diff --git a/ports/stm32f10x/mstimer-init.c b/ports/stm32f10x/mstimer-init.c
index 41184a72..6e594da4 100644
--- a/ports/stm32f10x/mstimer-init.c
+++ b/ports/stm32f10x/mstimer-init.c
@@ -29,64 +29,19 @@
 #include <stdint.h>
 #include "hardware.h"
 #include "bacnet/basic/sys/mstimer.h"
-#include "bacnet/basic/sys/debug.h"
 
 /* counter for the various timers */
 static volatile unsigned long Millisecond_Counter;
-static volatile struct mstimer_callback_data_t *Callback_Head;
-
-/**
- * Activate the LED
- */
-static void timer_debug_on(void)
-{
-    GPIO_WriteBit(GPIOB, GPIO_Pin_13, Bit_SET);
-}
-
-/**
- * Deactivate the LED
- */
-static void timer_debug_off(void)
-{
-    GPIO_WriteBit(GPIOB, GPIO_Pin_13, Bit_RESET);
-}
-
-/**
- * Toggle the state of the debug LED
- */
-static void timer_debug_toggle(void)
-{
-    static bool state = false;
-
-    if (state) {
-        timer_debug_off();
-        state = false;
-    } else {
-        timer_debug_on();
-        state = true;
-    }
-}
 
 /**
  * Handles the interrupt from the timer
  */
 void SysTick_Handler(void)
 {
-    struct mstimer_callback_data_t *cb;
-
     /* increment the tick count */
     Millisecond_Counter++;
-    cb = (struct mstimer_callback_data_t *)Callback_Head;
-    while (cb) {
-        if (mstimer_expired(&cb->timer)) {
-            cb->callback();
-            if (mstimer_interval(&cb->timer) > 0) {
-                mstimer_reset(&cb->timer);
-            }
-        }
-        cb = cb->next;
-    }
-    timer_debug_toggle();
+    /* run any callbacks */
+    mstimer_callback_handler();
 }
 
 /**
@@ -99,58 +54,16 @@ unsigned long mstimer_now(void)
     return Millisecond_Counter;
 }
 
-/**
- * Configures and enables a repeating callback function
- *
- * @param cb - pointer to a #mstimer_callback_data_t
- * @param callback - pointer to a #timer_callback_function function
- * @param milliseconds - how often to call the function
- *
- * @return true if successfully added and enabled
- */
-void mstimer_callback(struct mstimer_callback_data_t *new_cb,
-    mstimer_callback_function callback,
-    unsigned long milliseconds)
-{
-    struct mstimer_callback_data_t *cb;
-
-    if (new_cb) {
-        new_cb->callback = callback;
-        mstimer_set(&new_cb->timer, milliseconds);
-    }
-    if (Callback_Head) {
-        cb = (struct mstimer_callback_data_t *)Callback_Head;
-        while (cb) {
-            if (!cb->next) {
-                cb->next = new_cb;
-                break;
-            } else {
-                cb = cb->next;
-            }
-        }
-    } else {
-        Callback_Head = new_cb;
-    }
-}
-
 /**
  * Timer setup for 1 millisecond timer
  */
 void mstimer_init(void)
 {
-    GPIO_InitTypeDef GPIO_InitStructure;
-
-    GPIO_StructInit(&GPIO_InitStructure);
-    /* Configure the Receive LED */
-    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_13;
-    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
-    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
-    GPIO_Init(GPIOB, &GPIO_InitStructure);
-
     /* Setup SysTick Timer for 1ms interrupts  */
     if (SysTick_Config(SystemCoreClock / 1000)) {
-        /* Capture error */
-        while (1)
-            ;
+        while (1) {
+            /* config error? return  1 = failed, 0 = successful */
+        }
     }
+    NVIC_EnableIRQ(SysTick_IRQn);
 }
diff --git a/ports/stm32f4xx/mstimer-init.c b/ports/stm32f4xx/mstimer-init.c
index baa3fc80..2faeab9f 100644
--- a/ports/stm32f4xx/mstimer-init.c
+++ b/ports/stm32f4xx/mstimer-init.c
@@ -34,36 +34,16 @@
 
 /* counter for the various timers */
 static volatile unsigned long Millisecond_Counter;
-static volatile struct mstimer_callback_data_t *Callback_Head;
-
-/**
- * Toggle the state of the debug LED
- */
-static void timer_debug_toggle(void)
-{
-    led_toggle(LED_LD1);
-}
 
 /**
  * Handles the interrupt from the timer
  */
 void SysTick_Handler(void)
 {
-    struct mstimer_callback_data_t *cb;
-
     /* increment the tick count */
     Millisecond_Counter++;
-    cb = (struct mstimer_callback_data_t *)Callback_Head;
-    while (cb) {
-        if (mstimer_expired(&cb->timer)) {
-            cb->callback();
-            if (mstimer_interval(&cb->timer) > 0) {
-                mstimer_reset(&cb->timer);
-            }
-        }
-        cb = cb->next;
-    }
-    timer_debug_toggle();
+    /* run any callbacks */
+    mstimer_callback_handler();
 }
 
 /**
@@ -76,40 +56,6 @@ unsigned long mstimer_now(void)
     return Millisecond_Counter;
 }
 
-/**
- * Configures and enables a repeating callback function
- *
- * @param cb - pointer to a #mstimer_callback_data_t
- * @param callback - pointer to a #timer_callback_function function
- * @param milliseconds - how often to call the function
- *
- * @return true if successfully added and enabled
- */
-void mstimer_callback(struct mstimer_callback_data_t *new_cb,
-    mstimer_callback_function callback,
-    unsigned long milliseconds)
-{
-    struct mstimer_callback_data_t *cb;
-
-    if (new_cb) {
-        new_cb->callback = callback;
-        mstimer_set(&new_cb->timer, milliseconds);
-    }
-    if (Callback_Head) {
-        cb = (struct mstimer_callback_data_t *)Callback_Head;
-        while (cb) {
-            if (!cb->next) {
-                cb->next = new_cb;
-                break;
-            } else {
-                cb = cb->next;
-            }
-        }
-    } else {
-        Callback_Head = new_cb;
-    }
-}
-
 /**
  * Timer setup for 1 millisecond timer
  */
@@ -117,9 +63,9 @@ void mstimer_init(void)
 {
     /* Setup SysTick Timer for 1ms interrupts  */
     if (SysTick_Config(SystemCoreClock / 1000)) {
-        /* Capture error */
-        while (1)
-            ;
+        while (1) {
+            /* config error? return  1 = failed, 0 = successful */
+        }
     }
     NVIC_EnableIRQ(SysTick_IRQn);
 }
diff --git a/ports/xplained/mstimer-init.c b/ports/xplained/mstimer-init.c
index 06bec0b6..0c93e512 100644
--- a/ports/xplained/mstimer-init.c
+++ b/ports/xplained/mstimer-init.c
@@ -35,32 +35,31 @@
 #include "tc.h"
 #include "bacnet/basic/sys/mstimer.h"
 
-/* define which timer counter we are using */
-#define MY_TIMER TCE0
+/* define which timer for the counter we are using */
+#define MS_TIMER_COUNTER TCE0
+/* define which timer for the callback we are using */
+#define MS_TIMER_CALLBACK TCE1
 
 /* counter for the base timer */
 static volatile uint32_t Millisecond_Counter;
-/* callback data head of list */
-static volatile struct mstimer_callback_data_t *Callback_Head;
 
 /**
- * Handles the interrupt from the timer
+ * Handles an interrupt from a hardware millisecond timer
  */
-static void my_callback(void)
+static void my_counter_handler(void)
 {
-    struct mstimer_callback_data_t *cb;
-
     Millisecond_Counter++;
-    cb = (struct mstimer_callback_data_t *)Callback_Head;
-    while (cb) {
-        if (mstimer_expired(&cb->timer)) {
-            cb->callback();
-            if (mstimer_interval(&cb->timer) > 0) {
-                mstimer_reset(&cb->timer);
-            }
-        }
-        cb = cb->next;
-    }
+}
+
+/**
+ * Handles an interrupt from a hardware millisecond timer
+ */
+static void my_callback_handler(void)
+{
+    /* callback might go too long; prevent re-entrency by disable IRQ source */
+    tc_set_overflow_interrupt_level(&MS_TIMER_CALLBACK, TC_INT_LVL_OFF);
+    mstimer_callback_handler();
+    tc_set_overflow_interrupt_level(&MS_TIMER_CALLBACK, TC_INT_LVL_LO);
 }
 
 /**
@@ -72,49 +71,13 @@ unsigned long mstimer_now(void)
 {
     uint32_t timer_value; /* return value */
 
-    tc_set_overflow_interrupt_level(&MY_TIMER, TC_INT_LVL_OFF);
+    tc_set_overflow_interrupt_level(&MS_TIMER_COUNTER, TC_INT_LVL_OFF);
     timer_value = Millisecond_Counter;
-    tc_set_overflow_interrupt_level(&MY_TIMER, TC_INT_LVL_LO);
+    tc_set_overflow_interrupt_level(&MS_TIMER_COUNTER, TC_INT_LVL_LO);
 
     return timer_value;
 }
 
-/**
- * Configures and enables a repeating callback function
- *
- * @param new_cb - pointer to #mstimer_callback_data_t
- * @param callback - pointer to a #timer_callback_function function
- * @param milliseconds - how often to call the function
- *
- * @return true if successfully added and enabled
- */
-void mstimer_callback(struct mstimer_callback_data_t *new_cb,
-    mstimer_callback_function callback,
-    unsigned long milliseconds)
-{
-    struct mstimer_callback_data_t *cb;
-
-    tc_set_overflow_interrupt_level(&MY_TIMER, TC_INT_LVL_OFF);
-    if (new_cb) {
-        new_cb->callback = callback;
-        mstimer_set(&new_cb->timer, milliseconds);
-    }
-    if (Callback_Head) {
-        cb = (struct mstimer_callback_data_t *)Callback_Head;
-        while (cb) {
-            if (!cb->next) {
-                cb->next = new_cb;
-                break;
-            } else {
-                cb = cb->next;
-            }
-        }
-    } else {
-        Callback_Head = new_cb;
-    }
-    tc_set_overflow_interrupt_level(&MY_TIMER, TC_INT_LVL_LO);
-}
-
 /**
  * Timer setup for 1 millisecond timer
  */
@@ -122,13 +85,25 @@ void mstimer_init(void)
 {
     unsigned long period;
 
-    tc_enable(&MY_TIMER);
-    tc_set_overflow_interrupt_callback(&MY_TIMER, my_callback);
-    tc_set_wgm(&MY_TIMER, TC_WG_NORMAL);
-    tc_write_count(&MY_TIMER, 1);
-    period = sysclk_get_peripheral_bus_hz(&MY_TIMER);
+    tc_enable(&MS_TIMER_COUNTER);
+    tc_set_overflow_interrupt_callback(&MS_TIMER_COUNTER,
+        mstimer_counter_handler);
+    tc_set_wgm(&MS_TIMER_COUNTER, TC_WG_NORMAL);
+    tc_write_count(&MS_TIMER_COUNTER, 1);
+    period = sysclk_get_peripheral_bus_hz(&MS_TIMER_COUNTER);
     period /= 1000;
-    tc_write_period(&MY_TIMER, period);
-    tc_set_overflow_interrupt_level(&MY_TIMER, TC_INT_LVL_LO);
-    tc_write_clock_source(&MY_TIMER, TC_CLKSEL_DIV1_gc);
+    tc_write_period(&MS_TIMER_COUNTER, period);
+    tc_set_overflow_interrupt_level(&MS_TIMER_COUNTER, TC_INT_LVL_LO);
+    tc_write_clock_source(&MS_TIMER_COUNTER, TC_CLKSEL_DIV1_gc);
+
+    tc_enable(&MS_TIMER_CALLBACK);
+    tc_set_overflow_interrupt_callback(&MS_TIMER_CALLBACK,
+        my_callback_handler);
+    tc_set_wgm(&MS_TIMER_CALLBACK, TC_WG_NORMAL);
+    tc_write_count(&MS_TIMER_CALLBACK, 1);
+    period = sysclk_get_peripheral_bus_hz(&MS_TIMER_CALLBACK);
+    period /= 1000;
+    tc_write_period(&MS_TIMER_CALLBACK, period);
+    tc_set_overflow_interrupt_level(&MS_TIMER_CALLBACK, TC_INT_LVL_LO);
+    tc_write_clock_source(&MS_TIMER_CALLBACK, TC_CLKSEL_DIV1_gc);
 }
diff --git a/src/bacnet/basic/sys/mstimer.c b/src/bacnet/basic/sys/mstimer.c
index 38a2fef3..9fabb4c1 100644
--- a/src/bacnet/basic/sys/mstimer.c
+++ b/src/bacnet/basic/sys/mstimer.c
@@ -22,6 +22,60 @@
 #include <stdint.h>
 #include "bacnet/basic/sys/mstimer.h"
 
+/* callback data head of list */
+static struct mstimer_callback_data_t *Callback_Head;
+
+/**
+ * Handles an interrupt from a hardware millisecond timer
+ */
+void mstimer_callback_handler(void)
+{
+    struct mstimer_callback_data_t *cb;
+
+    cb = (struct mstimer_callback_data_t *)Callback_Head;
+    while (cb) {
+        if (mstimer_expired(&cb->timer)) {
+            cb->callback();
+            if (mstimer_interval(&cb->timer) > 0) {
+                mstimer_reset(&cb->timer);
+            }
+        }
+        cb = cb->next;
+    }
+}
+
+/**
+ * Configures and enables a repeating callback function
+ *
+ * @param new_cb - pointer to #mstimer_callback_data_t
+ * @param callback - pointer to a #timer_callback_function function
+ * @param milliseconds - how often to call the function
+ */
+void mstimer_callback(struct mstimer_callback_data_t *new_cb,
+    mstimer_callback_function callback,
+    unsigned long milliseconds)
+{
+    struct mstimer_callback_data_t *cb;
+
+    if (new_cb) {
+        new_cb->callback = callback;
+        mstimer_set(&new_cb->timer, milliseconds);
+    }
+    if (Callback_Head) {
+        cb = (struct mstimer_callback_data_t *)Callback_Head;
+        while (cb) {
+            if (!cb->next) {
+                cb->next = new_cb;
+                break;
+            } else {
+                cb = cb->next;
+            }
+        }
+    } else {
+        Callback_Head = new_cb;
+    }
+}
+
 /**
  * @brief Set a timer for a time sometime in the future
  *
diff --git a/src/bacnet/basic/sys/mstimer.h b/src/bacnet/basic/sys/mstimer.h
index 997e0134..3a1a8c42 100644
--- a/src/bacnet/basic/sys/mstimer.h
+++ b/src/bacnet/basic/sys/mstimer.h
@@ -35,8 +35,9 @@ struct mstimer {
   unsigned long interval;
 };
 
+/* optional callback function form */
 typedef void (*mstimer_callback_function) (void);
-/* callback data structure */
+/* optional callback data structure */
 struct mstimer_callback_data_t;
 struct mstimer_callback_data_t {
     struct mstimer timer;
@@ -62,15 +63,18 @@ BACNET_STACK_EXPORT
 unsigned long mstimer_elapsed(struct mstimer *t);
 BACNET_STACK_EXPORT
 unsigned long mstimer_interval(struct mstimer *t);
-/* HAL implementation */
-BACNET_STACK_EXPORT
-unsigned long mstimer_now(void);
+/* optional callback timer support for embedded systems */
 BACNET_STACK_EXPORT
 void mstimer_callback(
     struct mstimer_callback_data_t *cb,
     mstimer_callback_function callback,
     unsigned long milliseconds);
 BACNET_STACK_EXPORT
+void mstimer_callback_handler(void);
+/* HAL implementation */
+BACNET_STACK_EXPORT
+unsigned long mstimer_now(void);
+BACNET_STACK_EXPORT
 void mstimer_init(void);
 
 #ifdef __cplusplus