Assignment 7: Summer circuit

Due: 5:00pm, Friday, October 7. Value: 40 pts. Submit to Sauron.

The circuit-summer.circ file implements a simple keypad in Logisim; download this file to use as a starting point for your assignment. (You may need to right-click the link and select Save Link As… or Save Target As….)

You can see six wires sticking out at right.

• The topmost wire connects to the display, shown as a circle with a dash in it; you should send into this wire the 16-bit binary value holding what the calculator's display should show.
• The next 4-bit wire is the value of the current digit being pressed (at times when any of the 10 digit keys is pressed).
• The next wire is 1 when one of the 10 digit keys is pressed.
• The next wire is 1 when the + key is pressed.
• The next wire is 1 when the C key is pressed.
• The next wire is 1 when any of the 12 keys is pressed, but it is delayed for quite a while to allow your circuit to perform any calculations needed before this wire is used as the clock to a register or flip-flop.

Your job is to fill out the circuit so that it works like a 16-bit calculator that only does addition. For example, the following sequence shows how the display should update with different keys being pressed.

Button Display 4 4 5 45 6 456 + 456 3 3 + 459 5 5 4 54 1 541

Button Display (continued) + 1000 C 0 3 3 C 0 8 8 + 8 9 9 + 17

You may add any of the following components to your circuit:

Wiring library:	any component
Gates library:	any component
Plexers library:	multiplexer and demultiplexer only
Arithmetic library:	adder only
Memory library:	D flip-flop and register only
Input/Output library:	no component
Base library:	no component

You'll need to be familiar with Logisim's wire bundles for this assignment; you can read about them in the Wire bundles section of its documentation.

In building your circuit, I suggest including two 16-bit registers: one to hold the current number being entered, and another to hold the sum of the previous numbers entered. I also suggest using a single D flip-flop that tracks whether the last button pressed was + (in which case the sum of previous numbers should be displayed) or a digit (in which case the current number should be displayed). For the clock input on the registers and flip-flop, you would want to use the keypad's bottom output. The registers would each employ a multiplexer to control how the register should update with the keypress.

With each press of a digit button, you'll want the current number to update to 10 ⋅ current + digit (akin to the square.c portion of Assignment 2). To compute 10 ⋅ current, you'll want to use the same technique you employed in the mult10 function of Assignment 5.

I suggest that you start your circuit by first making it so that pressing multiple digit keys properly accumulates a number into a register (i.e., ignoring the + and C keys). Then you would enhance it to support + and C.

On the Linux computers, you can execute Logisim by entering the command logisim & at the terminal prompt. You can also download Logisim freely as directed on its Web page. It runs on Linux, Mac OS X, and Windows.

Submit your saved Logisim file to Sauron.