【正文】 AT89C51 InCircuit Programming This application note illustrates the incircuit programmability of the Atmel AT89C51 Flashbased microcontroller. Guidelines for the addition of incircuit programmability to AT89C51 applications are presented along with an application example and the modifications to it required to support incircuit programming. A method is then shown by which the AT89C51 microcontroller in the application can be reprogrammed remotely, over a mercial telephone line. The circuitry described in this application note supports five volt programming only, requiring the use of an AT89C51XX5. 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. 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 threestate outputs might be inserted between the application circuitry and the controller, with the buffer outputs threestated when programming is enabled. Alternately, a multiplexer might be used to select between signal sources, with signals applied to the controller by either the application circuitry or the programmer circuitry. Port Used for Output No circuit changes are required if the application circuitry can tolerate the state changes which occur at the port during programming. If the prior state of the application circuitry must be maintained during programming, a latch might be inserted between the controller and the application circuitry. The latch is enabled during programming, preserving the state of the application circuitry. An Application Example The AT89C51 application shown in Figure 1 is an implementation of a moving display. This application was selected for its simplicity and ability to show graphically the results of incircuit reprogramming. The text to be displayed is programmed into the controller as part of its firmware, and cannot be changed without reprogramming the device. The displayed text is presented in one of two modes selected by the fourposition DIP switch. In the first mode, one character at a time enters the display from the right and moves quickly to the left through each element of the display to its final position in the assembled message. In the second mode, the message moves through the display, from right to left, with the display acting as a window onto the message. This mode is familiar as the method often used in displays of stock prices. The output consists of four DL1414T, fourdigit, 17segment alphanumeric displays with integral decoders and drivers. This yields 16 total display elements, each capable of displaying digits 09, the upper case alphabet, and some punctuation characters. The displayable character codes are ASCII 20H5FH. A poweron reset circuit and a 6MHz crystal oscillator plete the application. Neither external program memory nor external data memory is used. Modifications to the Application to Support InCircuit Programming Figure 2 shows the application modified for incircuit programming. It is assumed that the programmer, when inactive, will neither drive nor excessively load the application. Since the application does not use external program memory, EA/VPP on the controller is connected to VCC. This meets the requirement for programming. The reset circuit has been modified by the addition of twotransistors, which allow RST on the controller to be forced high by the programmer. PSEN and ALE/PROG, unused in the basic application, areunder the direct control of the programmer. Programming requires programmer access to all of the four AT89C51 I/O ports, as documented in the data sheet. The programmer is connected directly to those controller pins which are unused by the application, while access to pins used by the application requires special treatment, as explained in the following paragraphs. The least significant four bits of the address generated by the programmer are multip