Circuits: VLSI

Puzzle 74 of 100

A latch from switches

Goal

Make Q follow D while EN is high. While EN is low, Q keeps the level D had when EN fell.

Die
26 by 14 squares
Layers
Metal, Polysilicon, Contact, N active, P active, N well, Metal 2, Via
Inputs
D, EN
Output
Q
Supply
VDD
Ground
GND

Go to the solution

New in this puzzle

A switched loop

Two inverters in a ring hold a level. One switch lets D into the ring while EN is high, and a second switch closes the ring while EN is low.

Hint

Draw a transmission gate from D to the input of the first inverter that conducts while EN is high. Draw a second one from the output of the second inverter back to that input that conducts while EN is low. EN runs on metal 2 along the lower track and its inverse along the upper one.

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 74, A latch from switches, 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 74, A latch from switches, 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 74, A latch from switches, 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

  • Metal125 squares
  • Polysilicon64 squares
  • Contact28 squares
  • N active22 squares
  • P active44 squares
  • N well104 squares
  • Metal 253 squares
  • Via13 squares
  • Input padD, EN
  • Output padQ
  • Supply padVDD
  • Ground padGND
  • Squares you draw, change, or erase79 squares

About the drawing

  • The drawing uses 279 of the die's 364 squares.
  • Part of the drawing is in place when the puzzle opens. To finish it, paint 61 empty squares and change 18 drawn squares.
  • It forms 10 transistors: 5 N transistors and 5 P transistors. Each one is a place where polysilicon crosses active silicon.
  • 28 contacts join metal to the polysilicon or active silicon under it.
  • 13 vias join metal 2 to the metal under it.
Starting drawing for puzzle 74, A latch from switches, on a numbered grid.
The drawing when the puzzle opens. The dashed outline marks the squares the solution changes.