Odd Oversampling

[ALuPin]

Hi newsgroup,

maybe someone of you out there has faced some similar problem:

I want to sample 16MHz data with a 125MHz clock.

But 125/16 = 7.8125

Is there some tricky method to perform this kind of oversampling ?

The only thing I know is the Bresenham algorithm, but could that
be the solution ?

Thank you in advance.

Rgds

 

[Mike Treseler]

I want to sample 16MHz data with a 125MHz clock.
Is there some tricky method to perform this kind of oversampling ?

There's a simple one if you have both clocks.
Write a synchronous process using 125MHz
as the clock and 16MHz as an input named "rate"
Synchronize "rate" and use it to generate a
rate_rising clock enable pulse.

-- Mike Treseler

 

[ALuPin]

It's easy enough with just 4 times oversampling. XAPP224 shows how to do it
at 400Mb+ data rates.
Cheers, Syms.

Please correct me if I got the problem wrong.

Is it right that I could define a counter (clocked with 125MHz)
and define one counter position as the sample point which is located
in the middle position neighborhood of the bit to sample ?
The counter would be quasi started if the first transition is
recognized. This transition could be recognized with a two stage flip
flop chain in the 125Mhz
clock domain.
The problem would be that the following sample points would walk
because
125/16 is a fraction number. Is that right ?

Could that sample point walk even then if the oversample factor 125/x
would
be an integer number for example 120MHz/12MHz when 120MHz having 0.05%
tolerance
and 20MHz having 0.02% tolerance ?


If I have 7 times oversampling would I need then an input stage
with seven stages (XAPP224)? What additional clocks would I need then
?
Or are four clocks sufficient?

Thank you in advance.

Rgds
André

 

[ALuPin]

One more question:

The quasi "oversample" solution in XAPP224 does imply that the sample
clock is the same as the clock from the incoming data stream.

But in my situation I have a clock that is 7-8 times faster.

Please clarify ...

Thank you.

Rgds
André

 

[Symon]

Hi André,
I'd probably do it like this.
Have a 7 bit counter that counts modulo 125 on your 125MHz clock. Call it
'counter'. If you get a transition on the data (which you've already sampled
into the 125MHz clock domain, right?), reset it to 0. When the counter says
4 | 12 | 19 | 27 | 35 | 43 | 51 | 58 | 66 | 74| 82 | 90 | 97 | 105 | 113 |
121 sample the data. (I'd check those numbers for yourself, my arithmetic
isn't what it used to be..) Of course with half the bits there will be a
transition so you'll only be sampling at big counts if you get a lot of
consecutive equal bits. Remember if the count gets to 124 the next count
should be 0, i.e. modulo 125.
Have fun, Syms.

 

[Peter Alfke]

Here is a circuit that generates 16 MHz from a 125 MHz clock:
Use a Xilinx DCM with simultaneous multiply by 16 and division by 25.
That gives you 80 MHz, which might be convenient for 5x oversampling.
Peter Alfke, Xilinx Applications

 

[Symon]

Personally I probably wouldn't do this, depending on the situation. I find
that having as few clocks as possible is the best strategy for FPGAs. I use
enables instead. So, if Andre already has a 125MHz system clock in his
device that he's using, he should stick with it rather than generating a new
clock domain. If, however, the 125MHz is only used in this circuit, I guess
making it into 80MHz might work.
YMMV, Syms.
p.s. I think all my numbers in that previous post were 1 too big for a
proper sync design.

 

[Mike Treseler]

But in my situation I have a clock that is 7-8 times faster.
Please clarify ...

Please reread my posting.
The idea is as clear as I have time to make it.

-- Mike Treseler

 

[Symon]

I just reread your post. Where does the 16 MHz clock come from? Rereading
Andre's original post, he only has 16 MHz (sic) data. ;-)
Cheers, Syms.

 

[ALuPin]

The 16MHz stands for an external data stream. I have no direct clock
recovery of that data stream. The 125MHz comes from a PLL. The input
clock of the PLL
is not the source clock of the data stream.

I will think about your suggestions.

Thank you to all.

Rgds
André