Puzzle 75 of 100

A weak feedback inverter

Goal

Make Q follow D while EN is high, and keep its level while EN is low. The feedback inverter in the drawing is too strong for the switch to overpower.

Die: 26 by 14 squares
Layers: Metal, Polysilicon, Contact, N active, P active, N well, Metal 2, Via
Inputs: D, EN
Output: Q
Supply: VDD
Ground: GND

Go to the solution

New in this puzzle

A weak keeper

An inverter can feed a latch's output back to its input without a second switch if it is weak. The switch must pass more current than the feedback inverter, so the feedback inverter has a long channel.

Hint

Lengthen the channel of the feedback inverter: paint polysilicon over the active silicon beside its gate, in both strips, so that the gate is three or four squares long. A long channel passes little current, so the switch wins.

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 75, A weak feedback inverter, 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 75, A weak feedback inverter, 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 75, A weak feedback inverter, 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

Metal126 squares
Polysilicon79 squares
Contact27 squares
N active22 squares
P active44 squares
N well104 squares
Metal 233 squares
Via7 squares
Input padD, EN
Output padQ
Supply padVDD
Ground padGND
Squares you draw, change, or erase21 squares

About the drawing

The drawing uses 274 of the die's 364 squares.
Part of the drawing is in place when the puzzle opens. To finish it, paint 6 empty squares and change 15 drawn squares.
It forms 10 transistors: 5 N transistors and 5 P transistors. Each one is a place where polysilicon crosses active silicon.
27 contacts join metal to the polysilicon or active silicon under it.
7 vias join metal 2 to the metal under it.

Starting drawing for puzzle 75, A weak feedback inverter, on a numbered grid. — The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.