diff --git a/nix/t1/conversion/sv-to-vcs-simulator.nix b/nix/t1/conversion/sv-to-vcs-simulator.nix
index edede9231..3f9d61118 100644
--- a/nix/t1/conversion/sv-to-vcs-simulator.nix
+++ b/nix/t1/conversion/sv-to-vcs-simulator.nix
@@ -6,8 +6,10 @@
 
 { mainProgram
 , rtl
+, vsrc
 , enableTrace ? false
 , vcsLinkLibs ? [ ]
+, topModule ? null
 }:
 
 assert lib.assertMsg (builtins.typeOf vcsLinkLibs == "list") "vcsLinkLibs should be list of strings";
@@ -22,7 +24,7 @@ stdenv.mkDerivation rec {
   dontPatchELF = true;
   enableCover = true;
 
-  src = rtl;
+  dontUnpack = true;
 
   vcsArgs = [
     "-sverilog"
@@ -31,8 +33,13 @@ stdenv.mkDerivation rec {
     "-y"
     "$DWBB_DIR/sim_ver"
     "+libext+.v"
-    "-file"
-    "filelist.f"
+    "-F"
+    "${rtl}/filelist.f"
+  ]
+  ++ lib.singleton vsrc
+  ++ lib.optionals (topModule != null) [
+    "-top"
+    topModule
   ]
   ++ lib.optionals enableCover [
     "-cm"
@@ -78,7 +85,7 @@ stdenv.mkDerivation rec {
     cp $mainProgram $out/lib
     cp -r $mainProgram.daidir $out/lib
     ${lib.optionalString enableCover ''
-      cp -r ./cm.vdb $out/lib 
+      cp -r ./cm.vdb $out/lib
     ''}
 
     # We need to carefully handle string escape here, so don't use makeWrapper
diff --git a/nix/t1/conversion/sv-to-verilator-emulator.nix b/nix/t1/conversion/sv-to-verilator-emulator.nix
index bc719d8a1..f71f44b3c 100644
--- a/nix/t1/conversion/sv-to-verilator-emulator.nix
+++ b/nix/t1/conversion/sv-to-verilator-emulator.nix
@@ -7,8 +7,10 @@
 
 { mainProgram
 , rtl
+, vsrc
 , enableTrace ? false
 , extraVerilatorArgs ? [ ]
+, topModule ? null
 , ...
 }@overrides:
 
@@ -19,15 +21,14 @@ rec {
 
   __noChroot = true;
 
-  src = rtl;
+  dontUnpack = true;
 
   nativeBuildInputs = [ verilator ];
 
   # zlib is required for Rust to link against
   propagatedBuildInputs = [ zlib ];
 
-  verilatorFilelist = "filelist.f";
-  verilatorTop = "TestBench";
+  verilatorFilelist = "${rtl}/filelist.f";
   verilatorThreads = 8;
   verilatorArgs = [
     "--cc"
@@ -44,8 +45,11 @@ rec {
     "-Wno-lint"
     "-F"
     verilatorFilelist
+  ]
+  ++ lib.singleton vsrc
+  ++ lib.optionals (topModule != null) [
     "--top"
-    verilatorTop
+    topModule
   ]
   ++ extraVerilatorArgs
   ++ lib.optionals enableTrace [
diff --git a/nix/t1/default.nix b/nix/t1/default.nix
index 73fc8b81a..b4c273387 100644
--- a/nix/t1/default.nix
+++ b/nix/t1/default.nix
@@ -97,13 +97,13 @@ lib.makeScope newScope
     # * verilatorTop: Top module of the system verilog, default using "TestBench"
     # * verilatorThreads: Threads for final verilating, default using 8
     # * verilatorArgs: Final arguments that pass to the verilator.
-    sv-to-verilator-emulator = t1Scope.callPackage ./conversion/sv-to-verilator-emulator.nix { stdenv = moldStdenv; };
+    sv-to-verilator-emulator = lib.makeOverridable (t1Scope.callPackage ./conversion/sv-to-verilator-emulator.nix { stdenv = moldStdenv; });
 
     # sv-to-vcs-simulator :: { mainProgram :: String, rtl :: Derivation, enableTrace :: Bool, vcsLinkLibs :: List<String> } -> Derivation
     #
     # sv-to-vcs-simulator will compile the given rtl, link with path specified in vcsLinksLibs to produce a VCS emulator.
     # enableTrace is false by default;
-    sv-to-vcs-simulator = t1Scope.callPackage ./conversion/sv-to-vcs-simulator.nix { };
+    sv-to-vcs-simulator = lib.makeOverridable (t1Scope.callPackage ./conversion/sv-to-vcs-simulator.nix { });
   }
     # Nix specification for t1 (with spike only) emulator
     # We don't expect extra scope for t1 stuff, so here we merge the t1 at t1Scope level.
diff --git a/nix/t1/t1.nix b/nix/t1/t1.nix
index 788c43aa3..c7631bb87 100644
--- a/nix/t1/t1.nix
+++ b/nix/t1/t1.nix
@@ -34,10 +34,10 @@ let
             (scopeBuilderFn topName generatorData scope)))
       strippedGeneratorData;
 in
-forEachTop (topName: generator: innerMostScope: {
+forEachTop (topName: generator: self: {
   inherit configName topName;
 
-  cases = innerMostScope.callPackage ../../tests { };
+  cases = self.callPackage ../../tests { };
 
   mlirbc = t1Scope.chisel-to-mlirbc {
     outputName = "${generator.fullClassName}.mlirbc";
@@ -46,13 +46,13 @@ forEachTop (topName: generator: innerMostScope: {
   };
 
   lowered-mlirbc = t1Scope.finalize-mlirbc {
-    outputName = "lowered-" + innerMostScope.mlirbc.name;
-    mlirbc = innerMostScope.mlirbc;
+    outputName = "lowered-" + self.mlirbc.name;
+    mlirbc = self.mlirbc;
   };
 
   rtl = t1Scope.mlirbc-to-sv {
     outputName = "${generator.fullClassName}-rtl";
-    mlirbc = innerMostScope.lowered-mlirbc;
+    mlirbc = self.lowered-mlirbc;
     mfcArgs = [
       "-O=release"
       "--disable-all-randomization"
@@ -75,13 +75,13 @@ forEachTop (topName: generator: innerMostScope: {
     exec ${t1Scope.omreader-unwrapped}/bin/omreader \
       ${lib.replaceStrings ["elaborator"] ["omreader"] generator.fullClassName} \
       \$cmd \
-      --mlirbc-file ${innerMostScope.lowered-mlirbc}/${innerMostScope.lowered-mlirbc.name} \
+      --mlirbc-file ${self.lowered-mlirbc}/${self.lowered-mlirbc.name} \
       $@
     EOF
 
     chmod +x $out/bin/omreader
   '';
-  rtlDesignMetadataJson = runCommand "get-rtl-design-metadata-from-om" { nativeBuildInputs = [ jq innerMostScope.omreader ]; } ''
+  rtlDesignMetadataJson = runCommand "get-rtl-design-metadata-from-om" { nativeBuildInputs = [ jq self.omreader ]; } ''
     jq --null-input \
       --arg march $(omreader march) \
       --arg extensions $(omreader extensions) \
@@ -94,19 +94,19 @@ forEachTop (topName: generator: innerMostScope: {
          "xlen": (if ($march|startswith("rv32")) then 32 else 64 end) }' \
       > $out
   '';
-  rtlDesignMetadata = with builtins; fromJSON (readFile innerMostScope.rtlDesignMetadataJson);
+  rtlDesignMetadata = with builtins; fromJSON (readFile self.rtlDesignMetadataJson);
 
   # ---------------------------------------------------------------------------------
   # VERILATOR
   # ---------------------------------------------------------------------------------
-  makeDifftest = innerMostScope.callPackage ../../difftest { };
+  makeDifftest = self.callPackage ../../difftest { };
 
-  verilator-dpi-lib = innerMostScope.makeDifftest {
+  verilator-dpi-lib = self.makeDifftest {
     outputName = "${topName}-verilator-dpi-lib";
     emuType = "verilator";
     moduleType = "dpi_${topName}";
   };
-  verilator-dpi-lib-trace = innerMostScope.makeDifftest {
+  verilator-dpi-lib-trace = self.makeDifftest {
     outputName = "${topName}-verilator-trace-dpi-lib";
     emuType = "verilator";
     moduleType = "dpi_${topName}";
@@ -115,48 +115,60 @@ forEachTop (topName: generator: innerMostScope: {
 
   verilator-emu = t1Scope.sv-to-verilator-emulator {
     mainProgram = "${topName}-verilated-simulator";
-    rtl = innerMostScope.rtl;
-    extraVerilatorArgs = [ "${innerMostScope.verilator-dpi-lib}/lib/libdpi_${topName}.a" ];
+    topModule = "TestBench";
+    rtl = self.rtl;
+    # Create a new nix store root with only the ClockGen file, to avoid any source change in
+    # t1 source root causing emulator to rebuild.
+    vsrc = with lib.fileset; toSource {
+      root = ../../${topName}/vsrc;
+      fileset = unions [ ../../${topName}/vsrc/ClockGen.sv ];
+    };
+    extraVerilatorArgs = [ "${self.verilator-dpi-lib}/lib/libdpi_${topName}.a" ];
   };
-  verilator-emu-trace = t1Scope.sv-to-verilator-emulator {
-    mainProgram = "${topName}-verilated-trace-simulator";
-    rtl = innerMostScope.rtl;
+  verilator-emu-trace = self.verilator-emu.override {
     enableTrace = true;
-    extraVerilatorArgs = [ "${innerMostScope.verilator-dpi-lib-trace}/lib/libdpi_${topName}.a" ];
+    mainProgram = "${topName}-verilated-trace-simulator";
+    extraVerilatorArgs = [ "${self.verilator-dpi-lib-trace}/lib/libdpi_${topName}.a" ];
   };
 
   # ---------------------------------------------------------------------------------
   # VCS
   # ---------------------------------------------------------------------------------
-  vcs-dpi-lib = innerMostScope.makeDifftest {
+  vcs-dpi-lib = self.makeDifftest {
     outputName = "${topName}-vcs-dpi-lib";
     emuType = "vcs";
     moduleType = "dpi_${topName}";
   };
-  vcs-dpi-lib-trace = innerMostScope.makeDifftest {
+  vcs-dpi-lib-trace = self.makeDifftest {
     outputName = "${topName}-vcs-dpi-trace-lib";
     emuType = "vcs";
     enableTrace = true;
     moduleType = "dpi_${topName}";
   };
 
-  offline-checker = innerMostScope.makeDifftest {
+  offline-checker = self.makeDifftest {
     outputName = "${topName}-offline-checker";
     moduleType = "offline_${topName}";
   };
 
   vcs-emu = t1Scope.sv-to-vcs-simulator {
     mainProgram = "${topName}-vcs-simulator";
-    rtl = innerMostScope.rtl;
-    vcsLinkLibs = [ "${innerMostScope.vcs-dpi-lib}/lib/libdpi_${topName}.a" ];
+    topModule = "TestBench";
+    rtl = self.rtl;
+    # Create a new nix store root with only the ClockGen file, to avoid any source change in
+    # t1 source root causing emulator to rebuild.
+    vsrc = with lib.fileset; toSource {
+      root = ../../${topName}/vsrc;
+      fileset = unions [ ../../${topName}/vsrc/ClockGen.sv ];
+    };
+    vcsLinkLibs = [ "${self.vcs-dpi-lib}/lib/libdpi_${topName}.a" ];
   };
-  vcs-emu-trace = t1Scope.sv-to-vcs-simulator {
-    mainProgram = "${topName}-vcs-trace-simulator";
-    rtl = innerMostScope.rtl;
+  vcs-emu-trace = self.vcs-emu.override {
     enableTrace = true;
-    vcsLinkLibs = [ "${innerMostScope.vcs-dpi-lib-trace}/lib/libdpi_${topName}.a" ];
+    mainProgram = "${topName}-vcs-trace-simulator";
+    vcsLinkLibs = [ "${self.vcs-dpi-lib-trace}/lib/libdpi_${topName}.a" ];
   };
 
-  run = innerMostScope.callPackage ./run { };
+  run = self.callPackage ./run { };
 }) # end of forEachTop
 )