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【正文】 facing to, for example, fluorescent displays, or can provide sufficient current to make interfacing other ponents straightforward. Some devices allow an I/O port to be configured as a system bus to allow offchip memory and I/O expansion. This facility is potentially useful as a product range develops, since successive enhancements may bee too big for onchip memory and it is undesirable not to build on the existing software base.Serial input/outputSerial munication with terminal devices is mon means of providing a link using a small number of lines. This sort of munication can also be exploited for interfacing special function chips or linking several microputers together .Both the mon asynchronous synchronous munication schemes require protocols that provide framing (start and stop) information .This can be implemented as a hardware facility or U(S) ART (Universal (synchronous) asynchronous receiver/transmitter) relieving the processor (and the applications programmer) of this lowlevel, timeconsuming, detail. t is merely necessary to selected a baudrate and possibly other options (number of stop bits, parity, etc.) and load (or read from) the serial transmitter (or receiver) buffer. Serialization of the data in the appropriate format is then handled by the hardware circuit.Timing/counter facilities Many application of singlechip microputers require accurate evaluation of elapsed real time .This can be determined by careful assessment of the execution time of each branch in a program but this rapidly bees inefficient for all but simplest programs .The preferred approach is to use timer circuit that can independently count precise time increments and generate an interrupt after a preset time has elapsed .This type of timer is usually arranged to be reloadable with the required count .The timer then decrements this value producing an interrupt or setting a flag when the counter reaches zero. Better timers then have the ability to automatically reload the initial count value. This relieves the programmer of the responsibility of reloading the counter and assessing elapsed time before the timer restarted ,which otherwise wound be necessary if continuous precisely timed interrupts were required (as in a clock ,for example).Sometimes associated with timer is an event counter. With this facility there is usually a special input pin , that can drive the counter directly. Timing ponents The clock circuitry of most microputers requires only simple timing ponents. If maximum performance is required, a crystal must be used to ensure the maximum clock frequency is approached but not exceeded. Many clock circuits also work with a resistor and capacitor as lowcost timing ponents or can be driven from an external source. This latter arrangement is useful is external synchronization of the microputer is required. Programming languages Higher level PLC programming languages have been around for some time, but lately their popularity has mushrooming. As Raymond Leveille, vice president amp。Automation .inc。s very difficult to write a trigonometric function using ladder logic. Languages gaining acceptance include Boolean, control system flowcharting, and such function c
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