL is compensating for the delay of the global clock
network used to reach those registers, by generating a clock that is
earlier in time by the same delay as the clock network.

The enhanced PLLs in Stratix have several compensation modes:

Normal mode: compensate for the delay of a clock distributed on a
global network -- chip-wide global or regional (quadrant) global.

Zero delay buffer: Compensate for the delay to an output IO.
Generally used when you want to send out a clock to your board that is
phase-aligned to the input clock (i.e. has no delay versus the input
clock).

External feedback: Lets you take the clock signal out to your board
via an output IO, and send it back into the Stratix chip via an input
IO. All the delay in this path will be compensating out by the PLL.

No compensation: Don't shift the clock back in time to compensate for
any delay.

What compensation is best depends on what you're trying to do. Normal
is the default, and if you have no reason to believe you need one of
the other modes, then you should just leave the compensation at the
default.

See http://www.altera.com/literature/hb/stx/ch_1_vol_2.pdf for more
details.

Hope this helps,

Vaughn
Altera