Ring Oscillator

A 5-stage ring oscillator on SKY130, from schematic to simulated frequency. This page doubles as the formatting reference: every construct the renderer understands is used somewhere below.

TL;DR: an odd number of inverters in a loop has no stable DC operating point, so it oscillates at roughly f = 1 / (2 * N * t_pd). Five stages of the standard inverter lands near 1.2 GHz at tt.

Overview

The ring oscillator is the hello world of analog design: it needs correct device models, a working simulator, and nothing else. If this page simulates cleanly, your toolchain setup is good and every later tutorial will run.

Inline styles for reference: bold, italic, both, inline code, struck, and an external link to the SKY130 PDK docs.

Prerequisites

• xschem and ngspice installed and on $PATH
• SKY130A PDK with $PDK_ROOT exported
  • built with open_pdks
  • includes the sky130_fd_pr primitives library
• Basic familiarity with SPICE netlists

Schematic

Keep the loop symmetric: identical W/L on every stage, and buffer the tap so the probe capacitance does not load the ring.

Netlist

* ring_osc.spice : 5-stage ring oscillator, sky130 tt
.lib $PDK_ROOT/sky130A/libs.tech/ngspice/sky130.lib.spice tt

.subckt inv in out vdd gnd
XM1 out in gnd gnd sky130_fd_pr__nfet_01v8 W=1   L=0.15
XM2 out in vdd vdd sky130_fd_pr__pfet_01v8 W=2.1 L=0.15
.ends

Xi1 n5 n1 vdd 0 inv
Xi2 n1 n2 vdd 0 inv
Xi3 n2 n3 vdd 0 inv
Xi4 n3 n4 vdd 0 inv
Xi5 n4 n5 vdd 0 inv

Vdd vdd 0 1.8
.ic v(n1)=0
.tran 10p 20n
.end

Run it

1. Export the environment
   1. export PDK_ROOT=/usr/local/share/pdk
2. Launch the simulation
   1. ngspice ring_osc.spice
   2. wait for the transient to finish
3. Measure the period on v(n5) and invert it

Results

Frequency tracks 1/t_pd, so expect roughly linear movement with VDD across corners.

Checklist

• schematic drawn, ERC clean
• transient runs at tt
• ss / ff corners swept
• layout drawn, DRC and LVS clean

Further reading

• SKY130 device documentation
• ngspice manual
• next up: toolchain setup