In Cirsim below, implement a 4-bit program counter. It will have these inputs: A Button named RESET
A Button named CLK
A single-bit input pin named B. This is set true for branch instructions.
A four-bit Bus input named BA. This is the branch address.
It has one output, a Bus output named PC. This is the output of the program counter.
This circuit implements a basic program counter. The function of the program counter is determined by the input B. When B is zero, the program counter increases by one for every clock cycle. It is stepping sequentially through memory. When B is true, the value on the input BA is clocked into the PC when the CLK input goes high. That is implementing a branch, where a new address has been selected for the program counter. When B goes low, the next clock will proceed to the next address after the BA address.
A program counter steps through the instructions in a program. When a branch occurs (like b some_address), the program counter is set to the address we branch to. It the continues counting from that point on.
Do not try to implement this functionality using Set and Reset. They are not appropriate since they are not synchronous. Any solution that uses Set and Reset to solve this problem is incorrect.
When the RESET button is pressed, reset the program counter to address 0000. This is not a synchronous operation and can use the Reset inputs (just wire the button to the inputs).
Click Test to test it, then Submit to submit the quiz question result.
Hints and Suggestions
It is easy to test that your solution is working without having to use the Test. Press reset and the PC output should to go 0000. Then ensure B is cleared. Pressing the CLK button 16 times should cycle through all 16 addresses and back to 0000. Then set B true and BA to 0101. Pressing CLK should set the program counter to match the value on input pin BA, 0101. Then turn off B and press CLK. It should go to the next address AFTER the new address we branched to: 0110. You can double-click on an input pin to change the value. Be sure to test each bit.
If you don't understand how the synchronous counter works, you will not be able to solve this task. You must also understand how to make a JK Flip-Flop work as if it were a D Flip-Flop. You should be able to figure that out from the characteristics table.
If you have more than four flip-flops in your circuit, you are doing it wrong.
The S input is NOT used is in any solution to this circuit! If you are using the S input, you are doing it wrong.
Consider turning on Show Output States to see what is going on.