Puzzle 50 of 100

Swapped controls

Goal

Repair the drawing so that Y follows A while S is low and B while S is high. One of the two transmission gates has its controls the wrong way around.

Die: 16 by 31 squares
Layers: Metal, Polysilicon, Contact, N active, P active, N well
Inputs: S, A, B
Output: Y
Supply: VDD, VDD
Ground: GND

Go to the solution

New in this puzzle

Gate polarity

A transmission gate conducts while its N gate is high and its P gate is low. With the two controls swapped, it conducts at exactly the times it should be off.

Hint

Point at each gate line to see the net it belongs to. The lower gate must conduct while S is high, so its N gate needs S and its P gate needs the inverse of S. Swap the two connections in the lower row.

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 50, Swapped controls, 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 50, Swapped controls, 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 50, Swapped controls, 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

Metal129 squares
Polysilicon86 squares
Contact27 squares
N active11 squares
P active22 squares
N well88 squares
Input padS, A, B
Output padY
Supply padVDD, VDD
Ground padGND
Squares you draw, change, or erase16 squares

About the drawing

The drawing uses 247 of the die's 496 squares.
Part of the drawing is in place when the puzzle opens. To finish it, paint 4 empty squares, change 9 drawn squares, and erase 3 squares.
It forms 6 transistors: 3 N transistors and 3 P transistors. Each one is a place where polysilicon crosses active silicon.
27 contacts join metal to the polysilicon or active silicon under it.

Starting drawing for puzzle 50, Swapped controls, on a numbered grid. — The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.