[rtl/core] fixed mtime_o low word to high word overflow inconsistency #…

…58

rtl/core/neorv32_top.vhd

@@ -941,8 +941,25 @@ begin
     mtime_irq  <= mtime_irq_i; -- use external machine timer interrupt
   end generate;
 
-  -- system time output --
-  mtime_o <= mtime_time when (IO_MTIME_EN = true) else (others => '0');
+
+  -- system time output LO --
+  mtime_sync: process(clk_i)
+  begin
+    if rising_edge(clk_i) then
+      -- buffer low word one clock cycle to compensate for MTIME's 1-cycle delay
+      -- when overflowing from low-word to high-word -> only relevant for processor-external devices
+      -- processor-internal devices (= the CPU) do not care about this delay offset as 64-bit MTIME.TIME
+      -- cannot be accessed within a single cycle
+      if (IO_MTIME_EN = true) then
+        mtime_o(31 downto 0) <= mtime_time(31 downto 0);
+      else
+        mtime_o(31 downto 0) <= (others => '0');
+      end if;
+    end if;
+  end process mtime_sync;
+
+  -- system time output HI --
+  mtime_o(63 downto 32) <= mtime_time(63 downto 32) when (IO_MTIME_EN = true) else (others => '0');
 
 
   -- Primary Universal Asynchronous Receiver/Transmitter (UART0) ----------------------------