Project 1: Adder Subtractor

This circuit is on Tinkercad: https://www.tinkercad.com/things/kDqc5U7gvIS-portfolio-1-adder-subtractor-combo/editel?sharecode=TY9X9o_KTmnd6K3TvmbwLwfDMG_hk_bWNQ8ve9JIeBo

This circuit is an adder subtractor combination with a switch to toggle between the two functions. This circuit has two standard four bit binary inputs and a toggle for the function, 0 or off being addition and 1 or on being subtraction. Inputs A4-A1 do not get altered and simply connect to the adder, however B4-B1 enter a Xor gate individually with the switch input, these four outputs then go back to the adder and if the switch is off, will not be changed, however if the switch is on, the bits will be flipped. In addition to this, the switch also acts as the carry in for the adder, to simply add one when in subtraction mode to finish swapping the number's sign. The output of this adder is then filtered to four Leds which read left to right with the MSB on the left along with an overflow led with the breadboard above containing overflow logic. The left hand side of this deals with addition and the right hand side deals with subtraction.

For addition overflow(blue RGB) it was simply A4, B4 and Sum4' anded +(or) A4', B4' and Sum 4 anded. For subtraction overflow(Green) it was (A4 xor B4)anded(S4 xor Cout). These were then anded with the subtractor switch so only subtraction or addition overflow would display depending on the switches position.

This circuit demonstrates both addition and subtraction together as well as knowledge of swapping between positive and negative numbers in binary.

Below is a simplified schematic of the circuit. The main adder subtractor cell is shown on the top with the appropriate overflow logic underneath with the addition/subtraction switch acting as an enable pin for each of the overflows.