FEATURE: Add `read_vcd -begin_time`/`-end_time` activity windowing (#466)

* Add read_vcd -begin_time/-end_time activity windowing.

Limit VCD transition and duty counting to an optional time window so
activity annotation can ignore regions outside the interval of interest.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Format VcdCount::setFilter parameters one per line.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Address VCD begin/end review: VcdTime, sentinel, rename.

Use VcdTime and vcd_null_time instead of int64_t/-1, rename
VcdCount filter bounds to begin/end_time, rename the regression to
vcd_begin_end_time, and document read_vcd -begin_time/-end_time in
ChangeLog.txt.

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: James Cherry <34749589+jjcherry56@users.noreply.github.com>

doc/ChangeLog.txt

@@ -4,6 +4,15 @@ OpenSTA Timing Analyzer Release Notes
 This file summarizes user visible changes for each release.
 See ApiChangeLog.txt for changes to the STA api.
 
+2026/07/20
+----------
+
+The read_vcd command supports -begin_time / -end_time to limit
+activity annotation to a VCD time window.
+
+  read_vcd [-scope scope] [-mode mode_name]
+           [-begin_time begin_time] [-end_time end_time] filename
+
 2026/05/01
 ----------
 

power/Power.i

@@ -22,6 +22,8 @@
 // 
 // This notice may not be removed or altered from any source distribution.
 
+%include "stdint.i"
+
 %{
 #include "power/Power.hh"
 
@@ -29,12 +31,16 @@
 #include "Sdc.hh"
 #include "Sta.hh"
 #include "power/SaifReader.hh"
+#include "power/VcdParse.hh"
 #include "power/VcdReader.hh"
 
 using namespace sta;
 
 %}
 
+// Match power/VcdParse.hh vcd_null_time for Tcl defaults.
+%constant int64_t vcd_null_time = -1;
+
 %inline %{
 
 void
@@ -201,11 +207,13 @@ clock_min_period(const char *mode_name)
 void
 read_vcd_file(const char *filename,
               const char *scope,
-              const char *mode_name)
+              const char *mode_name,
+              int64_t begin_time,
+              int64_t end_time)
 {
   Sta *sta = Sta::sta();
   sta->ensureLibLinked();
-  readVcdActivities(filename, scope, mode_name, sta);
+  readVcdActivities(filename, scope, mode_name, begin_time, end_time, sta);
 }
 
 ////////////////////////////////////////////////////////////////
@@ -228,4 +236,12 @@ report_activity_annotation_cmd(bool report_unannotated,
                                   report_annotated);
 }
 
+
+void
+clear_power()
+{
+  Power *power = Sta::sta()->power();
+  power->clear();
+}
+
 %} // inline

power/Power.tcl

@@ -238,16 +238,18 @@ proc read_power_activities { args } {
     set scope $keys(-scope)
   }
   sta_warn 305 "read_power_activities is deprecated. Use read_vcd."
-  read_vcd_file $filename $scope
+  read_vcd_file $filename $scope [cmd_mode_name] \
+    $sta::vcd_null_time $sta::vcd_null_time
 }
 
 ################################################################
 
-define_cmd_args "read_vcd" { [-scope scope] [-mode mode_name] filename }
+define_cmd_args "read_vcd" \
+  {[-scope scope] [-mode mode_name] [-begin_time begin_time] [-end_time end_time] filename}
 
 proc read_vcd { args } {
   parse_key_args "read_vcd" args \
-    keys {-scope -mode_name} flags {}
+    keys {-scope -mode -begin_time -end_time} flags {}
 
   check_argc_eq1 "read_vcd" $args
   set filename [file nativename [lindex $args 0]]
@@ -259,7 +261,15 @@ proc read_vcd { args } {
   if { [info exists keys(-mode)] } {
     set mode_name $keys(-mode)
   }
-  read_vcd_file $filename $scope $mode_name
+  set begin_time $sta::vcd_null_time
+  if { [info exists keys(-begin_time)] } {
+    set begin_time $keys(-begin_time)
+  }
+  set end_time $sta::vcd_null_time
+  if { [info exists keys(-end_time)] } {
+    set end_time $keys(-end_time)
+  }
+  read_vcd_file $filename $scope $mode_name $begin_time $end_time
 }
 
 ################################################################

power/VcdParse.cc

@@ -42,8 +42,13 @@ namespace sta {
 
 void
 VcdParse::read(const char *filename,
-               VcdReader *reader)
+               VcdReader *reader,
+               VcdTime begin_time,
+               VcdTime end_time)
 {
+  begin_time_ = begin_time;
+  end_time_ = end_time;
+
   stream_ = gzopen(filename, "r");
   if (stream_) {
     Stats stats(debug_, report_);
@@ -51,6 +56,11 @@ VcdParse::read(const char *filename,
     reader_ = reader;
     file_line_ = 0;
     stmt_line_ = 0;
+
+    // If user specified a start time, set it now.
+    if (begin_time != vcd_null_time) {
+      reader_->setTimeMin(begin_time);
+    }
     std::string token = getToken();
     while (!token.empty()) {
       if (token == "$date")
@@ -87,7 +97,10 @@ VcdParse::read(const char *filename,
           report_->fileError(806, filename_, file_line_, "time out of range {}",
                              token.substr(1));
         }
+        // Set time min to start time if it is not set at beginning
+        if (begin_time == vcd_null_time) {
           reader_->setTimeMin(time_);
+        }
         prev_time_ = time_;
       }
       else if (token[0] == '$')
@@ -238,8 +251,14 @@ VcdParse::parseVarValues()
     }
     token = getToken();
   }
+
+  // Set time_max to end_time if specified, otherwise use actual parsed time
+  if (end_time_ != vcd_null_time) {
+    reader_->setTimeMax(end_time_);
+  } else {
     reader_->setTimeMax(time_);
   }
+}
 
 std::string
 VcdParse::readStmtString()

power/VcdParse.hh

@@ -36,6 +36,9 @@ namespace sta {
 using VcdTime = int64_t;
 using VcdScope = std::vector<std::string>;
 
+// Sentinel for an unset begin/end time window bound.
+constexpr VcdTime vcd_null_time = -1;
+
 enum class VcdVarType {
   wire,
   reg,
@@ -64,7 +67,9 @@ public:
   VcdParse(Report *report,
            Debug *debug);
   void read(const char *filename,
-            VcdReader *reader);
+            VcdReader *reader,
+            VcdTime begin_time,
+            VcdTime end_time);
 
 private:
   void parseTimescale();
@@ -87,6 +92,11 @@ private:
 
   VcdTime time_ = 0;
   VcdTime prev_time_ = 0;
+
+  // Arguments to VcdParse
+  VcdTime begin_time_ = vcd_null_time;
+  VcdTime end_time_ = vcd_null_time;
+
   VcdScope scope_;
 
   Report *report_;

power/VcdReader.cc

@@ -52,48 +52,89 @@ public:
   VcdTime highTime(VcdTime time_max) const;
   void incrCounts(VcdTime time,
                   char value);
-  void incrCounts(VcdTime time,
-                  int64_t value);
   void addPin(const Pin *pin);
   const PinSeq &pins() const { return pins_; }
 
+  static void setFilter(VcdTime begin,
+                        VcdTime end);
+
 private:
+  VcdTime clippedIntervalStart() const;
   PinSeq pins_;
-  VcdTime prev_time_ = -1;
+  VcdTime prev_time_ = vcd_null_time;
   char prev_value_ = '\0';
   VcdTime high_time_ = 0;
   double transition_count_ = 0;
+
+  static VcdTime begin_time_;
+  static VcdTime end_time_;
 };
 
+// Define static members
+VcdTime VcdCount::begin_time_ = vcd_null_time;
+VcdTime VcdCount::end_time_ = vcd_null_time;
+
 void
 VcdCount::addPin(const Pin *pin)
 {
   pins_.push_back(pin);
 }
 
+VcdTime
+VcdCount::clippedIntervalStart() const
+{
+  // Clip prev_time_ to begin_time if signal went high before the window.
+  return (begin_time_ != vcd_null_time && prev_time_ < begin_time_)
+          ? begin_time_ : prev_time_;
+}
+
+void
+VcdCount::setFilter(VcdTime begin,
+                    VcdTime end)
+{
+  begin_time_ = begin;
+  end_time_ = end;
+}
+
 void
 VcdCount::incrCounts(VcdTime time,
                      char value)
 {
-  // Initial value does not coontribute to transitions or high time.
+  // Determine if this time point is within the filter window
-  if (prev_time_ != -1) {
+  bool in_window = (begin_time_ == vcd_null_time || time >= begin_time_)
-    if (prev_value_ == '1')
+                   && (end_time_ == vcd_null_time || time <= end_time_);
-      high_time_ += time - prev_time_;
+
+  // Initial value does not contribute to transitions or high time.
+  if (prev_time_ != vcd_null_time && in_window) {
+    if (prev_value_ == '1') {
+      VcdTime interval_start = clippedIntervalStart();
+      if (time > interval_start)
+        high_time_ += time - interval_start;
+    }
     if (value != prev_value_)
       transition_count_ +=
           (value == 'X' || value == 'Z' || prev_value_ == 'X' || prev_value_ == 'Z')
           ? .5
           : 1.0;
   }
+  // Update state for transitions before or within the window.
+  // This prevents values after window boundaries corrupting high time.
+  if (end_time_ == vcd_null_time || time <= end_time_) {
     prev_time_ = time;
     prev_value_ = value;
   }
+}
 
 VcdTime
 VcdCount::highTime(VcdTime time_max) const
 {
-  if (prev_value_ == '1')
+  if (prev_value_ == '1') {
-    return high_time_ + time_max - prev_time_;
+    VcdTime interval_start = clippedIntervalStart();
+    if (time_max > interval_start)
+      return high_time_ + time_max - interval_start;
+    else
+      return high_time_;
+  }
   else
     return high_time_;
 }
@@ -180,12 +221,14 @@ VcdCountReader::setTimeUnit(std::string_view ,
 void
 VcdCountReader::setTimeMin(VcdTime time)
 {
+  debugPrint(debug_, "read_vcd", 1, "setTimeMin called with time {}", time);
   time_min_ = time;
 }
 
 void
 VcdCountReader::setTimeMax(VcdTime time)
 {
+  debugPrint(debug_, "read_vcd", 1, "setTimeMax called with time {}", time);
   time_max_ = time;
 }
 
@@ -335,6 +378,8 @@ class ReadVcdActivities : public StaState
 public:
   ReadVcdActivities(std::string_view filename,
                     std::string_view scope,
+                    VcdTime begin_time,
+                    VcdTime end_time,
                     const Sdc *sdc,
                     Sta *sta);
   void readActivities();
@@ -345,6 +390,8 @@ private:
                       double transition_count);
 
   const std::string filename_;
+  VcdTime begin_time_;
+  VcdTime end_time_;
 
   std::set<const Pin *> annotated_pins_;
   VcdCountReader vcd_reader_;
@@ -359,20 +406,26 @@ void
 readVcdActivities(std::string_view filename,
                   std::string_view scope,
                   std::string_view mode_name,
+                  VcdTime begin_time,
+                  VcdTime end_time,
                   Sta *sta)
 {
   const Mode *mode = sta->findMode(mode_name);
   const Sdc *sdc = mode->sdc();
-  ReadVcdActivities reader(filename, scope, sdc, sta);
+  ReadVcdActivities reader(filename, scope, begin_time, end_time, sdc, sta);
   reader.readActivities();
 }
 
 ReadVcdActivities::ReadVcdActivities(std::string_view filename,
                                      std::string_view scope,
+                                     VcdTime begin_time,
+                                     VcdTime end_time,
                                      const Sdc *sdc,
                                      Sta *sta) :
   StaState(sta),
   filename_(filename),
+  begin_time_(begin_time),
+  end_time_(end_time),
   vcd_reader_(scope,
               sdc_network_,
               report_,
@@ -391,7 +444,9 @@ ReadVcdActivities::readActivities()
   if (clks.empty())
     report_->error(820, "No clocks have been defined.");
 
-  vcd_parse_.read(filename_.c_str(), &vcd_reader_);
+  // Set the time window filter once globally
+  VcdCount::setFilter(begin_time_, end_time_);
+  vcd_parse_.read(filename_.c_str(), &vcd_reader_, begin_time_, end_time_);
 
   if (vcd_reader_.timeMax() > 0)
     setActivities();

power/VcdReader.hh

@@ -26,6 +26,8 @@
 
 #include <string>
 
+#include "VcdParse.hh"
+
 namespace sta {
 
 class Sta;
@@ -34,6 +36,8 @@ void
 readVcdActivities(std::string_view filename,
                   std::string_view scope,
                   std::string_view mode_name,
+                  VcdTime begin_time,
+                  VcdTime end_time,
                   Sta *sta);
 
 } // namespace sta

test/regression_vars.tcl

@@ -169,6 +169,7 @@ record_public_tests {
   report_json2
   suppress_msg
   user_properties
+  vcd_begin_end_time
   verilog_attribute
   verilog_well_supplies
   verilog_specify

test/vcd_begin_end_time.ok

@@ -0,0 +1,60 @@
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.0666665 0.667
+u_inv/A 0.0666665 0.333
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.1 1.000
+u_inv/A 0.1 0.000
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.2 0.000
+u_inv/A 0.2 1.000
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.1 1.000
+u_inv/A 0.1 0.000
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.25 0.500
+u_inv/A 0.25 0.500
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.125 0.750
+u_inv/A 0.125 0.250
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.125 0.250
+u_inv/A 0.125 0.750
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.25 0.500
+u_inv/A 0.25 0.500
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.125 0.250
+u_inv/A 0.125 0.750
+
+Annotated 2 pin activities.
+Pin Name Activity Duty Cycle
+--------------------------------------------------------
+u_inv/Y 0.125 0.750
+u_inv/A 0.125 0.250
+

test/vcd_begin_end_time.tcl

@@ -0,0 +1,70 @@
+# Report pin activities
+proc report_activities { } {
+  set pins [get_pins -hierarchical *]
+  set clk_freq [expr 1.0 / (10 * 1e-12)]
+  puts "Pin Name Activity Duty Cycle"
+  puts "--------------------------------------------------------"
+  foreach pin $pins {
+    set prop [get_property $pin activity]
+    set transitions_per_sec [lindex $prop 0]
+    set duty [lindex $prop 1]
+    set activity [expr double($transitions_per_sec) / [expr $clk_freq * 2]]
+    puts "[get_full_name $pin] $activity $duty"
+  }
+  puts ""
+}
+
+# Setup
+read_liberty asap7_invbuf.lib.gz
+read_verilog vcd_begin_end_time.v
+link_design top
+
+# Define clock period in ps
+create_clock -name vclk -period 10
+
+# Full VCD reading works (normal behavior)
+# VCD changes at time 50 and 100 (inverter)
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top
+report_activities
+
+# Read VCD from start to first transition point
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -end_time 50
+report_activities
+
+# Read VCD from first transition point to second transition point
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 50 -end_time 100
+report_activities
+
+# Read VCD from second transition point to end
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 100
+report_activities
+
+# Read VCD around the first transition point
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 40 -end_time 60
+report_activities
+
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 20 -end_time 60
+report_activities
+
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 40 -end_time 80
+report_activities
+
+# Read VCD around the second transition point (should mirror the first)
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 90 -end_time 110
+report_activities
+
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 70 -end_time 110
+report_activities
+
+sta::clear_power
+read_vcd vcd_begin_end_time.vcd -scope top -begin_time 90 -end_time 130
+report_activities

test/vcd_begin_end_time.v

@@ -0,0 +1,14 @@
+`timescale 1ps/1ps
+
+module top (
+    input  wire A,
+    input  wire clk,
+    output wire Y
+);
+
+    INVx2_ASAP7_75t_R u_inv (
+        .A(A),
+        .Y(Y)
+    );
+
+endmodule

test/vcd_begin_end_time.vcd

@@ -0,0 +1,26 @@
+$date
+   Mon Mar 16 2026
+$end
+$version
+   VCD Test File
+$end
+$timescale
+   1ps
+$end
+$scope module top $end
+$var wire 1 ! A $end
+$var wire 1 " Y $end
+$upscope $end
+$enddefinitions $end
+#0
+$dumpvars
+0!
+1"
+$end
+#50
+1!
+0"
+#100
+0!
+1"
+#150