【正文】 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 multiplexed onto port one of the controller with the data from the DIP switch. Note that the four resistors added at the switch are not required in the basic application, since the AT89C51 provides internal pullups on port one. During the normal operation of the application, controller ports zero and two provide data and control signals (respectively) to the displays. During programming and program verification, the programmer asserts control of port zero and part of port two. The programmer is connected to ports zero and two without buffering, since, when inactive, its presence does not affect the normal operation of the application. A transparent latch has been added between port two of the controller and the display control inputs. The latch holds the display control signals inactive during programming, which eliminates erratic operation of the displays due to programmer activity on ports zero and two. No isolation of the display data inputs is required, since data applied to the inputs is ignored when the control signals are inactive. The AT89C51 reset circuit, input multiplexer and output latch are controlled by a single signal generated by the programmer. During programming, reset is asserted, the multiplexer switches inputs, and the latch freezes the display control lines. To ensure that the display control lines are in a known state before they are latched, an AT89C51 external interrupt is used to allow the programmer to signal the application before asserting reset. The application firmware responds to the interrupt by displaying a message and deactivating the display control lines. After programming, when reset is deasserted, the controller ports are high as the latch bees transparent. Since the display control inputs are inactive high, the display contents are not disturbed until the new program writes the display. Although not essential to this application, it might be imperative in some applications that the state of the peripheral circuitry not be disturbed during programming. The Programmer The programmer (Figure 3) generates the addresses, data and control signals necessary to program the AT89C51 embedded in the application. The programmer circui