diff --git a/platform/include/platform/internal/SysTimer.h b/platform/include/platform/internal/SysTimer.h
index 229d535695..ac4357bdae 100644
--- a/platform/include/platform/internal/SysTimer.h
+++ b/platform/include/platform/internal/SysTimer.h
@@ -191,7 +191,7 @@ public:
      */
     std::chrono::duration<int, period> unacknowledged_ticks() const
     {
-        return std::chrono::duration<int, period>(core_util_atomic_load_u8(&_unacknowledged_ticks));
+        return std::chrono::duration<int, period>(core_util_atomic_load_u32(&_unacknowledged_ticks));
     }
 
     /** Get the current tick count
@@ -248,7 +248,7 @@ protected:
     const highres_time_point _epoch;
     highres_time_point _time;
     uint64_t _tick;
-    uint8_t _unacknowledged_ticks;
+    uint32_t _unacknowledged_ticks;
     bool _wake_time_set;
     bool _wake_time_passed;
     bool _wake_early;
diff --git a/platform/source/SysTimer.cpp b/platform/source/SysTimer.cpp
index 2345e68bdd..6286cb739a 100644
--- a/platform/source/SysTimer.cpp
+++ b/platform/source/SysTimer.cpp
@@ -191,10 +191,15 @@ void SysTimer<Period, 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();
     }
 }