diff --git a/common/hw-time-timer.ads b/common/hw-time-timer.ads
index cfe2e0b..90978df 100644
--- a/common/hw-time-timer.ads
+++ b/common/hw-time-timer.ads
@@ -14,11 +14,12 @@
 
 private package HW.Time.Timer
    with
-      Abstract_State => ((Timer_State with Part_Of => HW.Time.State),
+      Abstract_State => ((Timer_State with
+                           Part_Of => HW.Time.State,
+                           External => Async_Writers),
                          (Abstract_Time with
                            Part_Of => HW.Time.State,
-                           External => Async_Writers)),
-      Initializes => (Timer_State)
+                           External => Async_Writers))
 is
 
    -- Returns the highest point in time that has definitely passed.
@@ -37,6 +38,7 @@
 
    function Hz return T
    with
+      Volatile_Function,
       Global => (Input => Timer_State);
 
 end HW.Time.Timer;
diff --git a/common/hw-time.adb b/common/hw-time.adb
index 28db95d..b81d343 100644
--- a/common/hw-time.adb
+++ b/common/hw-time.adb
@@ -34,25 +34,28 @@
    with
       Refined_Global => (Input => (Timer.Abstract_Time, Timer.Timer_State))
    is
+      Hz : constant T := Timer.Hz;
       Current : constant T := Timer.Raw_Value_Max;
    begin
-      return Current + (T (US) * Timer.Hz + 999_999) / 1_000_000;
+      return Current + (T (US) * Hz + 999_999) / 1_000_000;
    end US_From_Now;
 
    function MS_From_Now (MS : Natural) return T
    with
       Refined_Global => (Input => (Timer.Abstract_Time, Timer.Timer_State))
    is
+      Hz : constant T := Timer.Hz;
       Current : constant T := Timer.Raw_Value_Max;
    begin
-      return Current + (T (MS) * Timer.Hz + 999) / 1_000;
+      return Current + (T (MS) * Hz + 999) / 1_000;
    end MS_From_Now;
 
    function Now_US return Int64
    with
       Refined_Global => (Input => (Timer.Abstract_Time, Timer.Timer_State))
    is
-      MHz : constant T := Timer.Hz / 1_000_000;
+      Hz : constant T := Timer.Hz;
+      MHz : constant T := Hz / 1_000_000;
       Current : constant T := Timer.Raw_Value_Min;
    begin
       return Int64 (Current and (2 ** 63 - 1))
@@ -65,7 +68,7 @@
    with
       Refined_Global => (Input => (Timer.Abstract_Time))
    is
-      Current: T;
+      Current : T;
    begin
       loop
          Current := Timer.Raw_Value_Min;