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Microcontrollers

Software Examples to Understand and Use the Microcontroller Instruction Set.

Assembler Tutorials for both MPLAB Simulation and Downloading to the MMM development Board.

Code TUTMM1.ASM

The first piece of code simply sets all the PORTB pins as OUTPUTS and sequentially turns on each of the PORTB LED's in a simple loop. i.e. 00000001; 00000011; 00000111; 00001111; 00011111; 00111111; 01111111; 11111111, then clears all outputs and starts again.

This code should be first simulated to step through to get a feel for what is happening with the registers.

Then it can be downloaded to the MatrixMultiMedia Development Board, where it can run SLOWLY in RC mode to demonstrate the incrementing LED's on PORTB. (If the downloaded code simply lights up all the PortB LED's, then you may find that the 'configuration' is probably set to crystal mode and it is running too fast for the eye to discern anything. For this code make sure the configuration is set to RC operation and the switches S2 and S4 (on the target board) will need setting accordingly. S2 ought to be in RC mode (to the right!) and S4 ought to be in slow mode for this code (to the left!) to step through slow enough for one to see).

Fig 1 Flowchart for TUTMM1 code

Code TUTMM2.ASM

The second piece of simple assembler code produces a single lit LED that starts at the right hand side of PORTB LED's (LB0) and moves left until it reaches the eighth and final LED of PORTB(LB7), then the loop repeats. The code uses aliases, bit-names and 'conditional loops' which demonstrate how the 'carry bit' rotates into the register. (i.e. When register rolls over from 1111 1111 to 0000 0000, the CARRY bit is set.)

This code can again be simulated within the MPLAB environment to step through the lines of code and watch the registers change and observe the code loop-pattern. Once the machine code has been downloaded to the MMM development board, it will run (when operated in slow RC-mode) and can be seen operating through its loops.

Development Board Settings for RC operation - switches S2 set to RC mode (to the right!) and S4 set to SLOW mode (to the left!)

Fig 2 Flowchart for TUTMM2 code

Fig 2a Pseudo-code for TUTMM2 code

The CARRY FLAG is set when Port B register rotates left from 1000 0000 and overflows to 0000 0000 (i.e. bit 0 of the STATUS register is set).

Code TUTMM3.ASM

The third piece of simple assembler code produces a decrementing single illuminated LED moving from left to right on the PORTB LED's (from LB7 to LB0). It utilises the Rotate Right Instruction and checks the condition of the ZERO bit to determine when PORTB goes from 0000 0001 to 0000 0000. (when the RESULT of an (arithmetic or logic) operation is ZERO, Z bit (bit2) of the STATUS register is set.) This code achieves the opposite direction of rotation for the PORTB LED's, compared to TUTMM2.ASM code.

This code can again be simulated within the MPLAB environment to step through the lines of code and watch the registers change and observe the code loop-pattern. Once the machine code has been downloaded to the MMM development board, it will run (when operated in slow RC-mode) and can be seen operating through its loops.

Development Board Settings for RC operation - switches S2 set to RC mode (to the right!) and S4 set to SLOW mode (to the left!)

Fig 3 Flowchart for TUTMM3 code

Fig 3a Pseudo-code for TUTMM3 code

When PORTB rotates from 0000 0001 to 0000 0000, we need to loop back around to the start position again (via loop1). To achieve this if we then move the contents of PORTB onto itself ('MOVF PORTB, F'), it will generate a ZERO Flag (bit 2 of the Status register is set), as the result of this operation is ZERO.

Code TUTMM4.ASM

The fourth piece of simple assembler code displays both incrementing (followed by decrementing) decimal value representation on the PORTB LED's. The code uses loops while checking the ZERO status bit. It illustrates both UP and DOWN counts. This code can be sped up by rotating the 'RV1' pot (to the left of the S2 & S4 switches on the development board). This code can also be used to experiment with running up to a set BREAK POINT (see Practical Session 4), to allow the code to run to a specifc set point in the code before stepping through the code-area of interest.

Like the previous examples this code can again be simulated within the MPLAB environment then the machine code can be downloaded to the MMM development board.

Development Board Settings for RC operation - switches S2 set to RC mode (to the right!) and S4 set to SLOW mode (to the left!)

Fig 4 Flowchart for TUTMM4 code

Fig 4a Pseudo-code for TUTMM4 code

Code TUTMM5.ASM

The fifth piece of simple assembler code uses a single switch on Port A to increment the Port B LED count

Fig 5 Flowchart for TUTMM5 code

Fig 5a Pseudo-code for TUTMM5 code

Like the previous examples, this code can, again, be simulated within the MPLAB environment then the machine code can be downloaded to the MMM development board. For simulation on this example try using the following STIMULUS FILE:-

Development Board Settings for RC operation - switches S2 set to RC mode (to the right!) and S4 set to SLOW mode (to the left!). If this code is run in XT mode (with the clock crytal) it will run too fast for the eye to see!

[Link to Further Examples of Using Assembler Instructions and appreciation via simulation (tutorials 6 to 11)]

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Updated 06.07.07 ML

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