diff --git a/test_regress/t/t_instrument.cpp b/test_regress/t/t_instrument.cpp
index ea67c40b0..ef8af0cf7 100644
--- a/test_regress/t/t_instrument.cpp
+++ b/test_regress/t/t_instrument.cpp
@@ -9,34 +9,43 @@
 #include <verilated.h>
 #include <verilated_vcd_c.h>
 
-#include <fstream>
+#include <iostream>
 
 #include VM_PREFIX_INCLUDE
 
-unsigned long long main_time = 0;
+vluint64_t main_time = 0;
 double sc_time_stamp() { return main_time; }
 
 int main(int argc, char** argv) {
     Verilated::debug(0);
     Verilated::commandArgs(argc, argv);
 
-    std::unique_ptr<VM_PREFIX> top{new VM_PREFIX{"top"}};
-
-    std::ofstream logFile("obj_vlt/t_instrument/simulation_output.log");
-    if (!logFile.is_open()) {
-        printf("Error: Could not open log file\n");
-        return 1;
-    }
+    const std::unique_ptr<VM_PREFIX> top{new VM_PREFIX{"TOP"}};
 
     while (main_time <= 100) {
+        if (main_time < 20) {
+            top->in1a = 5;
+            top->in2a = 10;
+            top->in1b = 20;
+            top->in2b = 30;
+        } else if (main_time >= 20 && main_time < 63) {
+            top->in1a = 0;
+            top->in2a = 5;
+            top->in1b = 15;
+            top->in2b = 25;
+        } else if (main_time > 78) {
+            top->in1a = 10;
+            top->in2a = 15;
+            top->in1b = 25;
+            top->in2b = 35;
+        }
         top->eval();
-        logFile << "$time: " << main_time << " | "
-                << "Output outa: " << static_cast<int>(top->outa) << " | "
-                << "Output outb: " << static_cast<int>(top->outb) << std::endl;
+        std::cout << "$time: " << main_time << " | "
+                  << "Output outa: " << static_cast<int>(top->outa) << " | "
+                  << "Output outb: " << static_cast<int>(top->outb) << std::endl;
         ++main_time;
     }
     top->final();
-    top.reset();
     printf("*-* All Finished *-*\n");
     return 0;
 }
diff --git a/test_regress/t/t_instrument.out b/test_regress/t/t_instrument.out
index 7ba39071f..7f87814a2 100644
--- a/test_regress/t/t_instrument.out
+++ b/test_regress/t/t_instrument.out
@@ -1,13 +1,13 @@
-$time: 0 | Output outa: 0 | Output outb: 50
-$time: 1 | Output outa: 0 | Output outb: 50
-$time: 2 | Output outa: 0 | Output outb: 50
-$time: 3 | Output outa: 0 | Output outb: 50
-$time: 4 | Output outa: 0 | Output outb: 50
-$time: 5 | Output outa: 0 | Output outb: 50
-$time: 6 | Output outa: 0 | Output outb: 50
-$time: 7 | Output outa: 0 | Output outb: 50
-$time: 8 | Output outa: 0 | Output outb: 50
-$time: 9 | Output outa: 0 | Output outb: 50
+$time: 0 | Output outa: 15 | Output outb: 1
+$time: 1 | Output outa: 15 | Output outb: 1
+$time: 2 | Output outa: 15 | Output outb: 1
+$time: 3 | Output outa: 15 | Output outb: 50
+$time: 4 | Output outa: 15 | Output outb: 50
+$time: 5 | Output outa: 15 | Output outb: 50
+$time: 6 | Output outa: 15 | Output outb: 50
+$time: 7 | Output outa: 15 | Output outb: 50
+$time: 8 | Output outa: 15 | Output outb: 50
+$time: 9 | Output outa: 15 | Output outb: 50
 $time: 10 | Output outa: 0 | Output outb: 50
 $time: 11 | Output outa: 0 | Output outb: 50
 $time: 12 | Output outa: 0 | Output outb: 50
@@ -18,84 +18,85 @@ $time: 16 | Output outa: 0 | Output outb: 50
 $time: 17 | Output outa: 0 | Output outb: 50
 $time: 18 | Output outa: 0 | Output outb: 50
 $time: 19 | Output outa: 0 | Output outb: 50
-$time: 20 | Output outa: 0 | Output outb: 50
-$time: 21 | Output outa: 0 | Output outb: 50
-$time: 22 | Output outa: 0 | Output outb: 50
-$time: 23 | Output outa: 0 | Output outb: 50
-$time: 24 | Output outa: 0 | Output outb: 50
-$time: 25 | Output outa: 0 | Output outb: 50
-$time: 26 | Output outa: 0 | Output outb: 50
-$time: 27 | Output outa: 0 | Output outb: 50
-$time: 28 | Output outa: 0 | Output outb: 50
-$time: 29 | Output outa: 0 | Output outb: 50
-$time: 30 | Output outa: 0 | Output outb: 50
-$time: 31 | Output outa: 0 | Output outb: 50
-$time: 32 | Output outa: 0 | Output outb: 50
-$time: 33 | Output outa: 0 | Output outb: 50
-$time: 34 | Output outa: 0 | Output outb: 50
-$time: 35 | Output outa: 0 | Output outb: 50
-$time: 36 | Output outa: 0 | Output outb: 50
-$time: 37 | Output outa: 0 | Output outb: 50
-$time: 38 | Output outa: 0 | Output outb: 50
-$time: 39 | Output outa: 0 | Output outb: 50
-$time: 40 | Output outa: 0 | Output outb: 50
-$time: 41 | Output outa: 0 | Output outb: 50
-$time: 42 | Output outa: 0 | Output outb: 50
-$time: 43 | Output outa: 0 | Output outb: 50
-$time: 44 | Output outa: 0 | Output outb: 50
-$time: 45 | Output outa: 0 | Output outb: 50
-$time: 46 | Output outa: 0 | Output outb: 50
-$time: 47 | Output outa: 0 | Output outb: 50
-$time: 48 | Output outa: 0 | Output outb: 50
-$time: 49 | Output outa: 0 | Output outb: 50
-$time: 50 | Output outa: 0 | Output outb: 50
-$time: 51 | Output outa: 0 | Output outb: 50
-$time: 52 | Output outa: 0 | Output outb: 50
-$time: 53 | Output outa: 0 | Output outb: 50
-$time: 54 | Output outa: 0 | Output outb: 50
-$time: 55 | Output outa: 0 | Output outb: 50
-$time: 56 | Output outa: 0 | Output outb: 50
-$time: 57 | Output outa: 0 | Output outb: 50
-$time: 58 | Output outa: 0 | Output outb: 50
-$time: 59 | Output outa: 0 | Output outb: 50
-$time: 60 | Output outa: 0 | Output outb: 50
-$time: 61 | Output outa: 0 | Output outb: 50
-$time: 62 | Output outa: 0 | Output outb: 50
-$time: 63 | Output outa: 0 | Output outb: 50
-$time: 64 | Output outa: 0 | Output outb: 50
-$time: 65 | Output outa: 0 | Output outb: 50
-$time: 66 | Output outa: 0 | Output outb: 50
-$time: 67 | Output outa: 0 | Output outb: 50
-$time: 68 | Output outa: 0 | Output outb: 50
-$time: 69 | Output outa: 0 | Output outb: 50
-$time: 70 | Output outa: 0 | Output outb: 50
-$time: 71 | Output outa: 0 | Output outb: 50
-$time: 72 | Output outa: 0 | Output outb: 50
-$time: 73 | Output outa: 0 | Output outb: 50
-$time: 74 | Output outa: 0 | Output outb: 50
-$time: 75 | Output outa: 0 | Output outb: 50
-$time: 76 | Output outa: 0 | Output outb: 50
-$time: 77 | Output outa: 0 | Output outb: 50
-$time: 78 | Output outa: 0 | Output outb: 50
-$time: 79 | Output outa: 0 | Output outb: 50
-$time: 80 | Output outa: 0 | Output outb: 50
-$time: 81 | Output outa: 0 | Output outb: 50
-$time: 82 | Output outa: 0 | Output outb: 50
-$time: 83 | Output outa: 0 | Output outb: 50
-$time: 84 | Output outa: 0 | Output outb: 50
-$time: 85 | Output outa: 0 | Output outb: 50
-$time: 86 | Output outa: 0 | Output outb: 50
-$time: 87 | Output outa: 0 | Output outb: 50
-$time: 88 | Output outa: 0 | Output outb: 50
-$time: 89 | Output outa: 0 | Output outb: 50
-$time: 90 | Output outa: 0 | Output outb: 50
-$time: 91 | Output outa: 0 | Output outb: 50
-$time: 92 | Output outa: 0 | Output outb: 50
-$time: 93 | Output outa: 0 | Output outb: 50
-$time: 94 | Output outa: 0 | Output outb: 50
-$time: 95 | Output outa: 0 | Output outb: 50
-$time: 96 | Output outa: 0 | Output outb: 50
-$time: 97 | Output outa: 0 | Output outb: 50
-$time: 98 | Output outa: 0 | Output outb: 50
-$time: 99 | Output outa: 0 | Output outb: 50
-$time: 100 | Output outa: 0 | Output outb: 50
+$time: 20 | Output outa: 5 | Output outb: 40
+$time: 21 | Output outa: 5 | Output outb: 40
+$time: 22 | Output outa: 5 | Output outb: 40
+$time: 23 | Output outa: 5 | Output outb: 40
+$time: 24 | Output outa: 5 | Output outb: 40
+$time: 25 | Output outa: 5 | Output outb: 40
+$time: 26 | Output outa: 5 | Output outb: 40
+$time: 27 | Output outa: 5 | Output outb: 40
+$time: 28 | Output outa: 5 | Output outb: 40
+$time: 29 | Output outa: 5 | Output outb: 40
+$time: 30 | Output outa: 5 | Output outb: 40
+$time: 31 | Output outa: 5 | Output outb: 40
+$time: 32 | Output outa: 5 | Output outb: 1
+$time: 33 | Output outa: 5 | Output outb: 1
+$time: 34 | Output outa: 5 | Output outb: 1
+$time: 35 | Output outa: 5 | Output outb: 1
+$time: 36 | Output outa: 5 | Output outb: 1
+$time: 37 | Output outa: 5 | Output outb: 1
+$time: 38 | Output outa: 5 | Output outb: 1
+$time: 39 | Output outa: 5 | Output outb: 1
+$time: 40 | Output outa: 5 | Output outb: 1
+$time: 41 | Output outa: 5 | Output outb: 1
+$time: 42 | Output outa: 5 | Output outb: 1
+$time: 43 | Output outa: 5 | Output outb: 1
+$time: 44 | Output outa: 5 | Output outb: 1
+$time: 45 | Output outa: 5 | Output outb: 1
+$time: 46 | Output outa: 5 | Output outb: 1
+$time: 47 | Output outa: 5 | Output outb: 1
+$time: 48 | Output outa: 5 | Output outb: 1
+$time: 49 | Output outa: 5 | Output outb: 1
+$time: 50 | Output outa: 5 | Output outb: 1
+$time: 51 | Output outa: 5 | Output outb: 1
+$time: 52 | Output outa: 5 | Output outb: 1
+$time: 53 | Output outa: 5 | Output outb: 1
+$time: 54 | Output outa: 5 | Output outb: 1
+$time: 55 | Output outa: 5 | Output outb: 1
+$time: 56 | Output outa: 5 | Output outb: 1
+$time: 57 | Output outa: 5 | Output outb: 1
+$time: 58 | Output outa: 5 | Output outb: 1
+$time: 59 | Output outa: 5 | Output outb: 1
+$time: 60 | Output outa: 5 | Output outb: 1
+$time: 61 | Output outa: 5 | Output outb: 1
+$time: 62 | Output outa: 5 | Output outb: 1
+$time: 63 | Output outa: 5 | Output outb: 1
+$time: 64 | Output outa: 5 | Output outb: 1
+$time: 65 | Output outa: 5 | Output outb: 1
+$time: 66 | Output outa: 5 | Output outb: 1
+$time: 67 | Output outa: 5 | Output outb: 1
+$time: 68 | Output outa: 5 | Output outb: 1
+$time: 69 | Output outa: 5 | Output outb: 40
+$time: 70 | Output outa: 5 | Output outb: 40
+$time: 71 | Output outa: 5 | Output outb: 40
+$time: 72 | Output outa: 5 | Output outb: 40
+$time: 73 | Output outa: 5 | Output outb: 40
+$time: 74 | Output outa: 5 | Output outb: 40
+$time: 75 | Output outa: 5 | Output outb: 40
+$time: 76 | Output outa: 5 | Output outb: 40
+$time: 77 | Output outa: 5 | Output outb: 40
+$time: 78 | Output outa: 5 | Output outb: 40
+$time: 79 | Output outa: 25 | Output outb: 60
+$time: 80 | Output outa: 25 | Output outb: 60
+$time: 81 | Output outa: 25 | Output outb: 60
+$time: 82 | Output outa: 25 | Output outb: 60
+$time: 83 | Output outa: 25 | Output outb: 60
+$time: 84 | Output outa: 25 | Output outb: 60
+$time: 85 | Output outa: 0 | Output outb: 60
+$time: 86 | Output outa: 0 | Output outb: 60
+$time: 87 | Output outa: 0 | Output outb: 60
+$time: 88 | Output outa: 0 | Output outb: 60
+$time: 89 | Output outa: 0 | Output outb: 60
+$time: 90 | Output outa: 0 | Output outb: 60
+$time: 91 | Output outa: 0 | Output outb: 60
+$time: 92 | Output outa: 0 | Output outb: 60
+$time: 93 | Output outa: 0 | Output outb: 60
+$time: 94 | Output outa: 0 | Output outb: 60
+$time: 95 | Output outa: 0 | Output outb: 60
+$time: 96 | Output outa: 0 | Output outb: 60
+$time: 97 | Output outa: 0 | Output outb: 60
+$time: 98 | Output outa: 0 | Output outb: 60
+$time: 99 | Output outa: 0 | Output outb: 60
+$time: 100 | Output outa: 0 | Output outb: 60
+*-* All Finished *-*
diff --git a/test_regress/t/t_instrument.py b/test_regress/t/t_instrument.py
index 668cf3856..34ef9eccc 100755
--- a/test_regress/t/t_instrument.py
+++ b/test_regress/t/t_instrument.py
@@ -15,13 +15,11 @@ test.top_filename = "t/t_instrument.v"
 sim_filename = "t/" + test.name + ".cpp"
 dpi_filename = "t/t_instrumentDPI.cpp"
 vlt_filename = "t/" + test.name + ".vlt"
-log_filename = "obj_vlt/t_instrument/simulation_output.log"
 
-test.compile(make_top_shell=False,
-             make_main=False,
-             v_flags2=["--trace --exe --instrument", sim_filename, vlt_filename, dpi_filename])
-test.execute()
-
-test.files_identical(log_filename, test.golden_filename)
+test.compile(
+    make_top_shell=False,
+    make_main=False,
+    v_flags2=["--trace --timing --exe --instrument", sim_filename, vlt_filename, dpi_filename])
+test.execute(expect_filename=test.golden_filename)
 
 test.passes()
diff --git a/test_regress/t/t_instrument.v b/test_regress/t/t_instrument.v
index 674f8293e..5a7a8a2b8 100644
--- a/test_regress/t/t_instrument.v
+++ b/test_regress/t/t_instrument.v
@@ -5,17 +5,17 @@
 // SPDX-License-Identifier: CC0-1.0
 
 module top_module(
-    output logic [7:0] outa,
-    output logic [7:0] outb
+    input   reg     [7:0] in1a,
+    input   reg     [7:0] in2a,
+    input   reg     [7:0] in1b,
+    input   reg     [7:0] in2b,
+    output  logic   [7:0] outa,
+    output  logic   [7:0] outb
 );
-    logic [7:0] in1a = 8'd5;
-    logic [7:0] in2a = 8'd10;
-
-    logic [7:0] in1b = 8'd20;
-    logic [7:0] in2b = 8'd30;
 
     module_a a1 (.in1(in1a), .in2(in2a), .out(outa));
     module_a a2 (.in1(in1b), .in2(in2b), .out(outb));
+
 endmodule
 
 module module_a(
diff --git a/test_regress/t/t_instrument.vlt b/test_regress/t/t_instrument.vlt
index e96535c09..b0b8351f4 100644
--- a/test_regress/t/t_instrument.vlt
+++ b/test_regress/t/t_instrument.vlt
@@ -6,4 +6,6 @@
 
 `verilator_config
 
-instrument -model "instrument_var" -id 0 -target "top_module.a1.b1.out"
+instrument -callback "instrument_var" -id 0 -target "top_module.a1.b1.out"
+
+instrument -callback "instrument_var" -id 1 -target "top_module.a2.out"
diff --git a/test_regress/t/t_instrumentDPI.cpp b/test_regress/t/t_instrumentDPI.cpp
index 78eb0a723..b5e565435 100644
--- a/test_regress/t/t_instrumentDPI.cpp
+++ b/test_regress/t/t_instrumentDPI.cpp
@@ -8,18 +8,23 @@
 
 #include <verilated.h>
 
-#include <iostream>
 #include <svdpi.h>
 
-extern "C" short instrument_var(int id, const svLogic* x) {
+extern "C" int instrument_var(int id, int trigger, const svLogic* x) {
     switch (id) {
-    case 0: return 0;
+    case 0:
+        if ((VL_TIME_Q() >= 10 && VL_TIME_Q() < 20) || VL_TIME_Q() >= 85) {
+            return 0;
+        } else {
+            return *x;
+        }
+        //return 0;
     case 1:
-        // Stuck at 1 Fault Injection
-        return 1;
-    case 2:
-        // Inverter/Bit flip Fault injection (provisional)
-        return !x;
+        if ((VL_TIME_Q() >= 0 && VL_TIME_Q() < 3) || (VL_TIME_Q() >= 32 && VL_TIME_Q() < 69)) {
+            return 1;
+        } else {
+            return *x;
+        }
     default: return *x;
     }
 }