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Contemporary Design Execution
by Robert P. Bisey

Start ý The History ý Interfacing ý Interface Board Circuit Descriptions ý The Microcontroller Board • Program Logic • Software • Code Transmission Protocol • Something Old, Something New..• Sources and PDF

PROGRAM LOGIC

Figure 2 shows the general flow of the program logic that must occur to successfully generate a Morse code transmission. If the transmit/test push button is not pushed or the external CODE TRANSMIT signal does not occur, the program follows the No decision path and loops back endlessly onto itself, waiting for the push-button/external CODE TRANSMIT signal to occur.

If either of these two events do occur, the Yes decision path is initiated, which causes the Y-index register of the 68HC11 to point to the address of the first character in the Morse code string to be transmitted. This is immediately followed by a logic signal, which appears at the SBC-2A board, driving the input of a standby/enable transistor whose logic 0 TRANSMIT ENABLE output enables the repeater/transmitter to begin transmitting the code sequence. The instant this enabling signal occurs, a short delay of approximately 0.5 s elapses to ensure that the repeater/transmitter has adequate time to fully turn on from Standby/Power down mode and shift into Transmit mode before the first code element is actually transmitted.

After this delay, the program flow drops down to the top of a vertical string of five Yes/No decision diamonds, which tests for the presence of a specific character in the text string about to be transmitted. Namely, a hyphen, period, forward slash, backslash, or the letter X, which corresponds to a dash, dot, character space, word space, and end of string, respectively, would be found. For example, if the first character encountered in the text string is a period, because a dash in the first decision diamond is not a period, the program "falls through" the first decision diamond by taking the No path to the input of the second diamond. Because of the period in the text string of the second diamond, the program flow takes the Yes branch to execute a subroutine program segment, generating a Morse code Dot.

After the Dot subroutine has finished generating a Dot time, the subroutine bypasses the remaining decision diamonds and returns to the main program flow, initiating a program segment, which causes the Y-Index register to point to the next code element to be transmitted. Thus, the process continues so long as the letter X, the end of string character, is not encountered.

If the letter X is encountered, however, the program initiates another brief 0.5-s delay via the state change of the standby/enable transistor, returning the repeater/transmitter to Standby mode. The program then terminates at the Start node waiting for the transmit/test push button or external CODE TRANSMIT signal to occur to retransmit the entire Morse code string all over again.

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