16. Timer Driver


The Timer Driver is superfluous and should be replaced by the RTEMS counter support. Ask on the mailing list if you plan to write a 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.

16.1. 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.

16.1.1. benchmark_timer_initialize

Initialize the timer source.

void benchmark_timer_initialize(void)
  initialize the benchmark timer

16.1.2. 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.

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.

16.1.3. 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.

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”.

16.2. 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.