Puzzle 85 of 100

Two-bit equality

Goal

Make Y high while the number A1 A0 equals B1 B0, that is, while A1 equals B1 and A0 equals B0. The comparison of A0 with B0 is drawn.

Die: 39 by 22 squares
Layers: Metal, Polysilicon, Contact, N active, P active, N well, Metal 2, Via
Inputs: A0, B0, A1, B1
Output: Y
Supply: VDD
Ground: GND

Go to the solution

New in this puzzle

Comparing numbers

Two numbers are equal when every pair of matching bits is equal. An exclusive OR of each pair is high where that pair differs, so the numbers are equal while all of these outputs are low.

Hint

The drawn exclusive OR is high while A0 and B0 differ. Build a second one for A1 and B1, then a NOR gate of the two outputs: Y is high only while neither pair differs.

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 85, Two-bit equality, 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 85, Two-bit equality, 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 85, Two-bit equality, 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

Metal300 squares
Polysilicon120 squares
Contact84 squares
N active56 squares
P active104 squares
N well195 squares
Metal 2244 squares
Via47 squares
Input padA0, B0, A1, B1
Output padY
Supply padVDD
Ground padGND
Squares you draw, change, or erase283 squares

About the drawing

The drawing uses 603 of the die's 858 squares.
Part of the drawing is in place when the puzzle opens. To finish it, paint 198 empty squares and change 85 drawn squares.
It forms 24 transistors: 12 N transistors and 12 P transistors. Each one is a place where polysilicon crosses active silicon.
84 contacts join metal to the polysilicon or active silicon under it.
47 vias join metal 2 to the metal under it.

Starting drawing for puzzle 85, Two-bit equality, on a numbered grid. — The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.