Puzzle 90 of 100

A ripple carry

Goal

Add CI to the number A1 A0: make S1 S0 the two-bit sum, and CO high while the sum is four. The half adder for bit 0 is complete, and the one for bit 1 is drawn.

Die: 50 by 20 squares
Layers: Metal, Polysilicon, Contact, N active, P active, N well, Metal 2, Via
Inputs: A0, CI, A1
Outputs: S0, S1, CO
Supply: VDD
Ground: GND

Go to the solution

New in this puzzle

Carry from bit to bit

An adder of several bits passes the carry of each bit to the next bit up. The carry ripples from the lowest bit to the highest, so the top bits settle last.

Hint

Bit 1 adds A1 to the carry of bit 0. Bring A1 and the carry of the first half adder to the gate lines of the second one, then join its sum to S1 and its carry to CO.

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 90, A ripple carry, 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 90, A ripple carry, 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 90, A ripple carry, 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

Metal336 squares
Polysilicon112 squares
Contact70 squares
N active36 squares
P active72 squares
N well150 squares
Metal 2262 squares
Via55 squares
Input padA0, CI, A1
Output padS0, S1, CO
Supply padVDD
Ground padGND
Squares you draw, change, or erase116 squares

About the drawing

The drawing uses 606 of the die's 1000 squares.
Part of the drawing is in place when the puzzle opens. To finish it, paint 82 empty squares and change 34 drawn squares.
It forms 28 transistors: 14 N transistors and 14 P transistors. Each one is a place where polysilicon crosses active silicon.
70 contacts join metal to the polysilicon or active silicon under it.
55 vias join metal 2 to the metal under it.

Starting drawing for puzzle 90, A ripple carry, on a numbered grid. — The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.