# split frequency

Discussion in 'VHDL' started by Martin Kokelmann, Feb 15, 2005.

1. ### Martin KokelmannGuest

hello,

i want to split a frequency.For example:

The clock is given with 3.9152 MHz and as output there have to be 10KHz.

How can I implement it?

From 3.9152 MHz to 1 Hz is no problem.

Is this a attempt?

Martin

Martin Kokelmann, Feb 15, 2005

2. ### Allan HerrimanGuest

On Tue, 15 Feb 2005 14:59:13 +0100, Martin Kokelmann
<> wrote:

>hello,
>
>i want to split a frequency.For example:
>
>The clock is given with 3.9152 MHz and as output there have to be 10KHz.
>
>How can I implement it?
>
> From 3.9152 MHz to 1 Hz is no problem.
>
>Is this a attempt?

Firstly, are you sure you mean 3.9152MHz, and not 4.9152MHz?
4.9152MHz is a common crystal frequency, used for baud rate
generators.

In general, you can divide any frequency down to any lower frequency,
as long as the ratio of the two frequencies isn't an irrational
number.
So, yes, your problem is not difficult to solve.

Performing such a frequency division will result in adding jitter to
the signal. This jitter will have an amplitude that may be as large
as 1 period of the input clock (255ns here).

Can your application tolerate this much jitter?

The coding is trivial. This web page:
http://fractional-divider.tripod.com/
has a script which will write the code for you.

Regards,
Allan

Allan Herriman, Feb 15, 2005

3. ### JezwoldGuest

You can actualy divide by an approximation of an irrational number by
using whats known as a fractional divider,there are two commonly used
types one is a phase accumulator type the other is a dual modulus type.
IA search on google should help or i have a copy of a perl script and
test bench which makes design easy
Jez(removethis)
if you would like a copy.

Jezwold, Feb 15, 2005
4. ### Allan HerrimanGuest

On 15 Feb 2005 11:59:21 -0800, "Jezwold" <> wrote:

>You can actualy divide by an approximation of an irrational number by
>using whats known as a fractional divider,there are two commonly used
>types one is a phase accumulator type the other is a dual modulus type.

"approximation of irrational number" = rational number

Allan

Allan Herriman, Feb 16, 2005