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
Circuitry added to support AT89C51 incircuit programming should appear transparent to the application when programming is not taking place.
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.
本應用指南說明了Atmel AT89C51是可在線可編程的微控制器。它為電路編程提出了相應的例子，程序的修改需要在線編程的支持。這類顯示方法在應用程序中的AT89C51單片機可通過電話線遠程控制。該應用指南所描述的電路只支持5v電壓下編程。此應用軟件可以到Atmel進行下載。
總論
當不在進行程序設計的時候，在電路設計中的AT89C51設計將變得透明化。
在編程期間必須重視EA/VPP這一腳。在不使用外部程序存儲器的應用程序中，這腳可能會永久接到VCC。應用程序使用的外部程序存儲器要求這一腳為低電平才能正常運行。