EXAMPLE SQUARE WAVE - Mechanical Engineering例方波-机械工程.docVIP

Example Square wave OBJECTIVE This example has the following objectives: Review the use parallel ports Illustrate the generation of a square wave using Port B output and a waiting sequence Demonstrate the use of a subroutine to shorten the length of a program Examine the relation between displayed simulation time and clock cycles in THRSim11 simulator Figure 1 Square wave schematics showing the half-wave duration, ?t, and the low (L) and high (H) states. programs This example uses two programs that produce the same output – the generation of a square wave of programmable wavelength. The wavelength is programmed using the variable DELAY. The square-wave electrical signal is output through second pin of Port B, i.e., pin PB1. The difference between the first and second program consist in the use of a subroutine. The use of a subroutine compresses the code and shortens the total length of the program. Program sq_wav1 Instructions Define variable DELAY at $0000 Load REGBAS in reg. X Send low signal Load in accA Store accA in Port B Load DELAY into accB Decrease accB in a loop until zero Send high signal Load in accA Store accA in Port B Load DELAY into accB Decrease accB in a loop until zero Branch back to (iii) SWI Flowchart Code DELAY RMB 1 * Start main program ORG PROGRAM START LDX #REGBAS * Begin typing your code * Send low signal BEGIN LDAA # STAA PORTB,X LDAB DELAY LABEL1 SUBB #1 BHI LABEL1 * Send high signal LDAA # STAA PORTB,X LDAB DELAY LABEL2 SUBB #1 BHI LABEL2 BRA BEGIN SWI Flowchart and code The program flowchart is show to the right of the program instructions. Note the ‘send low signal’ and ‘send high signal’ blocks. Also note the two wait blocks, one sending the low signal, the other after sending the high signal. The wait blocks are not detailed, since they have been covered in a previous example and can be directly cloned from there (the ‘code reuse’ concept). The essential code for this program is sh