diff --git a/arch/Kconfig b/arch/Kconfig
index 7cb6af18657..71049ff51ba 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -59,6 +59,7 @@ config ARCH_SH
 
 config ARCH_SIM
 	bool "Simulation"
+	select ARCH_HAVE_TICKLESS
 	---help---
 		Linux/Cywgin user-mode simulation.
 
diff --git a/arch/sim/src/up_idle.c b/arch/sim/src/up_idle.c
index 8d04000c4cb..783b57eeabd 100644
--- a/arch/sim/src/up_idle.c
+++ b/arch/sim/src/up_idle.c
@@ -1,5 +1,5 @@
 /****************************************************************************
- * up_idle.c
+ * arch/sim/src/up_idle.c
  *
  *   Copyright (C) 2007-2009, 2011-2012, 2014 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
diff --git a/arch/sim/src/up_tickless.c b/arch/sim/src/up_tickless.c
index 56202e7c1e6..57a7935347f 100644
--- a/arch/sim/src/up_tickless.c
+++ b/arch/sim/src/up_tickless.c
@@ -1,5 +1,5 @@
 /****************************************************************************
- * arch/sim/up_tickless.c
+ * arch/sim/src/up_tickless.c
  *
  *   Copyright (C) 2014 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
@@ -41,13 +41,11 @@
  *
  *   void up_timer_initialize(void): Initializes the timer facilities.  Called
  *     early in the intialization sequence (by up_intialize()).
- *   int up_timer_gettime(FAR struct timespec *tp):  Returns the current
+ *   int up_timer_gettime(FAR struct timespec *ts):  Returns the current
  *     time from the platform specific time source.
  *   int up_timer_cancel(void):  Cancels the interval timer.
- *   int up_timer_start(FAR const struct timespec *tp): Start (or re-starts)
+ *   int up_timer_start(FAR const struct timespec *ts): Start (or re-starts)
  *     the interval timer.
- *   int up_timer_resolution(FAR struct timespec *tp): Returns the
- *     resolution of the interval timer.
  *
  * The RTOS will provide the following interfaces for use by the platform-
  * specific interval timer implementation:
@@ -138,17 +136,21 @@ void up_timer_initialize(void)
  *
  * Description:
  *   Return the elapsed time since power-up (or, more correctly, since
- *   up_timer_initialize() was called).  This function is functionally equivalent
- *   to:
+ *   up_timer_initialize() was called).  This function is functionally
+ *   equivalent to:
  *
- *      int clock_gettime(clockid_t clockid, FAR struct timespec *tp);
+ *      int clock_gettime(clockid_t clockid, FAR struct timespec *ts);
  *
  *   when clockid is CLOCK_MONOTONIC.
  *
+ *   This function provides the basis for reporting the current time and
+ *   also is used to eliminate error build-up from small erros in interval
+ *   time calculations.
+ *
  *   Provided by platform-specific code and called from the RTOS base code.
  *
  * Input Parameters:
- *   tp - Provides the location in which to return the up-time.
+ *   ts - Provides the location in which to return the up-time.
  *
  * Returned Value:
  *   Zero (OK) is returned on success; a negated errno value is returned on
@@ -162,10 +164,10 @@ void up_timer_initialize(void)
  *
  ****************************************************************************/
 
-int up_timer_gettime(FAR struct timespec *tp)
+int up_timer_gettime(FAR struct timespec *ts)
 {
-  tp->tv_sec  = g_elapsed_time.tv_sec;
-  tp->tv_nsec = g_elapsed_time.tv_nsec;
+  ts->tv_sec  = g_elapsed_time.tv_sec;
+  ts->tv_nsec = g_elapsed_time.tv_nsec;
   return OK;
 }
 
@@ -173,12 +175,21 @@ int up_timer_gettime(FAR struct timespec *tp)
  * Name: up_timer_cancel
  *
  * Description:
- *   Cancel the interval timer.  up_timer_expiration() will not be called.
+ *   Cancel the interval timer and return the time remaining on the timer.
+ *   These two steps need to be as nearly atomic as possible.
+ *   up_timer_expiration() will not be called unless the timer is restarted
+ *   with up_timer_start().
+ *
+ *   If, as a race condition, the timer has already expired when this
+ *   function is called, then that pending interrupt must be cleared so
+ *   that up_timer_start() and the remaining time of zero should be
+ *   returned.
  *
  *   Provided by platform-specific code and called from the RTOS base code.
  *
  * Input Parameters:
- *   None
+ *   ts - Location to return the remaining time.  Zero should be returned
+ *        if the timer is not active.
  *
  * Returned Value:
  *   Zero (OK) is returned on success; a negated errno value is returned on
@@ -192,8 +203,23 @@ int up_timer_gettime(FAR struct timespec *tp)
  ****************************************************************************/
 
 #ifdef CONFIG_SCHED_TICKLESS
-int up_timer_cancel(void)
+int up_timer_cancel(FAR struct timespec *ts)
 {
+  /* Return the time remaining on the simulated timer */
+
+  if (g_timer_active)
+    {
+      ts->tv_sec  = g_interval_delay.tv_nsec;
+      ts->tv_nsec = g_interval_delay.tv_sec;
+    }
+  else
+    {
+      ts->tv_sec  = 0;
+      ts->tv_nsec = 0;
+    }
+
+  /* Disable and reset the simulated timer */
+
   g_interval_delay.tv_nsec = 0;
   g_interval_delay.tv_sec  = 0;
   g_timer_active           = false;
@@ -211,7 +237,7 @@ int up_timer_cancel(void)
  *   Provided by platform-specific code and called from the RTOS base code.
  *
  * Input Parameters:
- *   tp - Provides the time interval until up_timer_expiration() is called.
+ *   ts - Provides the time interval until up_timer_expiration() is called.
  *
  * Returned Value:
  *   Zero (OK) is returned on success; a negated errno value is returned on
@@ -225,82 +251,14 @@ int up_timer_cancel(void)
  ****************************************************************************/
 
 #ifdef CONFIG_SCHED_TICKLESS
-int up_timer_start(FAR const struct timespec *tp)
+int up_timer_start(FAR const struct timespec *ts)
 {
-  g_interval_delay.tv_nsec = tp->tv_nsec;
-  g_interval_delay.tv_sec  = tp->tv_sec;
+  g_interval_delay.tv_nsec = ts->tv_nsec;
+  g_interval_delay.tv_sec  = ts->tv_sec;
   g_timer_active           = true;
 }
 #endif
 
-/****************************************************************************
- * Name: up_timer_remaining
- *
- * Description:
- *   Return the time remaining until the next timer expiratino.
- *
- *   Provided by platform-specific code and called from the RTOS base code.
- *
- * Input Parameters:
- *   tp - Location to return the remaining time.  Zero should be returned
- *        if the timer is not active.
- *
- * Returned Value:
- *   Zero (OK) is returned on success; a negated errno value is returned on
- *   any failure.
- *
- * Assumptions:
- *   May be called from interrupt level handling or from the normal tasking
- *   level.  Interrupts may need to be disabled internally to assure
- *   non-reentrancy.
- *
- ****************************************************************************/
-
-int up_timer_remaining(FAR struct timespec *tp)
-{
-  if (g_timer_active)
-    {
-      tp->tv_sec  = g_interval_delay.tv_nsec;
-      tp->tv_nsec = g_interval_delay.tv_sec;
-    }
-  else
-    {
-      tp->tv_sec  = 0;
-      tp->tv_nsec = 0;
-    }
-
-  return OK;
-}
-
-/****************************************************************************
- * Name: up_timer_resolution
- *
- * Description:
- *   Returns the resolution of the interval timer.  This defines the minimal
- *   time separation between two events and is used for scheduling and for
- *   setting up timed events.
- *
- *   Provided by platform-specific code and called from the RTOS base code.
- *
- * Input Parameters:
- *   tp - Provides the location in which to return the timer resolution.
- *
- * Returned Value:
- *
- * Assumptions:
- *   May be called from interrupt level handling or from the normal tasking
- *   level.  Interrupts may need to be disabled internally to assure
- *   non-reentrancy.
- *
- ****************************************************************************/
-
-int up_timer_resolution(FAR struct timespec *tp)
-{
-  tp->tv_sec  = TICK_SEC;
-  tp->tv_nsec = TICK_NSEC;
-  return 0;
-}
-
 /****************************************************************************
  * Name: up_timer_update
  *