[time-nuts] Of rubidium life and piggy-bank anemia....

[Magnus Danielson]

From: Peter Vince <pvince at theiet.org>
Subject: Re: [time-nuts] Of rubidium life and piggy-bank anemia....
Date: Sun, 02 Dec 2007 19:26:46 +0000
Message-ID: <65348.1196623606 at uk2.net>

> 
> On Sun Dec  2 17:59 , Magnus Danielson <magnus at rubidium.dyndns.org> sent:
> 
> >The problem is that he calls the DFFs latches. The problem with the term
> >latches is that they sound like a good generic term, but really isn't.
> 
> At the risk of showing my ignorance, could I ask you to explain the difference
> please?

A latch is a device in which the enable input allows data to flow freely
through the latch, much as a buffer, when the enable input is active. As the
enable goes inactive, it retains the last input level (or what it perceived as
the last input level considering metastability).

A D flip-flop updates its output on the rising (or falling) edge of the clock
input.

> > In message 4751D4F9.4020207 at xtra.co.nz>, Bruce Griffiths writes:
> >   
> > The latches do NOT span clock domains, they are in the same domain
> > as the counter.
> 
> Sorry again, could you briefly explain what is meant by "clock domains"?

A clock domain is a group of logic and DFFs clocked by a common clock source.
Within the clock-domain all logic transitions occur synchronously to the clock
of the clock domain. For modern ASIC and FPGA designs this allows for a much
simplified analysis when combined with the ban of latches, since all logic
signals, gate delays and routing delays can be statically analyzed and this
allows for powerfull automatic placement and routing mechanisms. The respecting
the setup time prior to the clock edge avoids metastability, and the term slack
refers to the margin to the setup time.

Between clock domains we assume asynchronous aspects, meaning that we risc
metastability cause upset of values. By having an asynchronous signal being
clocked into the clock domain using two DFFs clocked with the clock domain
clock, the metastability issue becomes virtually removed.

When designing ASICs and FPGAs you specifically divide the design into clock
domains and sometimes one chooses to clock signals into a higher clocks in
order to avoid many different clock domains. For FPGAs you have a limited
amount of clock networks, but for very small clock domains you can handle it
through a local clock domain setup, but it is a bit trickier to handle.

Cheers,
Magnus