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+.. comment SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. COMMENT: COPYRIGHT (c) 1988-2002.
+.. COMMENT: On-Line Applications Research Corporation (OAR).
+.. COMMENT: All rights reserved.
+
+Timer Driver
+############
+
+The timer driver is primarily used by the RTEMS Timing Tests.  This driver
+provides as accurate a benchmark timer as possible.  It typically reports its
+time in microseconds, CPU cycles, or bus cycles.  This information can be very
+useful for determining precisely what pieces of code require optimization and
+to measure the impact of specific minor changes.
+
+The gen68340 BSP also uses the Timer Driver to support a high performance mode
+of the on-CPU UART.
+
+Benchmark Timer
+===============
+
+The RTEMS Timing Test Suite requires a benchmark timer.  The RTEMS Timing Test
+Suite is very helpful for determining the performance of target hardware and
+comparing its performance to that of other RTEMS targets.
+
+This section describes the routines which are assumed to exist by the RTEMS
+Timing Test Suite.  The names used are *EXACTLY* what is used in the RTEMS
+Timing Test Suite so follow the naming convention.
+
+benchmark_timer_initialize
+--------------------------
+
+Initialize the timer source.
+
+.. code-block:: c
+
+    void benchmark_timer_initialize(void)
+    {
+      initialize the benchmark timer
+    }
+
+Read_timer
+----------
+
+The ``benchmark_timer_read`` routine returns the number of benchmark time units
+(typically microseconds) that have elapsed since the last call to
+``benchmark_timer_initialize``.
+
+.. code-block:: c
+
+    benchmark_timer_t benchmark_timer_read(void)
+    {
+      stop time = read the hardware timer
+      if the subtract overhead feature is enabled
+        subtract overhead from stop time
+      return the stop time
+    }
+
+Many implementations of this routine subtract the overhead required to
+initialize and read the benchmark timer.  This makes the times reported more
+accurate.
+
+Some implementations report 0 if the harware timer value change is sufficiently
+small.  This is intended to indicate that the execution time is below the
+resolution of the timer.
+
+benchmark_timer_disable_subtracting_average_overhead
+----------------------------------------------------
+
+This routine is invoked by the "Check Timer" (``tmck``) test in the RTEMS
+Timing Test Suite.  It makes the ``benchmark_timer_read`` routine NOT subtract
+the overhead required to initialize and read the benchmark timer.  This is used
+by the ``tmoverhd`` test to determine the overhead required to initialize and
+read the timer.
+
+.. code-block:: c
+
+    void benchmark_timer_disable_subtracting_average_overhead(bool find_flag)
+    {
+      disable the subtract overhead feature
+    }
+
+The ``benchmark_timer_find_average_overhead`` variable is used to indicate the
+state of the "subtract overhead feature".
+
+gen68340 UART FIFO Full Mode
+============================
+
+The gen68340 BSP is an example of the use of the timer to support the UART
+input FIFO full mode (FIFO means First In First Out and roughly means
+buffer). This mode consists in the UART raising an interrupt when n characters
+have been received (*n* is the UART's FIFO length). It results in a lower
+interrupt processing time, but the problem is that a scanf primitive will block
+on a receipt of less than *n* characters. The solution is to set a timer that
+will check whether there are some characters waiting in the UART's input
+FIFO. The delay time has to be set carefully otherwise high rates will be
+broken:
+
+- if no character was received last time the interrupt subroutine was entered,
+  set a long delay,
+
+- otherwise set the delay to the delay needed for ``n`` characters receipt.