Alternative for NB4L6254 (Clock Switch that eliminates runt pulses)

Hi.

I'm working on an ECL circuit that switches an 1.5GHz clock off for some nanoseconds and than turns it on again.

The switched clock must not contain runt pulses (truncated clock cycles).

Onsemi's NB4L6254 has a feature they call "Eliminates Output Runt PulseGeneration and Metastability".

Are there similar parts available?

Regards, Norbert N.

Reply to
norbert.notz
Loading thread data ...

There are two interesting cases in this area. One is switching between 2 clock sources. The other is just turning a clock on and off.

The latter is simple. Just clock a FF (with the correct edge) and use that to AND with your clock. It's all simple digital logic. You have to check the setup/hold/prop to make sure it works. That check is more complicated than just meeting setup for the next cycle. You have to make sure that setup lets the whole clock pulse through.

(Stand on your head and use OR rather than AND if you want the clock to park high rather than low...)

Switching between clocks without letting any runts through is a bit tricky. It requires a few cycles of dead time while switching. The old clock has to say "done" and turn off. The new clock has to run that "done" signal through a synchronizer and then turn itself on.

--
These are my opinions, not necessarily my employer\'s.  I hate spam.
Reply to
Hal Murray

Thank you very much for your suggestions. What type of FF do you mean?

Reply to
norbert.notz

Could you please explain your suggestion a bit more detailed? A D-FF would halve the the clock frequency.

Reply to
norbert.notz

Hi Hal,

I understand your suggestion. In this circuit the D-FF's propagation delay is USED (desired) to generate a runt pulse free clock. I want to turn off/on a 1.5GHz (T =3D 666 ps) clock. As D-FF I consider to use the MC100EP29, the MC100LVEP05 as AND.

Because of T =3D 666 ps the D-FF's propagation delay has to be in the range 333 ps - 666 ps. The MC100EP29 has a propagation delay 350 ps -

550 ps (25=B0C - 85=B0C), so it may work.

The problem I see is the asynchronism between the switch-signal (D- FF's D-input) and the clock: Setup and Hold times could be violated driving the D-FF into metastable state.

Do you have practical experience using this circuit build with ECL- gates?

Regards, Norbert N.

Reply to
norbert.notz

Hi Hal,

I understand your suggestion. In this circuit the D-FF's propagation delay is USED (desired) to generate a runt pulse free clock. I want to turn off/on a 1.5GHz (T =3D 666 ps) clock. As D-FF I consider to use the MC100EP29, the MC100LVEP05 as AND.

Because of T =3D 666 ps the D-FF's propagation delay has to be in the range 333 ps - 666 ps. The MC100EP29 has a propagation delay 350 ps -

550 ps (25=B0C - 85=B0C), so it may work.

The problem I see is the asynchronism between the switch-signal (D- FF's D-input) and the clock: Setup and Hold times could be violated driving the D-FF into metastable state.

Do you have practical experience using this circuit build with ECL- gates?

Regards, Norbert N.

Reply to
norbert.notz

It's not a simple circuit. I think Xilinx had an app note about it.

The basic idea is a FF and AND gate to make each side, and an OR gate to merge them together. Call the FFs EnableClkA and EnableClkB.

You have to turn them on cleanly and turn them off cleanly. They get clocked by their own clock.

With 2 FFs, you have 4 states. 10 and 01 are normal operational modes. 11 is illegal. 00 is a transition.

You also have an external (asynchronous) signal to select which clock to use. It needs 2 synchronizers, one for each clock domain.

To switch from ClkA to ClkB, you have to turn EnableClkA off then turn EnableClkB on. From ClkB's view, EnableClkA is anynchronous so you need a synchronizer in that path.

So if you are running with EnableClkA on, and the control signal says switch to ClkB, first you turn EnableClkA off, then EnableClkB can get turned on.

--
These are my opinions, not necessarily my employer\'s.  I hate spam.
Reply to
Hal Murray

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.