Assignment 7: Summer circuit

Due: 5:00pm, Thursday, October 10. Value: 30 pts.

The summer.circ file implements a simple keypad in Logisim, shown at right. [Link] (You may need to right-click the link and select “Save Link As…” or “Save Target As…”.) You can see six wires sticking out the circuit's right side.

• The topmost wire (initially gray) 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. Every flip-flop and register should have this wire for its clock input, and you should not pass this wire through any gates before it reaches a clock input.

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 left-shift and addition circuits. This brings up the question of how to perform a left-shift in Logisim. The circuit at right illustrates a four-bit shift using two 16-bit splitters and a constant-0 component; given the input of 0xFA15, it yields 0xA150.

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.