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Old 05-04-2006, 09:02 AM
Dave
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Default Phase alignment of DCMs on different boards/devices

Hi group,

Scenario: Multiple identical boards with a Virtex-II each all being
fed the same clock signal which is being fed to a DCM on each device.

Lets assume the clock signal has no skew between the FPGAs and all
FPGAs get powered/configured at the same time (so the clock signals
enter the DCMs in phase).

Will the DCMs achieve lock at the same time and therefore produce
output clocks that are in phase relative to each other on all 4 FPGAs?
Or will the lock time vary? The datasheet says anything up to for
example 120us (for 24-30MHz input, using DLL op) but does this mean a
potential phase difference of 120us between the outputs of the DCMs?

What about when multiplying/dividing or using the CLKFX outputs? Does
that make a difference?

Many thanks for your time,

Dave

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Old 05-04-2006, 10:04 AM
Rene Tschaggelar
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Default Re: Phase alignment of DCMs on different boards/devices

Dave wrote:

> Hi group,
>
> Scenario: Multiple identical boards with a Virtex-II each all being
> fed the same clock signal which is being fed to a DCM on each device.
>
> Lets assume the clock signal has no skew between the FPGAs and all
> FPGAs get powered/configured at the same time (so the clock signals
> enter the DCMs in phase).
>
> Will the DCMs achieve lock at the same time and therefore produce
> output clocks that are in phase relative to each other on all 4 FPGAs?
> Or will the lock time vary? The datasheet says anything up to for
> example 120us (for 24-30MHz input, using DLL op) but does this mean a
> potential phase difference of 120us between the outputs of the DCMs?



Don't count on them being equally fast to lock.
There is no "in phase" in distributed systems,
strictly speaking. The phase is the delay between
clock cycles, so the max phase error is plus minus
half the clock.

You can synch multiple devices to within a clock cycle,
provided the system is less distributed than a
clockcycle is fast, with a trigger pulse. To achieve better
synchronization, you need some hardware delay or PLL.
Most highend FPGAs do have internal cClock PLLs.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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  #3 (permalink)  
Old 05-04-2006, 04:02 PM
Peter Alfke
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Default Re: Phase alignment of DCMs on different boards/devices

You must distinguish between the relatively long time that th DLL takes
to achieve lock (many microseconds) and the very small phase error on
the outputs (picoseconds, which is million times shorter) once the DLL
is locked

So, the different DLL will most likely achieve lock at different times,
but once in lock, they will each perform to data sheet specs, which
means picoseconds of difference on the outputs

Another matter is that frequency division can obviously generate
out-of-step outputs between different chips.

Peter Alfke, Xilinx

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Old 05-05-2006, 08:03 AM
Dave
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Default Re: Phase alignment of DCMs on different boards/devices

> You must distinguish between the relatively long time that th DLL takes
> to achieve lock (many microseconds) and the very small phase error on
> the outputs (picoseconds, which is million times shorter) once the DLL
> is locked
>
> So, the different DLL will most likely achieve lock at different times,
> but once in lock, they will each perform to data sheet specs, which
> means picoseconds of difference on the outputs


Hi Peter,

I would have thought that if all DCMs start attempting lock at exactly
the same time then they would achieve it at the same time. What causes
them to achieve lock at differnt times assuming the input to each is
exactly in phase and they start working at the same time? Is it down
to differences between the silicon or some other inherent property of
the DCMs loop structures?

Many thanks,

Dave

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  #5 (permalink)  
Old 05-05-2006, 09:05 AM
Rene Tschaggelar
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Posts: n/a
Default Re: Phase alignment of DCMs on different boards/devices

Dave wrote:

>>You must distinguish between the relatively long time that th DLL takes
>>to achieve lock (many microseconds) and the very small phase error on
>>the outputs (picoseconds, which is million times shorter) once the DLL
>>is locked
>>
>>So, the different DLL will most likely achieve lock at different times,
>>but once in lock, they will each perform to data sheet specs, which
>>means picoseconds of difference on the outputs

>
>
> Hi Peter,
>
> I would have thought that if all DCMs start attempting lock at exactly
> the same time then they would achieve it at the same time. What causes
> them to achieve lock at differnt times assuming the input to each is
> exactly in phase and they start working at the same time? Is it down
> to differences between the silicon or some other inherent property of
> the DCMs loop structures?


Even if the silicon was identical to the Angstroem,
you couldn't guarantee they have the same temperature
nor that they see the same supply voltage with the
same impedance.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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