Quick question on non-overlapping clock generators

Status
Not open for further replies.
D

diarmuid

Full Member level 2
Joined
Aug 4, 2011
Messages
143
Helped
10
Reputation
20
Reaction score
9
Trophy points
1,308
Location
Dublin, Ireland
Visit site
Activity points
2,429
Hello All,

Im not so experienced with non-overlapping clock generators so have 2 probably quite basic questions on them:

Going by the topology of two cross-coupled NOR gates with even numbers of inverters inserted in their feedback paths:

- Are the inverters inserted to increase the non-overlapping durations?

- Could you not just use an inverter string consisting of an odd number of inverters instead?

Thanks and all the best,

Diarmuid
 

You might see alternatives with NAND and odd # inverters.
The inverters set the majority of the nonoverlap dwell but
the phasing has to give the logic what it needs for the
active phase to be logically unique.
 
I done a bit more reading up on the subject and yes you are right, the degree of non-overlap is determined by the number of
inverters in the feedback chain (even for NOR, odd for NAND). More inverters increase non-overlap but at the cost of reduced
duty cycle.

However, what about just using an inverter string (consisting of an odd number of inverters)?

From sims I ran I see this produces small non-overlap. I get ~ 1ns non-overlap using an inverter string of 9 inverters. There
must be some snag to this that Im not seeing. Anything obvious?

Thanks,

Diarmuid
 
So you've got ~ 100pS/stage prop delays; roughly right,
I guess, for a low fanout short channel technology. You
might add dummy loading to stretch the delay total without
unacceptable degradation to risetime / jitter. For long
nonoverlap times I will create a RC delay block (with
hysteresis for anti-chatter) with a couple inverters' worth
of squaring / boosting afterward, substituting this for a
much longer inverter chain. There's an area trade between
inverters and resistor / capacitor sizes that you want to
explore, in this decision.
 
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…