Puzzle 78 of 100

Divide by two

Goal

Make Q change level at every rising edge of CLK, so that it repeats every two clock periods. Only the period of Q is judged.

Die: 40 by 14 squares
Layers: Metal, Polysilicon, Contact, N active, P active, N well, Metal 2, Via
Input: CLK
Output: Q
Supply: VDD
Ground: GND

Go to the solution

New in this puzzle

Changing at every edge

A flip-flop whose inverted output feeds its input changes level at every rising edge of the clock. Its output repeats at half the frequency of the clock.

An unknown start

A flip-flop has no defined level at power on, so Q may start high or low. This puzzle therefore judges the time between rising edges of Q rather than its level.

Hint

Add an inverter after Q and join its output to the input of the first switch, on metal 2 along the middle track. At each rising edge the flip-flop then takes the inverse of its own output.

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 78, Divide by two, 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 78, Divide by two, 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 78, Divide by two, 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

Metal201 squares
Polysilicon128 squares
Contact43 squares
N active35 squares
P active70 squares
N well160 squares
Metal 279 squares
Via11 squares
Input padCLK
Output padQ
Supply padVDD
Ground padGND
Squares you draw, change, or erase75 squares

About the drawing

The drawing uses 446 of the die's 560 squares.
Part of the drawing is in place when the puzzle opens. To finish it, paint 59 empty squares and change 16 drawn squares.
It forms 16 transistors: 8 N transistors and 8 P transistors. Each one is a place where polysilicon crosses active silicon.
43 contacts join metal to the polysilicon or active silicon under it.
11 vias join metal 2 to the metal under it.

Starting drawing for puzzle 78, Divide by two, on a numbered grid. — The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.