Arbitrary Clock Frequencies From Base Clock

[abhisheknag]

Hello,

I have a board with the Xilinx V2 Pro (xc2vp30). I am designing a BPSK
demodulator on this board. For the demodulator, I need to generate
arbitrary clock frequencies from 8 MHz to 10 MHz in 100 kHz increments
from the base clock. How can I do this? Does anyone have any VHDL code
I can use for this?

Thanks and regards,

Abhishek

 

[robertdb]

Hi,

I do this using this simple Numeric Controlled Oscillator -NCO- code I
found in here: http://www.analogservices.com/vhdlmod.htm
However the generated clk is not pure in the sense that some periods
are a bit longer than others.

Robert

(e-mail address removed) ha escrit:

 

[Rtafas]

What is your master clock? If it is n*100khz, you may use a counter,
the simplest way is count until you reach the "divider number -1" and
generate an enable pulse. You will not get a 50% duty cycle clock (but
you can count to generate it twice higher than you need and divide by
two). The jitter will be the one from your oscilator.

A DCO (or NCO) is good too, but only if jitter is not a problem (it is
said that jitter is a master clock cycle plus the oscillator, but I
have never found such calculations).

Also, you may need to lock this clock, so, locked loop is the way (PLL,
DPLL or ADPLL).

Also, a good reading about counters and digital synchronous design are
a very good starting points to understand the codes you may found
(IMHO, it is fundamental). Google for it!

Regards,

Rtafas

 

[Ian Muncaster]

Following on from Rtafas,
if your master clock is n*50khz then
you should be able to achieve a 50:50 duty cycle clock by loading a count
and a half count value then when counting if your below the half count out =
'1' and when above half count out = '0' this should achieve a near 50:50
duty cycle clock.

Hope this is of help.

REGARDS IAN.

 

[pygmalion]

Hello all,

Thank you for the responses. For the time being, I've decided to use
the DCMs on the FPGA I am using. However, I have a feeling I'll require
more "arbitrary" clocks later into the project. Then, I am going to use
some of the many suggestions I've received in this thread.

Thanks and regards,

Abhishek

 

[Ricardo]

DCM? fixed output then. Note that it has a minimun working frequency
(about 1Mhz for Spartan3, if I am correct).

If it's ok for your design, good then!

Regards

Tafas