Designing ring oscillator layout

[Gullubaba]

Hi ! I am designing a ring oscillator in 0.18 um CMOS process. To simulate this circuit (in schematic mode) in Mentor Graphics, I need to put an initial condition ( e.g. 1) for start-up the circuit and due to oscillator circuit, no input is required here.

My question is: "During circuit layout, do I need to put an input port to start-up the circuit ? If yes, then this port will be used initially through which a pulse or trigger with sufficient amplitude need to be injected. Right?

If not then how to start-up the CMOS ring oscillator circuit practically in layout ?

Please help me with your expert views.

Thanks in advance...

 

[D.A.(Tony)Stewart]

Ring Oscillators are simply a ring of inverters cascaded with the last stage fed into the 1st stage. YOu need an odd number to make it oscillate and you may want to see the signal at a lower frequency than the maximum of a single stage so cascade 5 or more odd number of inverters. YOu can cascade as many as you want.

 

[FvM]

Startup pulse may be a requirement in simulation, it isn't for the real hardware.

 

[dick_freebird]

You want a prime number of stages to ensure that you
can't get a higher order oscillation (a possibility with, say,
a 15-stage RO - will oscillate indefinitely at 1/(15*tpd),
1/(5*tpd) or 1/(3*tpd) depending on how the "traveling
wave" initializes or is modified by switching noises.

Building in a good stiff decoupling on-chip is important
to getting ideal fosc measurements - if the supply is
collapsing, against your nH-range bondwire feed, the
oscillation frequency will be compromised or even
quenched / destabilized. You'd be well off to simulate
with these "external" parasitics, not just the close-in
extracted ones, with a reasonably inductive power
source.

You also want to rebuffer your output, I'd suggest with
a resistor divider or source follower to 50 ohms Zout and,
if practical, give this a separate supply and ground so as
not to perturb the RO with external load currents.