PICAssemblyLanguageforthe
                    CompleteBeginner
                      MichaelA.Covington
                     Artificial Intelligence Center
                     TheUniversityofGeorgia
                     Athens,Georgia 30602-7415
                     http://www.ai.uga.edu/mc
              This article appeared in Electronics Now Magazine in 1999 and is
              reprinted here by permission. Some web addresses have been up-
              dated but the content has not; you will find that MPLAB, for instance,
              nowlooks somewhatdifferent.
              You may print out this article for personal use but not for further pub-
              lication.
                  c
              Copyright 
 1999 Gernsback Publications, Inc.
                  c
              Copyright 
 1999, 2004 Michael A. Covington.
            These days, the field of electronics is divided into “haves” and “have-
           nots” – people who can program microcontrollers and people who can’t.
           If you’re one of the “have-nots,” this article is for you.
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           Microcontrollers areone-chipcomputersdesignedtocontrolotherequip-
          ment, and almost all electronic equipment now uses them. The average
          American home now contains about 100 computers, almost all of which
          are microcontrollers hidden within appliances, clocks, thermostats, and
          evenautomobile engines.
           Although some microcontrollers can be programmed in C or BASIC,
          youneedassemblylanguagetogetthebestresultswiththeleastexpensive
          micros. The reason is that assembly language lets you specify the exact
          instructions that the CPU will follow; you can control exactly how much
          timeandmemoryeachstepoftheprogramwilltake. Onatinycomputer,
          this can be important. What’s more, if you’re not already an experienced
          programmer, you may well find that assembly language is simpler than
          BASICorC.Inmanywaysit’smorelike designing a circuit than writing
          software.
           The trouble with assembly language is that it’s different for each kind
          of CPU.There’soneassemblylanguageforPentiums,anotherforPICmi-
          crocontrollers, still another for Motorola 68000s, and so forth. There are
          even slight differences from one model of PIC to another. And that leads
          to a serious problem – each assembly-language manual seems to assume
          that you already know the assembly language for some other processor!
          So as you look from one manual to another in puzzlement, there’s no way
          to get started.
           That’s the problem this article will address. I won’t teach you all of
          PICassembly language, just enough to get you started. For concreteness,
          I’ll use just one processor, the PIC16F84. To be very precise, I’ll use the
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                     PIC16F84-04P, which operates up to 4 MHz and is housed in a plastic DIP
                     package.1 ThisisaproductofMicrochip,Inc. (Chandler,Arizona),andit’s
                     closely related to the rest of the PIC family – which, however, I’ll ignore to
                     prevent confusion.
                         Todotheexperimentsdescribedinthisarticle,you’llneedoneormore
                     PIC16F84-04P chips; we strongly recommend having more than one so
                     you can rule out a damaged PIC if your circuit doesn’t work. You’ll also
                     needtheotherpartsforthecircuitsyouwanttobuild(seetheschematics).
                     Andyou’ll needaPC-compatiblepersonalcomputer, the MPASMassem-
                     bler software (which you can download from http://www.microchip.com),
                     and a PIC programmer such as Ramsey Electronics’ “PICPRO-1” or the
                     NOPPP programmer published in this magazine, September 1998, and
                     described at http://www.covingtoninnovations.com/noppp. The PIC16F8X
                     datasheet,actually a122-pagemanual,willalsocomeinhandy;it’scalled
                     PIC16F8X because it covers both PIC16F84 and PIC14F83, and you can
                     downloaditorrequestaprintedcopyfromMicrochip.
                     1 PART1-MEETTHEPIC
                     1.1   What’sinsideaPIC?
                     Figure 1 shows the pinout of the PIC16F84, and Figure 2 shows the most
                     important parts inside. The PIC is a tiny but complete computer. It has a
                     CPU(central processing unit), program memory (PROM), working mem-
                        1Noteadded2004: The10-MHzversionisnowmorecommonandwillworkinallthe
                     samecircuits.
                                                         3
                                                                  PIC16F84

                                                              1
 A2
      A1
 18

                                                              2
 A3
      A0
 17

                                                              3
 A4
     O1
 16

                                                              4
 MCLR
 O2
 15

                                                              5
 GND
    V+
 14

                                                              6
 B0
      B7
 13

                                                              7
 B1
      B6
 12

                                                              8
 B2
      B5
 11

                                                              9
 B3
      B4
 10

                                                     Figure 1: Pinout of PIC16F84.
                          ory (RAM),andtwoinput-outputports.
                              The CPU is, of course, the “brain” of the computer. It reads and exe-
                          cutesinstructionsfromtheprogrammemory. Asitdoesso,itcanstoreand
                          retrieve data in working memory (RAM). Some CPUs make a distinction
                          between “registers” located within the CPU and “RAM” located outside
                          it; the PIC doesn’t, and its general-purpose working RAM is also known
                          as“fileregisters.” Onthe’F84,thereare68bytesofgeneral-purposeRAM,
                          located at addresses hex 0C to hex 4F.
                              Besidesthegeneral-purposememory,thereisaspecial“workingregis-
                          ter” or “W register” where the CPU holds the data it’s working on. There
                          are also several special-function registers each of which controls the oper-
                                                                      4