diff --git a/bacnet-stack/ports/bdk-atxx4-mstp/timer.c b/bacnet-stack/ports/bdk-atxx4-mstp/timer.c
index 384c7161..b38677cc 100644
--- a/bacnet-stack/ports/bdk-atxx4-mstp/timer.c
+++ b/bacnet-stack/ports/bdk-atxx4-mstp/timer.c
@@ -303,6 +303,26 @@ void timer_interval_restart(
     }
 }
 
+/*************************************************************************
+* Description: Return the elapsed time
+* Returns: number of milliseconds elapsed
+* Notes: only up to 255ms elapsed
+**************************************************************************/
+uint8_t timer_milliseconds_delta(uint8_t start)
+{
+    return (timer_milliseconds_byte() - start);
+}
+
+/*************************************************************************
+* Description: Mark the start of a delta timer
+* Returns: mark timer starting tick
+* Notes: only up to 255ms elapsed
+**************************************************************************/
+uint8_t timer_milliseconds_mark(void)
+{
+    return timer_milliseconds_byte();
+}
+
 #ifdef TEST
 #include <assert.h>
 #include <string.h>
diff --git a/bacnet-stack/ports/bdk-atxx4-mstp/timer.h b/bacnet-stack/ports/bdk-atxx4-mstp/timer.h
index fb9bf0e0..f5a9aeaf 100644
--- a/bacnet-stack/ports/bdk-atxx4-mstp/timer.h
+++ b/bacnet-stack/ports/bdk-atxx4-mstp/timer.h
@@ -48,8 +48,8 @@ extern "C" {
         void);
     uint32_t timer_milliseconds(
         void);
-    uint32_t timer_milliseconds_set(
-        uint32_t value);
+    uint8_t timer_milliseconds_byte(
+        void);
 
     /* these functions are in the generic timer.c module */
 
@@ -103,6 +103,10 @@ extern "C" {
     void timer_interval_restart(
         struct itimer *t);
 
+    /* special for 8-bit microcontrollers - limited to 255ms */
+    uint8_t timer_milliseconds_delta(uint8_t start);
+    uint8_t timer_milliseconds_mark(void);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */