diff --git a/platform/source/SysTimer.cpp b/platform/source/SysTimer.cpp
index 0803849e25d..6ebfbf053ea 100644
--- a/platform/source/SysTimer.cpp
+++ b/platform/source/SysTimer.cpp
@@ -192,10 +192,15 @@ void SysTimer<US_IN_TICK, IRQ>::acknowledge_tick()
 {
     // Try to avoid missed ticks if OS's IRQ level is not keeping
     // up with our handler.
-    // 8-bit counter to save space, and also make sure it we don't
-    // try TOO hard to resync if something goes really awry -
-    // resync will reset if the count hits 256.
-    if (core_util_atomic_decr_u8(&_unacknowledged_ticks, 1) > 0) {
+    // This value should not get large during normal operation.
+    // However, when interrupts are not handled for a while
+    // (e.g. by debug halt or large critical sections) this
+    // number will get large very quickly. All these un-
+    // acknowledged ticks need to be processed because otherwise
+    // the OS timing will be off. Processing may take a while.
+    // For more info see: https://github.com/ARMmbed/mbed-os/issues/13801
+
+    if (core_util_atomic_decr_u32(&_unacknowledged_ticks, 1) > 0) {
         _set_irq_pending();
     }
 }
diff --git a/platform/source/SysTimer.h b/platform/source/SysTimer.h
index 3098a8e6142..73285a4ffdd 100644
--- a/platform/source/SysTimer.h
+++ b/platform/source/SysTimer.h
@@ -172,7 +172,7 @@ class SysTimer: private mbed::TimerEvent, private mbed::NonCopyable<SysTimer<US_
      */
     int unacknowledged_ticks() const
     {
-        return core_util_atomic_load_u8(&_unacknowledged_ticks);
+        return core_util_atomic_load_u32(&_unacknowledged_ticks);
     }
 
     /** Get the current tick count
@@ -227,7 +227,7 @@ class SysTimer: private mbed::TimerEvent, private mbed::NonCopyable<SysTimer<US_
     static void _clear_irq_pending();
     us_timestamp_t _time_us;
     uint64_t _tick;
-    uint8_t _unacknowledged_ticks;
+    uint32_t _unacknowledged_ticks;
     bool _wake_time_set;
     bool _wake_time_passed;
     bool _wake_early;