barcode reader project in asp.net OPTIMIZING PIC MICROCONTROLLER APPLICATIONS in Software

Creating Quick Response Code in Software OPTIMIZING PIC MICROCONTROLLER APPLICATIONS

OPTIMIZING PIC MICROCONTROLLER APPLICATIONS
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Figure 8.6 A synchronous serial bus with a serial-to-parallel interface chip or a simulated parallel bus can be used to increase the number of I/O pins available to the PIC microcontroller.
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you copy subroutines that cross over page boundaries into one consistent page, the actual program memory requirements will decrease. For optimizing le register and program memory usage, the best suggestion I can give you is to try to come up with as many possible solutions as you can. This also includes looking at other people s code for examples. Chances are that you will be able to reduce the requirements above and beyond the immediate needs of the application. Not having enough I/O pins can be a particularly troubling problem, but additional pins can be added to the circuit either via a synchronous serial bus and using serial-toparallel interface chips or by simulating a parallel bus, as shown in Fig. 8.6. The obvious drawbacks to using the synchronous serial bus is the time required for shifting bits and possible bit ripple as the bits get shifted in or out. This bit ripple can be avoided by placing a latch between the shift register and the I/O pins. The latch output is updated after the data has been shifted from the PIC microcontroller. To shift data out, the following code could be used:
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bcf DataBit btfsc SourceRegister, 0 bsf DataBit bsf ClockBit bcf ClockBit
These ve instructions for shifting out a bit can be reduced to four instructions by keeping a le register loaded with a shifted value and placing the data bit in bit 0 or bit 7 of the I/O port. The code for a data pin at RBO, with a clock, also in port B (and rising edge active), to shift out a bit rst has the port B value saved in shifted format and the clock bit low:
rrf PORTB, w andwf 0x0FF ^ (1 << (clock 1)) movwf PORTBSave
ASSEMBLY-LANGUAGE SOFTWARE TECHNIQUES
When the least signi cant bit of the data is to be shifted out, the four instructions
rrf SourceRegister, w rlf PORTBSave, w movwf PORTB bsf ClockBit
could be used. These four instructions can be put into a loop to shift out a byte:
movlw 8 movwf Count Loop: rrf SourceRegister, f rlf PORTBSave, w movwf PORTBB bsf ClockBit decfsz Count, f goto Loop
This code requires 16 instruction cycles less than if the original ve instructions were used in Loop:
movlw 8 movwf Count Loop: bcf DataBit btfsc SourceRegister, 0 bsf DataBit bsf ClockBit bcf ClockBit rrf SourceRegister, f decfsz Count, f goto Loop
Creating a parallel bus can be challenging to wire and may require a more substantial change to your application than the synchronous serial interface. Despite these drawbacks, data can be passed quickly and does not have the ripple of the serial method. Note that both these methods require additional chips that can drive the parts cost up to the point where a PIC microcontroller with more pins is more economical. The last issue for optimization is often the most dif cult to overcome, and this is meeting minimum timing speci cations. Often the only solution to this problem is to use a PIC microcontroller (or another microcontroller) with built-in hardware that provides the basic function of running the PIC microcontroller at a faster speed. This is not to say that the problem is insolvable; with a bit of work, you can work through your code to nd solutions to the problem. Remember to look at as many different solutions as possible, and remember that there can be some advantages to rearranging the hardware. As was shown in the data-shifting example, a substantial improvement in instruction cycles resulted in a nine-cycle loop for a 22 percent speed improvement of the application
A BAKER S DOZEN RULES
function. Always remember that you should be treating all issues and problems the same way in which you would if you had a failure in some hardware or software; approach the problem using the techniques discussed in Chap. 14.
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