Multiple clock domains in a FPGA (using DLL's)

[takkaya]

I am targeting a Spartan IIE (400E) device with four global clock
buffers (BUFG) and four DLL's.

But I need five clock domains 25MHz(clkin, DLL1), 50MHz (clk2x, DLL1)
and 30MHz (clkdv, DLL2), 60MHz (clkin, DLL2), 120MHz (clk2x, DLL2) in
my design.

I know that it is not allowed to use more than 2 BUFG per DLL.

I would like to use the DLL's to have a zero propagation delay and low
clock skew between output clock signals distributed throughout the
device. The primary global nets are driven by global buffers (25, 50,
60 and 120 MHz).

The 30 MHz clock domain (internal) is not so critical, so maybe I can
use a secondary global nets?

How can I drive the secondary global nets with an output of the DLL
e.g. the 30 MHz?

Thanks in advance.

 

[Nial Stewart]

takkaya <takkaya@hotmail.com> wrote in message
news:5c8d9153.0311110158.db6a06b@posting.google.com...

I am targeting a Spartan IIE (400E) device with four global clock
buffers (BUFG) and four DLL's.

But I need five clock domains 25MHz(clkin, DLL1), 50MHz (clk2x, DLL1)
and 30MHz (clkdv, DLL2), 60MHz (clkin, DLL2), 120MHz (clk2x, DLL2) in
my design.
I know that it is not allowed to use more than 2 BUFG per DLL.
I would like to use the DLL's to have a zero propagation delay and low
clock skew between output clock signals distributed throughout the
device. The primary global nets are driven by global buffers (25, 50,
60 and 120 MHz).
The 30 MHz clock domain (internal) is not so critical, so maybe I can
use a secondary global nets?
How can I drive the secondary global nets with an output of the DLL
e.g. the 30 MHz?
Thanks in advance.

Generate a /2 enable signal off the 60MHz clock and use this
with the 60MHz clock to drive logic at 30MHz.

You could give this a multi-cycle path constraint for P+R, but
60MHz isn't that fast so unless your device is very full you
could probably get away without this.


Nial

------------------------------------------------
Nial Stewart Developments Ltd
FPGA and High Speed Digital Design
www.nialstewartdevelopments.co.uk