Clocks and Timers

+Clock Frequency Definition + + +CYGNUM_HAL_RTC_NUMERATOR +CYGNUM_HAL_RTC_DENOMINATOR +CYGNUM_HAL_RTC_PERIOD + + + +These macros are defined in the CDL for each platform and supply the +necessary parameters to specify the frequency at which the clock +interrupts. These parameters are usually found in the CDL definitions +for the target platform, or in some cases the CPU variant. + + + +CYGNUM_HAL_RTC_NUMERATOR and +CYGNUM_HAL_RTC_DENOMINATOR specify the resolution +of the clock interrupt. This resolution involves two separate values, +the numerator and the denominator. The result of dividing the +numerator by the denominator should correspond to the number of +nanoseconds between clock interrupts. For example a numerator of +1000000000 and a denominator of 100 means that there are 10000000 +nanoseconds (or 10 milliseconds) between clock interrupts. Expressing +the resolution as a fraction minimizes clock drift even for +frequencies that cannot be expressed as a simple integer. For example +a frequency of 60Hz corresponds to a clock resolution of +16666666.66... nanoseconds. This can be expressed accurately as +1000000000 over 60. + + + +CYGNUM_HAL_RTC_PERIOD specifies the exact value +used to initialize the clock hardware, it is the value passed as a +parameter to HAL_CLOCK_INITIALIZE() and +HAL_CLOCK_RESET(). The exact meaning of the value +and the range of legal values therefore depends on the target +hardware, and the hardware documentation should be consulted for +further details. + + + +The default values for these macros in all HALs are calculated to give +a clock interrupt frequency of 100Hz, or 10ms between interrupts. To +change the clock frequency, the period needs to be changed, and the +resolution needs to be adjusted accordingly. As an example consider +the i386 PC target. The default values for these macros are: + + + +CYGNUM_HAL_RTC_NUMERATOR 1000000000 +CYGNUM_HAL_RTC_DENOMINATOR 100 +CYGNUM_HAL_RTC_PERIOD 11932 + + + +To change to, say, a 200Hz clock the period needs to be halved to +5966, and to compensate the denominator needs to be doubled to 200. To +change to a 1KHz interrupt rate change the period to 1193 and the +denominator to 1000. + + + +Some HALs make this process a little easier by deriving the period +arithmetically from the denominator. This calculation may also involve +the CPU clock frequency and possibly other factors. For example in the +ARM AT91 variant HAL the period is defined by the following +expression: + + + +((CYGNUM_HAL_ARM_AT91_CLOCK_SPEED/32) / CYGNUM_HAL_RTC_DENOMINATOR) + + + +In this case it is not necessary to change the period at all, just +change the denominator to select the desired clock frequency. However, +note that for certain choices of frequency, rounding errors in this +calculation may result in a small clock drift over time. This is +usually negligible, but if perfect accuracy is required, it may be +necessary to adjust the frequency or period by hand. + + +