Virtex4 FX12 dynamic clock divider

Hi everyone,

For my design, which is implemented in Virtex-4 FX12 (speed grade -10)
I need to get an adjustable (in operation) clock of frequency 30 to 66
MHz with the smallest possible increments. On the board I have 100MHz
oscillator from which I tried to get 400 MHz (the higher the frequency
the smaller the clock adjusting increments) using EDK 8.1's DCM (CLKFX
4/1). From this DCM I also power the PPC at 200MHz (CLK2X).
I coupled the 400MHz clock to clock divider using rising edge as a
process reference and an integer counter. This configuration does NOT
work. The first problem is the frequency - EDK can compile my design if
I lower the frequency to 200MHz, which is unaceptable. The other
problem is a structure of my clock divider - if I make a process with
rising edge detection the clock divider can only be even (clock is
always divided by a factor of two!).
How to build a fast dual edge (DDR) clock divider in VHDL?
Is there any other way to solve my problem?

Help wanted,

Guru

The best way to create clocks with tiny increments is using Direct
Digital Synthesis (DDS), a.k.a. phase accumulation. You just build a
long accumulator, clock it from your 100 MHz (200 MHz would be better),
and you can get at its MSB any frequency you want with any granularity
you want (I built a circuit with 1 Hz resolution, but you can also get
1 millihertz.)
The catch is jitter. You will create an unavoidable worst-case jitter
of one clock period (10 ns or 5 ns in your case.)
Reducing that jitter is very complicated.
Peter Alfke, Xilinx Applications

Guru schrieb:

> For my design, which is implemented in Virtex-4 FX12 (speed grade -10)
> I need to get an adjustable (in operation) clock of frequency 30 to 66
> MHz with the smallest possible increments. On the board I have 100MHz

What is "smallest possible increment" If I spent a hell of stuff and 
money on it, I will have microhertz resolution with jitter down to a few 
picoseconds.
Hey man, give us some numbers. Frequency resolution? Jitter?

> oscillator from which I tried to get 400 MHz (the higher the frequency
> the smaller the clock adjusting increments) using EDK 8.1's DCM (CLKFX
> 4/1). From this DCM I also power the PPC at 200MHz (CLK2X).
> I coupled the 400MHz clock to clock divider using rising edge as a
> process reference and an integer counter. This configuration does NOT
> work. The first problem is the frequency - EDK can compile my design if
> I lower the frequency to 200MHz, which is unaceptable. The other
> problem is a structure of my clock divider - if I make a process with
> rising edge detection the clock divider can only be even (clock is
> always divided by a factor of two!).
> How to build a fast dual edge (DDR) clock divider in VHDL?

Such a question from someone who calls himself Guru? ;-)

> Is there any other way to solve my problem?

There are many ways to skin a cat. One way is to have two counters, on 
counting on the rising edge, the other on the falling edge. Combining 
the two outputs in a clever fasion in a LUT can work. Or routing a 
signal to an output pin, make a connection to another pin back into the 
FPGA makes it possible to use the output DDR Flipflops. If you have a 
spare MGT, you can use it to make a real high frequncy DDS.

Regards
Falk

Peter Alfke wrote:
> The best way to create clocks with tiny increments is using Direct
> Digital Synthesis (DDS), a.k.a. phase accumulation.

Peter,

Original poster mentioned that he was using V4.   Can he use the
dynamic reconfiguration feature of the DCM?  If memory serves this
allows for changing the M and D values.


Regards,
Erik.

---
Erik Widding
President
Birger Engineering, Inc.

 (mail) 100 Boylston St #1070; Boston, MA 02116
(voice) 617.695.9233
  (fax) 617.695.9234
  (web) http://www.birger.com

I think he can, but this gets us into the subject that Falk addressed,
and I was too polite to mention:
Be more specific!
What is "smallest possible?"
How much jitter is acceptable?
If you can tolerate jitter, DDS is unbeatable.
M/D still leaves big holes...
Peter Alfke

Here are some more details:
The output clock is used for CMOS imaging sensor.
The input clock can be 200MHz (CLK2X from DCM0)
Desired clock increment is about 1 MHz.
Maximum jitter not specified.
2 DCMs free for now.

I think that DCM with dynamic FX ratios cannot produce such increments,
because
output frequency can only take fraction ratios according to input
clock. I think DDS is the best solution for my problem. The open
question is which frequency to take for input: 100, 200, 300 ..MHz?

Thank you all,

Guru

it is possible to have virtual clock DDS you can calculate several
phaseses per highest useable clock (200MHz) and use one output stage
that combines the result, IO DDR primitive, you can so have DDS clock
as if it would be 400MHz while still using max 200MHz real clock in the
FPGA

Antti

Guru schrieb:

> Here are some more details:
> The output clock is used for CMOS imaging sensor.
> The input clock can be 200MHz (CLK2X from DCM0)
> Desired clock increment is about 1 MHz.
> Maximum jitter not specified.

Ahhh, now we move forward!

> 2 DCMs free for now.

> I think that DCM with dynamic FX ratios cannot produce such increments,
> because
> output frequency can only take fraction ratios according to input
> clock. I think DDS is the best solution for my problem. The open
> question is which frequency to take for input: 100, 200, 300 ..MHz?

Lets look at the worst case. You want 66 MHz max. If we use some kind of 
  660 MHz master clock, we will have 1/10 UI jitter. Not too bad, not 
too good. I guess for the CMOS image sensor the jitter wont hurt, since 
it is read like a ram array, isnt it?
So what you need is a multiphase DDS using lets say 4 accus. The MSBs 
must be parallel-serial converted using a x2 Clock and a DDR output 
stage. Been there, done that. Do a search in the FPGA FAQ, its explained 
there.

Regards
Falk

Can IO DDR primitive output be used inside FPGA, because I need this
clock also for several processes inside FPGA, or I have to route it to
output I/O pin and then physically tie it to input I/O pin?
By using DDR primitive I can theoretically get only 2.5ns jitter (+
logic delay) at 200 MHz input clock.

Guru

Guru schrieb:
> Can IO DDR primitive output be used inside FPGA, because I need this
> clock also for several processes inside FPGA, or I have to route it to
> output I/O pin and then physically tie it to input I/O pin?

Hmmm? Maybe you can do a trick. Define an bidirectional inout port, 
drive the output with the DDR FF and use the input as the (generated) 
clock. This may work.

> By using DDR primitive I can theoretically get only 2.5ns jitter (+
> logic delay) at 200 MHz input clock.

Logic delay doesnt matter for jitter, the delay is just a static phase 
shift. If you use a 300 MHz clock, you can go down to 1.6 ns. What 
device are you using? AFAIK Viertex-II etc. go up to 840 Mbit/s in DDR 
data transfer, Spartan-3 up to 622 Mbit/s.

Regards
Falk