Circuits: VLSI

Puzzle 84 of 100

Equal bits

Goal

Make Y high while A and B are equal: both high or both low.

Die
24 by 24 squares
Layers
Metal, Polysilicon, Contact, N active, P active, N well, Metal 2, Via
Inputs
A, B
Output
Y
Supply
VDD
Ground
GND

Go to the solution

New in this puzzle

Equality

Two bits are equal while both are high or both are low. Equality is the inverse of the exclusive OR, so it is high exactly where the exclusive OR is low.

Hint

Equality is the inverse of the exclusive OR. Draw a NAND gate of A and B, then a compound gate that pulls Y low while A or B is high and the NAND output is also high.

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 84, Equal bits, 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 84, Equal bits, 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 84, Equal bits, 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

  • Metal144 squares
  • Polysilicon49 squares
  • Contact33 squares
  • N active21 squares
  • P active46 squares
  • N well120 squares
  • Metal 2126 squares
  • Via18 squares
  • Input padA, B
  • Output padY
  • Supply padVDD
  • Ground padGND
  • Squares you draw, change, or erase204 squares

About the drawing

  • The drawing uses 298 of the die's 576 squares.
  • Part of the drawing is in place when the puzzle opens. To finish it, paint 127 empty squares and change 77 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.
  • 18 vias join metal 2 to the metal under it.
Starting drawing for puzzle 84, Equal bits, on a numbered grid.
The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.