外文翻译单片机在编程电路中应用.docVIP

附　录 附录A　外文资料 AT89C51 In-Circuit Programming This application note illustrates the in-circuit programmability of the Atmel AT89C51 Flash-based microcontroller. Guidelines for the addition of in-circuit programmability to AT89C51 applications are presented along with an application example and the modifications to it required to support in-circuit programming. A method is then shown by which the AT89C51 microcontroller in the application can be reprogrammed remotely, over a commercial telephone line. The circuitry described in this application note supports five volt programming only, requiring the use of an AT89C51-XX-5. The standard AT89C51 requires 12 volts for programming. The software for this application may be obtained by downloading from Atmel’s. General Considerations EA/VPP must be held high during programming. In applications which do not utilize external program memory, this pin may be permanently strapped to VCC. Applications utilizing external program memory require that this pin be held low during normal operation. RST must be held active during programming. A means must be provided for overriding the application reset circuit, which typically asserts RST only briefly after power is applied. PSEN must be held low during programming, but must not be driven during normal operation. ALE/PROG is pulsed low during programming, but must not be driven during normal operation. During programming, AT89C51 I/O ports are used for the application of mode select, addresses and data, possibly requiring that the controller be isolated from the application circuitry. How this is done is application dependent and will be addressed here only in general terms. Port Used for Input During programming, the controller must be isolated from signals sourced by the application circuitry. A buffer with three state outputs might be inserted between the application circuitry and the controller, with the buffer outputs three-stated when programming is enabled. Alternately, a multip