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Information in this article applies to:

  • C51 Version 5.50
  • RTX51 Tiny Version 1.06


What is the purpose of the configuration parameter INT_CLOCK in RTX51 Tiny, and how does it relate to the execution speed of my multitasking firmware?


A timer is used to periodically generate interrupts, which provides the RTX51 Clock. In order to generate an interrupt, the timer must count up to 65535 (FFFFH) and then overflow to 0. After each interrupt the timer is reloaded with a value to start counting up from. This value is 65535 - INT_CLOCK. INT_CLOCK is, therefore, the number of machine cycles between interrupts.

For example, if INT_CLOCK is 10000, then the timer must count up 10000 machine cycles between interrupts. For a crystal frequency of 12MHz, each machine cycle takes 1 MicroSecond. Therefore, interrupts will occur every 10 MilliSeconds.

By lowering the value of INT_CLOCK, the speed of the RTX51 Clock may be increased. Each interrupt has some overhead (100-200 machine cycles) and setting the value too low results in all the processing time used for interrupt overhead rather than task execution.


Last Reviewed: Friday, July 15, 2005

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