Question

Consider the following assembly language code. The clock frequency is 4 MHz- and all initialization steps have been done correctly (like setting up digital I/O, the oscillator configuration, etc.) Constants Bit Pattern EQU H'20' LoopCtr EQU H'21' Max Count EQU .23; Main program loop MainLoop CLRF BitPattern CALL Output BSF BitPattern, 1 CALL Output RRF BitPattern CALL Output BSF BitPattern, 1 CALL Output GOTO MainLoop Output MOVF BitPattern, W MOVWF PORTB MOVLW MaxCount MOVWF LoopCtr Loop NOP DECFSZ LoopCtr GOTO Loop NOP RETURN a. What sequence of numbers appears in BitPattern (hex or decimal is fine, just make sure it is clear which one you are doing!) b. From the start of one of the CALL instructions to the end of the subsequent RETURN instruction, how many cycles are taken? c. Sketch the waveforms you would see if oscilloscope probes are attached to the pins associated with PORTB<0> and PORTB<1>. Indicate both the vertical and horizontal scales (voltes/div and time/div).

Answer 1

Solution: a. The sequence of numbers appearing in BitPattern . In main loop, CLRF BitPattern would clear "BitPattern" and hence it results in 0x00 BSF BitPattern, 1 would set bit 1 of BitPattern and hence it results in 0x01. RRF BitPattern would rotate right so it would give 0x00 BSF BitPattern would set bit 1 of BitPattern, so it would give 0x01 In output MOVF BitPattern, W would move content of BitPattern to W. and hence W has 0x01 . b. The number of cycles taken From beginning of first CALL instruction to end of subsequent RETURN instruction. There are 41 cycles taking place per loop. . *