feat: add example pc file and testbench

src/getting_started/onboarding/RiSC-16/pc.v

@@ -0,0 +1,38 @@
+module pc (
+    input clk,
+    input rst_n,
+    input [1:0] MUX_output, // Controlled by Control module
+    input [15:0] pc_plus1,
+    input [15:0] pc_plus1_imm,
+    input [15:0] alu_out, // The output from regB that has been passed through the ALU
+    output [15:0] nxt_instr
+);
+
+reg [15:0] pc_reg; // Internal Register to hold state of pc
+
+assign nxt_instr = pc_reg; // Assigns output to be linked to pc_reg
+
+always @(posedge clk or negedge rst_n) 
+begin
+    if (!rst_n) 
+    begin
+        // On reset, the program counter is initialized to the starting address 0.
+        pc_reg <= 16'h0000;
+    end 
+    else 
+    begin
+        // On a clock edge, the next value of the PC is determined by the MUX selector.
+        case (MUX_output)
+            2'b00:
+                pc_reg <= pc_plus1;
+            2'b01:
+                pc_reg <= pc_plus1_imm;
+            2'b10:
+                pc_reg <= alu_out;
+            default: 
+                pc_reg <= pc_plus1; // safety
+        endcase
+    end
+end
+
+endmodule

src/getting_started/onboarding/RiSC-16/pc_tb.v

@@ -0,0 +1,179 @@
+// Testbench for the Program Counter (pc) module
+`timescale 1ns / 1ps
+
+module pc_tb;
+
+    // Inputs to the DUT (Device Under Test)
+    reg clk;
+    reg rst_n;
+    reg [1:0] MUX_output;
+    reg [15:0] pc_plus1;
+    reg [15:0] pc_plus1_imm;
+    reg [15:0] alu_out;
+
+    // Output from the DUT
+    wire [15:0] nxt_instr;
+
+    // Instantiate the Program Counter module
+    pc uut (
+        .clk(clk),
+        .rst_n(rst_n),
+        .MUX_output(MUX_output),
+        .pc_plus1(pc_plus1),
+        .pc_plus1_imm(pc_plus1_imm),
+        .alu_out(alu_out),
+        .nxt_instr(nxt_instr)
+    );
+
+    // Clock generation: 100MHz clock (10ns period)
+    always #5 clk = ~clk;
+
+    // Test sequence
+    initial begin
+        // 1. Initial state setup
+        $display("T=%0t: [SETUP] Initializing testbench...", $time);
+        clk = 0;
+        rst_n = 1;
+        MUX_output = 2'b00;
+        pc_plus1 = 16'h0000;
+        pc_plus1_imm = 16'h0000;
+        alu_out = 16'h0000;
+        #10; // Wait for a moment
+
+        // 2. Test Case: Asynchronous Reset
+        $display("T=%0t: [TEST] Asserting active-low reset.", $time);
+        rst_n = 0;
+        #10; // Hold reset for a short period
+        if (nxt_instr == 16'h0000) begin
+            $display("T=%0t: [PASS] PC reset to 0x%h as expected.", $time, nxt_instr);
+        end else begin
+            $display("T=%0t: [FAIL] PC is 0x%h, expected 0x0000.", $time, nxt_instr);
+        end
+        #10;
+        rst_n = 1; // De-assert reset
+        $display("T=%0t: [SETUP] De-asserting reset.", $time);
+
+
+        // 3. Test Case: Sequential Increment (MUX_output = 00)
+        $display("\n T=%0t: [TEST] Testing sequential increment (MUX_output = 2'b00).", $time);
+        MUX_output = 2'b00;
+        pc_plus1 = 16'h0001;
+        @(posedge clk);
+        #1;
+        if (nxt_instr === 16'h0001) begin
+            $display("T=%0t: [PASS] PC incremented to 0x%h.", $time, nxt_instr);
+        end else begin
+            $display("T=%0t: [FAIL] PC is 0x%h, expected 0x0001.", $time, nxt_instr);
+        end
+
+        pc_plus1 = 16'h0002;
+        @(posedge clk);
+        #1;
+        if (nxt_instr === 16'h0002) begin
+            $display("T=%0t: [PASS] PC incremented to 0x%h.", $time, nxt_instr);
+        end else begin
+            $display("T=%0t: [FAIL] PC is 0x%h, expected 0x0002.", $time, nxt_instr);
+        end
+
+
+        // 4. Test Case: Branch with Immediate (MUX_output = 01)
+        $display("\n T=%0t: [TEST] Testing branch with immediate (MUX_output = 2'b01).", $time);
+        MUX_output = 2'b01;
+        pc_plus1_imm = 16'h1234;
+        // Set other inputs to different values to ensure they aren't selected
+        pc_plus1 = 16'hFFFF; 
+        alu_out = 16'hEEEE;
+        @(posedge clk);
+        #1;
+        if (nxt_instr === 16'h1234) begin
+            $display("T=%0t: [PASS] PC branched to immediate address 0x%h.", $time, nxt_instr);
+        end else begin
+            $display("T=%0t: [FAIL] PC is 0x%h, expected 0x1234.", $time, nxt_instr);
+        end
+
+
+        // 5. Test Case: Jump via ALU (MUX_output = 10)
+        $display("\n T=%0t: [TEST] Testing jump via ALU output (MUX_output = 2'b10).", $time);
+        MUX_output = 2'b10;
+        alu_out = 16'hABCD;
+        // Set other inputs to different values
+        pc_plus1 = 16'hFFFF;
+        pc_plus1_imm = 16'hEEEE;
+        @(posedge clk);
+        #1;
+        if (nxt_instr === 16'hABCD) begin
+            $display("T=%0t: [PASS] PC jumped to ALU address 0x%h.", $time, nxt_instr);
+        end else begin
+            $display("T=%0t: [FAIL] PC is 0x%h, expected 0xABCD.", $time, nxt_instr);
+        end
+
+        // 6. Test Case: Back-to-back operations
+        $display("\n T=%0t: [TEST] Testing mixed back-to-back operations.", $time);
+
+        // a. Increment
+        MUX_output = 2'b00;
+        pc_plus1 = 16'hABCE;
+        @(posedge clk);
+        #1;
+        $display("T=%0t: [INFO] Current PC: 0x%h (After increment)", $time, nxt_instr);
+
+        // b. Jump
+        MUX_output = 2'b10;
+        alu_out = 16'hBEEF;
+        @(posedge clk);
+        #1;
+        $display("T=%0t: [INFO] Current PC: 0x%h (After jump)", $time, nxt_instr);
+
+        // c. Branch
+        MUX_output = 2'b01;
+        pc_plus1_imm = 16'hCAFE;
+        @(posedge clk);
+        #1;
+        $display("T=%0t: [INFO] Current PC: 0x%h (After branch)", $time, nxt_instr);
+        if (nxt_instr !== 16'hCAFE) begin
+            $display("T=%0t: [FAIL] Back-to-back sequence failed. PC is 0x%h, expected 0xCAFE.", $time, nxt_instr);
+        end else begin
+             $display("T=%0t: [PASS] Back-to-back sequence successful.", $time);
+        end
+
+        // 7. Test Case: Undefined MUX select (2'b11)
+        // Since the module's case statement does not define 2'b11, it should perform the default
+        // of incrementing the PC
+        $display("\n T=%0t: [TEST] Testing undefined MUX select (2'b11).", $time);
+        MUX_output = 2'b11;
+        pc_plus1 = 16'h0009; // Change inputs to see if they are ignored
+        pc_plus1_imm = 16'h000A;
+        alu_out = 16'h000B;
+        @(posedge clk);
+        #1;
+        if (nxt_instr === 16'h0009) begin
+             $display("T=%0t: [PASS] PC correctly incremented to PC+1 at 16'h0009.", $time, nxt_instr);
+        end else begin
+             $display("T=%0t: [FAIL] PC changed to 16'h000A or 16'h000B, expected it to go to 16'h0009.", $time, nxt_instr);
+        end
+
+        // 8. Test Case: Asynchronous Reset again
+        $display("\n T=%0t: [TEST] Asserting active-low reset.", $time);
+        rst_n = 0;
+        #10; // Hold reset for a short period
+        if (nxt_instr == 16'h0000) begin
+            $display("T=%0t: [PASS] PC reset to 0x%h as expected.", $time, nxt_instr);
+        end else begin
+            $display("T=%0t: [FAIL] PC is 0x%h, expected 0x0000.", $time, nxt_instr);
+        end
+        #10;
+        rst_n = 1; // De-assert reset
+        $display("T=%0t: [SETUP] De-asserting reset.", $time);
+
+
+        $display("\n T=%0t: [INFO] All tests completed.", $time);
+        $finish; // End simulation
+    end
+
+    // Optional: Monitor to see signal changes at every time step
+    // initial begin
+    //     $monitor("T=%0t | clk=%b rst_n=%b | MUX_sel=%b | pc+1=%h pc+1+imm=%h alu_out=%h | nxt_instr_addr=%h",
+    //         $time, clk, rst_n, MUX_output, pc_plus1, pc_plus1_imm, alu_out, nxt_instr);
+    // end
+
+endmodule