Puzzle 55 of 100

A ring oscillator

Goal

Close the ring so that Y repeats with a period between 10 and 16 ns once EN is high.

Die: 24 by 15 squares
Layers: Metal, Polysilicon, Contact, N active, P active, N well
Input: EN
Output: Y
Supply: VDD
Ground: GND

New in this puzzle

A ring of inverting stages

A ring of an odd number of inverting stages never settles: each stage changes the next, and the change comes back inverted. The period is twice the time a change takes to go around the ring.

An enable

A NAND gate in the ring inverts its ring input only while EN is high. While EN is low its output stays high, so the ring rests in a known state at power on and starts when EN rises.

Guide

Place a contact on the outlined square beside the second inverter's output, and run polysilicon from it down under the GND rail to a second contact.
Run metal along the row under the rail to the NAND gate, and join it to the free gate line with a contact and polysilicon.

Hint

The NAND gate and the two inverters after it become a ring of three inverting stages when the second inverter's output returns to the NAND gate's free input. Take that output down under the GND rail on polysilicon, along the row below on metal, and back up into the free gate line.

Solution

This is the game's reference drawing for the puzzle. It meets the goal and passes the design rules. Other drawings can pass as well.

Top View of the solved die for puzzle 55, A ring oscillator, with the Side View below it. — Top View of the solved die, captured from the game. Paths that carry a high level at the moment shown are lit. The Side View below cuts through the row between the dashed lines.

The game window in Isometric View with the solved die for puzzle 55, A ring oscillator, the guide, and the scope. — The whole game window in Isometric View. The scope on the right compares each output with its target band, and the message above it confirms that the drawing passes.

Solution drawing for puzzle 55, A ring oscillator, drawn on a numbered grid with every layer in its game color and pattern. — The solution on the grid. Squares are numbered from the top left corner, so you can count them. The dashed outline marks the squares you draw, change, or erase; the rest is drawn when the puzzle opens.

Legend

Metal133 squares
Polysilicon66 squares
Contact33 squares
N active14 squares
P active28 squares
N well88 squares
Input padEN
Output padY
Supply padVDD
Ground padGND
Squares you draw, change, or erase23 squares

About the drawing

The drawing uses 231 of the die's 360 squares.
Part of the drawing is in place when the puzzle opens. To finish it, paint 20 empty squares and change 3 drawn squares.
It forms 10 transistors: 5 N transistors and 5 P transistors. Each one is a place where polysilicon crosses active silicon.
33 contacts join metal to the polysilicon or active silicon under it.

Starting drawing for puzzle 55, A ring oscillator, on a numbered grid. — The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.