diff --git a/litex/build/sim/config.py b/litex/build/sim/config.py
index 07a2be574..43eeb1565 100644
--- a/litex/build/sim/config.py
+++ b/litex/build/sim/config.py
@@ -3,13 +3,11 @@
 # License: BSD
 
 import json
+import math
 
 class SimConfig():
-    def __init__(self, default_clk=None):
+    def __init__(self):
         self.modules = []
-        self.default_clk = default_clk
-        if default_clk:
-            self.add_clocker(default_clk)
 
     def _format_interfaces(self, interfaces):
         if not isinstance(interfaces, list):
@@ -23,15 +21,18 @@ class SimConfig():
             new.append(obj)
         return new
 
-    def add_clocker(self, clk):
-        self.add_module("clocker", [], clocks=clk, tickfirst=True)
+    def _format_timebase(self):
+        clockers = [m for m in self.modules if m["module"] == "clocker"]
+        timebase_ps = _calculate_timebase_ps(clockers)
+        return {"timebase": int(timebase_ps)}
 
-    def add_module(self, name, interfaces, clocks=None, args=None, tickfirst=False):
+    def add_clocker(self, clk, freq_hz, phase_deg=0):
+        args = {"freq_hz": freq_hz, "phase_deg": phase_deg}
+        self.add_module("clocker", [], clocks=clk, tickfirst=True, args=args)
+
+    def add_module(self, name, interfaces, clocks="sys_clk", args=None, tickfirst=False):
         interfaces = self._format_interfaces(interfaces)
-        if clocks:
-            interfaces += self._format_interfaces(clocks)
-        else:
-            interfaces += [self.default_clk]
+        interfaces += self._format_interfaces(clocks)
         newmod = {
             "module": name,
             "interface": interfaces,
@@ -49,4 +50,53 @@ class SimConfig():
         return False
 
     def get_json(self):
-        return json.dumps(self.modules, indent=4)
+        assert "clocker" in (m["module"] for m in self.modules), \
+            "No simulation clocker found! Use sim_config.add_clocker() to define one or more clockers."
+        config = self.modules + [self._format_timebase()]
+        return json.dumps(config, indent=4)
+
+def _calculate_timebase_ps(clockers):
+    """Calculate timebase for a list of clocker modules
+
+    Clock edges happen at time instants:
+        t(n) = n * T/2 + P/360 * T
+    where: T - clock period, P - clock phase [deg]
+    We must be able to represent clock edges with the timebase B:
+        t(n) mod B = 0, for all n
+    In this function checks that:
+        ((T/2) mod B = 0) AND ((P/360 * T) mod B = 0)
+
+    Currently we allow only for integer periods (in ps), which it's quite restrictive.
+    """
+    # convert to picoseconds, 1ps is our finest timebase for dumping simulation data
+    periods_ps = [1e12 / c["args"]["freq_hz"] for c in clockers]
+    phase_shifts_ps = [p * c["args"]["phase_deg"]/360 for c, p in zip(clockers, periods_ps)]
+
+    # calculate timebase as greatest common denominator
+    timebase_ps = None
+    for period, phase_shift in zip(periods_ps, phase_shifts_ps):
+        if timebase_ps is None:
+            timebase_ps = int(period/2)
+        timebase_ps = math.gcd(timebase_ps, int(period/2))
+        timebase_ps = math.gcd(timebase_ps, int(phase_shift))
+
+    # check correctness
+    for clocker, period, phase_shift in zip(clockers, periods_ps, phase_shifts_ps):
+        def error(description):
+            return f"""
+SimConfig:
+{description}:
+  timebase = {timebase_ps}ps, period = {period}ps, phase_shift = {phase_shift}ps,
+  clocker[args] = {clocker["args"]}
+Adjust clock definitions so that integer multiple of 1ps can be used as a timebase.
+            """.strip()
+
+        assert int(period) == period, error("Non-integer period")
+        assert int(phase_shift) == phase_shift, error("Non-integer phase_shift")
+
+        assert (period/2 % timebase_ps) == 0, \
+            error("Could not find an integer timebase for period")
+        assert (phase_shift % timebase_ps) == 0, \
+            error("Could not find an integer timebase for phase shift")
+
+    return timebase_ps
diff --git a/litex/build/sim/core/modules.h b/litex/build/sim/core/modules.h
index ea83d9ebd..961042113 100644
--- a/litex/build/sim/core/modules.h
+++ b/litex/build/sim/core/modules.h
@@ -3,6 +3,8 @@
 #ifndef __MODULE_H_
 #define __MODULE_H_
 
+#include <stdint.h>
+#include <stdbool.h>
 #include "pads.h"
 
 struct interface_s {
@@ -25,7 +27,7 @@ struct ext_module_s {
   int (*new_sess)(void **, char *);
   int (*add_pads)(void *, struct pad_list_s *);
   int (*close)(void*);
-  int (*tick)(void*);
+  int (*tick)(void*, uint64_t);
 };
 
 struct ext_module_list_s {
@@ -33,8 +35,24 @@ struct ext_module_list_s {
   struct ext_module_list_s *next;
 };
 
-int litex_sim_file_to_module_list(char *filename, struct module_s **mod);
+struct clk_edge_t {
+  int last_clk;
+};
+
+int litex_sim_file_parse(char *filename, struct module_s **mod, uint64_t *timebase);
 int litex_sim_load_ext_modules(struct ext_module_list_s **mlist);
 int litex_sim_find_ext_module(struct ext_module_list_s *first, char *name , struct ext_module_list_s **found);
 
+inline bool clk_pos_edge(struct clk_edge_t *edge, int new_clk) {
+  bool is_edge = edge->last_clk == 0 && new_clk == 1;
+  edge->last_clk = new_clk;
+  return is_edge;
+}
+
+inline bool clk_neg_edge(struct clk_edge_t *edge, int new_clk) {
+  bool is_edge = edge->last_clk == 1 && new_clk == 0;
+  edge->last_clk = new_clk;
+  return is_edge;
+}
+
 #endif
diff --git a/litex/build/sim/core/modules/clocker/clocker.c b/litex/build/sim/core/modules/clocker/clocker.c
index cfd2f1039..785d0d719 100644
--- a/litex/build/sim/core/modules/clocker/clocker.c
+++ b/litex/build/sim/core/modules/clocker/clocker.c
@@ -1,11 +1,15 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <json-c/json.h>
 #include "error.h"
 #include "modules.h"
 
 struct session_s {
-  char *sys_clk;
+  char *clk;
+  char *name;
+  uint32_t freq_hz;
+  uint16_t phase_deg;
 };
 
 static int litex_sim_module_pads_get( struct pad_s *pads, char *name, void **signal)
@@ -34,6 +38,53 @@ out:
   return ret;
 }
 
+static int clocker_parse_args(struct session_s *s, const char *args)
+{
+  int ret = RC_OK;
+  json_object *args_json = NULL;
+  json_object *freq_json = NULL;
+  json_object *phase_json = NULL;
+
+  args_json = json_tokener_parse(args);
+  if (!args_json) {
+    ret = RC_JSERROR;
+    fprintf(stderr, "[clocker] Could not parse args: %s\n", args);
+    goto out;
+  }
+
+  if(!json_object_object_get_ex(args_json, "freq_hz", &freq_json))
+  {
+    ret = RC_JSERROR;
+    fprintf(stderr, "[clocker] \"freq_hz\" not found in args: %s\n", json_object_to_json_string(args_json));
+    goto out;
+  }
+
+  if(!json_object_object_get_ex(args_json, "phase_deg", &phase_json))
+  {
+    ret = RC_JSERROR;
+    fprintf(stderr, "[clocker] \"phase_deg\" not found in args: %s\n", json_object_to_json_string(args_json));
+    goto out;
+  }
+
+  s->freq_hz = json_object_get_uint64(freq_json);
+  s->phase_deg = json_object_get_uint64(phase_json);
+
+  if (s->freq_hz == 0) {
+    ret = RC_JSERROR;
+    fprintf(stderr, "[clocker] \"freq_hz\" must be different than 0\n");
+    goto out;
+  }
+
+  if (s->phase_deg >= 360) {
+    ret = RC_JSERROR;
+    fprintf(stderr, "[clocker] \"phase_deg\" must be in range [0, 360)\n");
+    goto out;
+  }
+out:
+  if(args_json) json_object_put(args_json);
+  return ret;
+}
+
 static int clocker_start()
 {
   printf("[clocker] loaded\n");
@@ -58,6 +109,7 @@ static int clocker_new(void **sess, char *args)
   }
   memset(s, 0, sizeof(struct session_s));
 
+  clocker_parse_args(s, args);
 out:
   *sess=(void*)s;
   return ret;
@@ -75,20 +127,34 @@ static int clocker_add_pads(void *sess, struct pad_list_s *plist)
   }
   pads = plist->pads;
 
-  if(!strcmp(plist->name, "sys_clk")) {
-    litex_sim_module_pads_get(pads, "sys_clk", (void**)&s->sys_clk);
+  ret = litex_sim_module_pads_get(pads, plist->name, (void**)&s->clk);
+  if (ret != RC_OK) {
+    goto out;
   }
 
-  *s->sys_clk=0;
-
+  s->name = plist->name;
+  *s->clk=0;
+  printf("[clocker] %s: freq_hz=%u, phase_deg=%u\n", s->name, s->freq_hz, s->phase_deg);
 out:
   return ret;
 }
 
-static int clocker_tick(void *sess)
+static int clocker_tick(void *sess, uint64_t time_ps)
 {
-  struct session_s *s=(struct session_s*)sess;
-  *s->sys_clk = ~(*s->sys_clk);
+  static const uint64_t ps_in_sec = 1000000000000ull;
+  struct session_s *s = (struct session_s*) sess;
+
+  uint64_t period_ps = ps_in_sec / s->freq_hz;
+  uint64_t phase_shift_ps = period_ps * s->phase_deg / 360;
+
+  // phase-shifted time relative to start of current period
+  uint64_t rel_time_ps = (time_ps - phase_shift_ps) % period_ps;
+  if (rel_time_ps < (period_ps/2)) {
+    *s->clk = 1;
+  } else {
+    *s->clk = 0;
+  }
+
   return 0;
 }
 
diff --git a/litex/build/sim/core/modules/ethernet/ethernet.c b/litex/build/sim/core/modules/ethernet/ethernet.c
index 30aca3f3b..3896cf9c7 100644
--- a/litex/build/sim/core/modules/ethernet/ethernet.c
+++ b/litex/build/sim/core/modules/ethernet/ethernet.c
@@ -200,14 +200,16 @@ out:
   return ret;
 }
 
-static int ethernet_tick(void *sess)
+static int ethernet_tick(void *sess, uint64_t time_ps)
 {
+  static struct clk_edge_t edge;
   char c;
   struct session_s *s = (struct session_s*)sess;
   struct eth_packet_s *pep;
 
-  if(*s->sys_clk == 0)
+  if(!clk_pos_edge(&edge, *s->sys_clk)) {
     return RC_OK;
+  }
 
   *s->tx_ready = 1;
   if(*s->tx_valid == 1) {
diff --git a/litex/build/sim/core/modules/jtagremote/jtagremote.c b/litex/build/sim/core/modules/jtagremote/jtagremote.c
index 6a42b3756..87c7c1598 100644
--- a/litex/build/sim/core/modules/jtagremote/jtagremote.c
+++ b/litex/build/sim/core/modules/jtagremote/jtagremote.c
@@ -208,14 +208,16 @@ out:
   return ret;
 
 }
-static int jtagremote_tick(void *sess)
+static int jtagremote_tick(void *sess, uint64_t time_ps)
 {
+  static struct clk_edge_t edge;
 	char c, val;
 	int ret = RC_OK;
 
   struct session_s *s = (struct session_s*)sess;
-  if(*s->sys_clk == 0)
+  if(!clk_pos_edge(&edge, *s->sys_clk)) {
     return RC_OK;
+  }
 
   s->cntticks++;
   if(s->cntticks % 10)
diff --git a/litex/build/sim/core/modules/serial2console/serial2console.c b/litex/build/sim/core/modules/serial2console/serial2console.c
index 114947f5f..678024525 100644
--- a/litex/build/sim/core/modules/serial2console/serial2console.c
+++ b/litex/build/sim/core/modules/serial2console/serial2console.c
@@ -145,10 +145,11 @@ out:
   return ret;
 }
 
-static int serial2console_tick(void *sess) {
+static int serial2console_tick(void *sess, uint64_t time_ps) {
+  static struct clk_edge_t edge;
   struct session_s *s = (struct session_s*)sess;
 
-  if(*s->sys_clk == 0) {
+  if(!clk_pos_edge(&edge, *s->sys_clk)) {
     return RC_OK;
   }
 
diff --git a/litex/build/sim/core/modules/serial2tcp/serial2tcp.c b/litex/build/sim/core/modules/serial2tcp/serial2tcp.c
index 8691d175b..990228603 100644
--- a/litex/build/sim/core/modules/serial2tcp/serial2tcp.c
+++ b/litex/build/sim/core/modules/serial2tcp/serial2tcp.c
@@ -210,14 +210,16 @@ out:
   return ret;
 
 }
-static int serial2tcp_tick(void *sess)
+static int serial2tcp_tick(void *sess, uint64_t time_ps)
 {
+  static struct clk_edge_t edge;
   char c;
   int ret = RC_OK;
 
   struct session_s *s = (struct session_s*)sess;
-  if(*s->sys_clk == 0)
+  if(!clk_pos_edge(&edge, *s->sys_clk)) {
     return RC_OK;
+  }
 
   *s->tx_ready = 1;
   if(s->fd && *s->tx_valid) {
diff --git a/litex/build/sim/core/modules/spdeeprom/spdeeprom.c b/litex/build/sim/core/modules/spdeeprom/spdeeprom.c
index 1d1584fee..0a80aeff2 100644
--- a/litex/build/sim/core/modules/spdeeprom/spdeeprom.c
+++ b/litex/build/sim/core/modules/spdeeprom/spdeeprom.c
@@ -65,7 +65,7 @@ struct session_s {
 static int spdeeprom_start();
 static int spdeeprom_new(void **sess, char *args);
 static int spdeeprom_add_pads(void *sess, struct pad_list_s *plist);
-static int spdeeprom_tick(void *sess);
+static int spdeeprom_tick(void *sess, uint64_t time_ps);
 // EEPROM simulation
 static void fsm_tick(struct session_s *s);
 static enum SerialState state_serial_next(struct session_s *s);
@@ -162,15 +162,16 @@ out:
   return ret;
 }
 
-static int spdeeprom_tick(void *sess)
+static int spdeeprom_tick(void *sess, uint64_t time_ps)
 {
+  static struct clk_edge_t edge;
   struct session_s *s = (struct session_s*) sess;
 
   if (s->sda_in == 0 || s->sda_out == 0 || s->scl == 0) {
       return RC_OK;
   }
 
-  if(*s->sys_clk == 0) {
+  if(!clk_pos_edge(&edge, *s->sys_clk)) {
     return RC_OK;
   }
 
diff --git a/litex/build/sim/core/modules/xgmii_ethernet/xgmii_ethernet.c b/litex/build/sim/core/modules/xgmii_ethernet/xgmii_ethernet.c
index 4474e1f2a..d8152e96c 100644
--- a/litex/build/sim/core/modules/xgmii_ethernet/xgmii_ethernet.c
+++ b/litex/build/sim/core/modules/xgmii_ethernet/xgmii_ethernet.c
@@ -233,12 +233,13 @@ unsigned int g_mask = 0xff;
 unsigned int g_idle = 0x07070707;
 #endif
 
-static int xgmii_ethernet_tick(void *sess)
+static int xgmii_ethernet_tick(void *sess, uint64_t time_ps)
 {
+  static struct clk_edge_t edge;
   struct session_s *s = (struct session_s*)sess;
   struct eth_packet_s *pep;
 
-  if(*s->sys_clk == 0) {
+  if(!clk_pos_edge(&edge, *s->sys_clk)) {
     s->preamble=0;
     return RC_OK;
   }
diff --git a/litex/build/sim/core/parse.c b/litex/build/sim/core/parse.c
index 6af8a198b..5b45cb778 100644
--- a/litex/build/sim/core/parse.c
+++ b/litex/build/sim/core/parse.c
@@ -228,11 +228,10 @@ static int json_to_module_list(json_object *obj, struct module_s **mod)
   for(i = 0; i < n; i++)
   {
     tobj = json_object_array_get_idx(obj, i);
+
     if(!json_object_object_get_ex(tobj, "module", &name))
     {
-      ret=RC_JSERROR;
-      eprintf("expected \"module\" in object (%s)\n", json_object_to_json_string(tobj));
-      goto out;
+      continue;
     }
 
     if(!json_object_object_get_ex(tobj, "interface", &interface))
@@ -288,6 +287,14 @@ static int json_to_module_list(json_object *obj, struct module_s **mod)
       m->tickfirst = json_object_get_boolean(tickfirst);
     }
   }
+
+  if (!m)
+  {
+      ret = RC_JSERROR;
+      eprintf("No modules found in config file:\n%s\n", json_object_to_json_string(obj));
+      goto out;
+  }
+
   *mod = first;
   first=NULL;
 
@@ -299,7 +306,68 @@ out:
   return ret;
 }
 
-int litex_sim_file_to_module_list(char *filename, struct module_s **mod)
+static int json_get_timebase(json_object *obj, uint64_t *timebase)
+{
+  json_object *tobj;
+  int ret=RC_OK;
+  int i, n;
+  uint64_t _timebase = 0;
+  json_object *json_timebase;
+
+  if(!obj || !timebase)
+  {
+    ret = RC_INVARG;
+    eprintf("Wrong arguments\n");
+    goto out;
+  }
+
+  if(!json_object_is_type(obj, json_type_array))
+  {
+    ret=RC_JSERROR;
+    eprintf("Config file must be an array\n");
+    goto out;
+  }
+
+  n = json_object_array_length(obj);
+  for(i = 0; i < n; i++)
+  {
+    tobj = json_object_array_get_idx(obj, i);
+
+    if(!json_object_object_get_ex(tobj, "timebase", &json_timebase))
+    {
+      continue;
+    }
+
+    if (_timebase != 0)
+    {
+      ret=RC_JSERROR;
+      eprintf("\"timebase\" found multiple times: in object (%s)\n", json_object_to_json_string(tobj));
+      goto out;
+    }
+
+    _timebase = json_object_get_uint64(json_timebase);
+    if (_timebase == 0)
+    {
+      ret=RC_JSERROR;
+      eprintf("\"timebase\" cannot be zero: in object (%s)\n", json_object_to_json_string(tobj));
+      goto out;
+    }
+  }
+
+  if (_timebase == 0)
+  {
+    ret=RC_JSERROR;
+    eprintf("No \"timebase\" found in config:\n%s\n", json_object_to_json_string(obj));
+    goto out;
+  }
+  *timebase = _timebase;
+
+out:
+  return ret;
+}
+
+
+int litex_sim_file_parse(char *filename, struct module_s **mod, uint64_t *timebase)
 {
   struct module_s *m=NULL;
   json_object *obj=NULL;
@@ -318,6 +386,12 @@ int litex_sim_file_to_module_list(char *filename, struct module_s **mod)
     goto out;
   }
 
+  ret = json_get_timebase(obj, timebase);
+  if(RC_OK != ret)
+  {
+    goto out;
+  }
+
   ret = json_to_module_list(obj, &m);
   if(RC_OK != ret)
   {
diff --git a/litex/build/sim/core/sim.c b/litex/build/sim/core/sim.c
index a4e9adeae..33e6edb34 100644
--- a/litex/build/sim/core/sim.c
+++ b/litex/build/sim/core/sim.c
@@ -31,6 +31,8 @@ struct session_list_s {
   struct session_list_s *next;
 };
 
+uint64_t timebase_ps = 1;
+uint64_t sim_time_ps = 0;
 struct session_list_s *sesslist=NULL;
 struct event_base *base=NULL;
 
@@ -63,7 +65,7 @@ static int litex_sim_initialize_all(void **sim, void *base)
   }
 
   /* Load configuration */
-  ret = litex_sim_file_to_module_list("sim_config.js", &ml);
+  ret = litex_sim_file_parse("sim_config.js", &ml, &timebase_ps);
   if(RC_OK != ret)
   {
     goto out;
@@ -181,16 +183,20 @@ static void cb(int sock, short which, void *arg)
     for(s = sesslist; s; s=s->next)
     {
       if(s->tickfirst)
-	s->module->tick(s->session);
+        s->module->tick(s->session, sim_time_ps);
     }
-    litex_sim_eval(vsim);
+
+    litex_sim_eval(vsim, sim_time_ps);
     litex_sim_dump();
+
     for(s = sesslist; s; s=s->next)
     {
       if(!s->tickfirst)
-	s->module->tick(s->session);
+        s->module->tick(s->session, sim_time_ps);
     }
 
+    sim_time_ps += timebase_ps;
+
     if (litex_sim_got_finish()) {
         event_base_loopbreak(base);
         break;
diff --git a/litex/build/sim/core/veril.cpp b/litex/build/sim/core/veril.cpp
index 914743604..b65aa8b67 100644
--- a/litex/build/sim/core/veril.cpp
+++ b/litex/build/sim/core/veril.cpp
@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
 #include "Vsim.h"
 #include "verilated.h"
 #ifdef TRACE_FST
@@ -16,13 +17,15 @@ VerilatedFstC* tfp;
 #else
 VerilatedVcdC* tfp;
 #endif
-long tfp_start;
-long tfp_end;
+uint64_t tfp_start;
+uint64_t tfp_end;
+uint64_t main_time = 0;
 
-extern "C" void litex_sim_eval(void *vsim)
+extern "C" void litex_sim_eval(void *vsim, uint64_t time_ps)
 {
   Vsim *sim = (Vsim*)vsim;
   sim->eval();
+  main_time = time_ps;
 }
 
 extern "C" void litex_sim_init_cmdargs(int argc, char *argv[])
@@ -34,7 +37,7 @@ extern "C" void litex_sim_init_tracer(void *vsim, long start, long end)
 {
   Vsim *sim = (Vsim*)vsim;
   tfp_start = start;
-  tfp_end = end;
+  tfp_end = end >= 0 ? end : UINT64_MAX;
   Verilated::traceEverOn(true);
 #ifdef TRACE_FST
       tfp = new VerilatedFstC;
@@ -45,20 +48,15 @@ extern "C" void litex_sim_init_tracer(void *vsim, long start, long end)
       sim->trace(tfp, 99);
       tfp->open("sim.vcd");
 #endif
+  tfp->set_time_unit("1ps");
+  tfp->set_time_resolution("1ps");
 }
 
 extern "C" void litex_sim_tracer_dump()
 {
-  static unsigned int ticks=0;
-  int dump = 1;
-  if (ticks < tfp_start)
-      dump = 0;
-  if (tfp_end != -1)
-      if (ticks > tfp_end)
-          dump = 0;
-  if (dump)
-      tfp->dump(ticks);
-  ticks++;
+  if (tfp_start <= main_time && main_time <= tfp_end) {
+    tfp->dump(main_time);
+  }
 }
 
 extern "C" int litex_sim_got_finish()
@@ -73,7 +71,6 @@ extern "C" void litex_sim_coverage_dump()
 }
 #endif
 
-vluint64_t main_time = 0;
 double sc_time_stamp()
 {
   return main_time;
diff --git a/litex/build/sim/core/veril.h b/litex/build/sim/core/veril.h
index 4944316e6..2daac15db 100644
--- a/litex/build/sim/core/veril.h
+++ b/litex/build/sim/core/veril.h
@@ -3,17 +3,19 @@
 #ifndef __VERIL_H_
 #define __VERIL_H_
 
+#include <stdint.h>
+
 #ifdef __cplusplus
 extern "C" void litex_sim_init_cmdargs(int argc, char *argv[]);
-extern "C" void litex_sim_eval(void *vsim);
-extern "C" void litex_sim_init_tracer(void *vsim, long start, long end)
+extern "C" void litex_sim_eval(void *vsim, uint64_t time_ps);
+extern "C" void litex_sim_init_tracer(void *vsim, long start, long end);
 extern "C" void litex_sim_tracer_dump();
 extern "C" int litex_sim_got_finish();
 #if VM_COVERAGE
 extern "C" void litex_sim_coverage_dump();
 #endif
 #else
-void litex_sim_eval(void *vsim);
+void litex_sim_eval(void *vsim, uint64_t time_ps);
 void litex_sim_init_tracer(void *vsim);
 void litex_sim_tracer_dump();
 int litex_sim_got_finish();
diff --git a/litex/tools/litex_sim.py b/litex/tools/litex_sim.py
index 4c33dbb72..c07cb3a98 100755
--- a/litex/tools/litex_sim.py
+++ b/litex/tools/litex_sim.py
@@ -343,7 +343,9 @@ def main():
     soc_kwargs     = soc_sdram_argdict(args)
     builder_kwargs = builder_argdict(args)
 
-    sim_config = SimConfig(default_clk="sys_clk")
+    sys_clk_freq = int(1e6)
+    sim_config = SimConfig()
+    sim_config.add_clocker("sys_clk", freq_hz=sys_clk_freq)
 
     # Configuration --------------------------------------------------------------------------------