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`1.3.2 Cloclt Oscillator Circuit and Clocking Unit (a)
`
`After the poster supply. the cloclt is the tree! latportarrr rrrrtr ofir .r_1-tsrerrr. A processor needs a eloelt
`oscillator circuit. The cloclt controls the various clocking requirements of the CPU. of the system
`timers and the CPU machine cycles. The machine cycles are for
`(i) fetching the codes and data from memory and then decoding and executing at the processor.
`and
`
`(ii) transferring the results to memory.
`'11): cloclt controls the time for executing an instntction. The cloclt circuit uses either a crystal
`{external to the processor) or a ceramic resonator (intemally associated with the processor} or an
`external oscillator IC attached to the processor. {a} The crystal resonator gives the highest stability in
`lrequency witlt temperature and drift in the circuit- The crystal in association with an appropriate
`resistance in parallel and a pair of series capacitance at both pins resonates at the frequency. which is
`either double or single times the crystal-frequency. Further. the crystal is kept as near as feasible to
`two pins of the processor. [b) The internal ceramic resonator. if available in a processor. saves the use
`of the external crystal and gives a reasonable though not very highly stable frequency. [A typical drift
`of the ceramic resonator is about ten minutes per month compared to the typical drift of l or 5 minutes
`per month ofa crystal]. {c} The external lC—based cloclt oscillator has a significantly higher power
`dissipation compared to the internal processor-resonator. However. it provides a higher driving capa-
`bility. which might be needed when the various circuits of embedded system are concurrently driven.
`For example. a multiprocessor system needs the cloclt circuit. which should give a high driving capa-
`bility and enables control of all the processors concurrently.
`
`{For the processing unI’o(a). a highly stable oscillator is required and tire processor cloclt-out
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`1.3.3 Real 'l1meCloelr (ETC) and Tlnrers for Various Timing and
`Counting Needsolthe System
`
`A timer circuit suitably configured is the syrrear-cloclt. also called real-tirne cloclt (RTC). An RTC is
`used by the schedulers and for real-time programming. An RTC is designed as follows: Assume a
`processor generates a cloclt output every 0.5 us. When a system timer is configured by a software
`insuuction to issue timeout after 200 inputs from the processor cloclt outputs. then there are H3000
`inteI1'upI.s(ticks] each second. The RTC ticking rate is then 10 ltHz and it interrupts every I00 us. The
`RTC is also used to obtain soflware-controlled delays and time-outs.
`More than one timer using the system cloclt {RTC} may be needed for the various timing and
`counting needs in a system. Refer to Section 3.2 for a description of timers and counters.
`
`1
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`ASSA 1012