【正文】 e serial interface, to which the modem responds with Hayesstyle ASCII numeric codes. The software is designed for use with Hayespatible modems, which includes the Prometheus ProModem 1200 used here. The serial interface, through which the programmer connects to the modem, supports two handshaking signals, DTR and DSR. On power up, the programmer asserts DTR, to which the modem responds by asserting DSR. If the modem should fail to respond to any mand, including the mand to hang up, the programmer deasserts DTR, which forces the modem to drop the line. The modem monitors the phone line while the programmer sleeps, waiting for an ining call. When a call is detected, the modem answers and attempts to establish munication with the caller. If a connection is established, the modem sends a code to the programmer, waking it up. The programmer verifies the connect code and begins polling for a valid packet header. Ining packets must arrive fewer than thirty seconds apart, or the modem drops the line (hangs up) and the programmer returns to sleep, waiting for the next call. If the caller hangs up, the thirty second period must expire before another call will be answered. Calls ining during the reset delay period are ignored. If a valid packet header is received prior to the expiration of the reset delay period, the programmer will attempt to read and validate the ining packet. At any time during packet reception, an invalid character, parity error or timeout during character reception will cause the partial packet to be declared invalid and discarded. Two packet types are defined: data and endoffile. A data packet contains five fields in addition to the packet header, one of which is a variable length data field. The data field contains program data to be written into the AT89C51 controller in the application. The load address field contains the address at which the data is to be written. The endoffile packet contains the same fields as the data packet, except that the data field is empty. This packet type has special meaning to the programmer, as explained below. Any packet which contains an invalid record type, record length or checksum is invalid. Program data accumulated during the processing of an invalid packet is discarded. The programmer sends a NAK to the transmitter to signal reception of an invalid packet and resumes polling for a valid packet header. Receipt of the first valid data packet causes the programmer to interrupt the application controller. The controller responds to the interrupt by abandoning execution of its usual program and displaying a message indicating that programming is taking place. If this is the first valid data packet since power was applied or an endoffile packet was received, the programmer asserts the control signals necessary to erase the program memory in the application controller. The programmer then places the controller in programming mode. The first and subsequent valid data packets are dissected as they are received and the data which they contain is programmed into the application controller at the address indicated in the packet load address field. After programming, the data is read back from the controller and verified against the received packet data. Successful verification indicates that programming was successful, causing the programmer to send ACK to the transmitter. The programmer then resumes polling for a valid packet header, subject to the thirty second reset delay. If programming fails, the programmer sends CAN to signal the transmitter to abort the file transfer. The modem drops the line and the programmer returns to sleep, waiting for the next call. The application controller is left in programming mode, preventing it from executing the inplete or invalid program which it contains. It is important to note that invalid packets are NEVER programmed into the application controller. To do so would require that the program memory in the controller be pletely erased before the error could be corrected, causing the nonrecoverable loss of all previous program data. Upon receipt of an endoffile packet, the programmer returns its control outputs to the inactive, power on state, allowing the application controller to begin execution of the new program. The programmer then resumes polling for a valid packet header, subject to the thirty second reset delay. If a valid packet is received prior to the expiration of the thirty second delay, another programming cycle begins, which can only be terminated by the reception of a valid endoffile packet. If the reset delay expires prior to the reception of a valid endoffile packet, the modem will drop the line and the programmer will return to sleep, waiting for the next call. In this case, the application controller is left in programming mode, preventing it from executing its program. To return the application to normal operation, another call must be received, and a valid program file uploaded, terminated by an endoffile packet． 單片機(jī)在編程電路中的應(yīng)用 本應(yīng)用指南說明了 Atmel AT