Circuits: VLSI

Puzzle 47 of 100

Exclusive OR from switches

Goal

Make Y high while exactly one of A and B is high.

Die
18 by 30 squares
Layers
Metal, Polysilicon, Contact, N active, P active, N well
Inputs
A, B
Output
Y
Supply
VDD
Ground
GND

Go to the solution

New in this puzzle

Choosing B or its inverse

Exclusive OR passes B unchanged while A is low and inverts it while A is high. A two-way switch that chooses between B and its inverse computes it with two transmission gates and two inverters.

Hint

Y equals B while A is low, and the inverse of B while A is high. Build a two-way switch that A controls: one transmission gate passes B, and the other, drawn the other way up, passes the inverse of B. The inverters are drawn, and their four lines end under the GND rail.

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 47, Exclusive OR from switches, 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 47, Exclusive OR from switches, 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 47, Exclusive OR from switches, 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
  • Contact28 squares
  • N active20 squares
  • P active30 squares
  • N well94 squares
  • Input padA, B
  • Output padY
  • Supply padVDD
  • Ground padGND
  • Squares you draw, change, or erase107 squares

About the drawing

  • The drawing uses 256 of the die's 540 squares.
  • Part of the drawing is in place when the puzzle opens. To finish it, paint 106 empty squares and change 1 drawn square.
  • It forms 8 transistors: 4 N transistors and 4 P transistors. Each one is a place where polysilicon crosses active silicon.
  • 28 contacts join metal to the polysilicon or active silicon under it.
Starting drawing for puzzle 47, Exclusive OR from switches, on a numbered grid.
The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.