                                   All About Clocks
                                  By Lauren A. Colby
                                    Frederick, MD.
          
          March 21, 1992
          
               For years, I've been fascinated by clocks and, when
          computers came out, I became very interested in computer clocks.
          As everybody knows, XT machines don't have a built-in clock. AT
          machines do. It is built into their CMOS ram.
          
               Unfortunately, the CMOS clocks don't keep very good time. I
          read somewhere that an AT CMOS clock can gain or lose as much as
          five minutes per month. Whoever wrote that was awfully
          charitable. Some of our AT machines in the office have been known
          to gain or lose five minutes in a week!
          
               Most high-priced AT clones will have a little clock trimmer
          on the mother board, to adjust the clock. Fiddling with that
          trimmer is not, however, a good idea!
          
               A couple of years ago, we had a machine which kept
          particularly poor time, so I resolved to adjust the trimmer.
          Everyday, I'd turn on the machine, move the trimmer clockwise or
          counter-clockwise, and make a record of what I did, to see
          whether I needed to slow down or speed up the clock.
          
               One day, I turned on the machine and found that it had
          gained 12 hours, just overnight. I went into Setup and found, to
          my horror that the CMOS clock was running at twice the normal
          speed. Evidently, I hadn't properly grounded myself when I
          adjusted the trimmer, and had wiped out a frequency divider in
          the clock, so that it no longer divided by two, and was running
          straight through, instead.
          
               The only way to save the machine was to install an auxiliary
          clock board, which I did. In fact, I now install an XT type clock
          board in all my machines. The 40 mhz 486 machine which I'm using
          right now has a little $10.00 XT clock board, because it keeps
          good time, while the CMOS clock does not.
          
               In that connection, don't believe the nonsense you may
          heard, that an XT clock board won't work in an AT machine without
          disabling the CMOS clock. The board will work just fine! All you
          have to do is plug it in, and read the time with the software
          that came with the board. The best ones use a program called
          TIMER which you invoke from your autoexec.bat file. If you have
          DOS 3.3 or better, invoking TIMER /S will set your DOS clock and
          the CMOS clock as well!
          
               Also, you don't necessarily have to use up a slot to install
          the board. Probably, you have an I/O board in your machine
          already (parallel, serial, game port) and it most likely has some
          cables coming out of it which go to sockets, installed in
          slotcovers.  If so, you can remove the slot cover from your clock
          board and plug the board into the bus, behind one of these
          socketed slot covers. That way, a slot which is normally useless
          becomes useful.
          
                Purists, and I am one, will naturally want to set any clock
          so that it keeps perfect time. The obvious way to do that is to
          count the clock oscillator with a frequency counter. That,
          however, is easier said than done. The output from the
          oscillator, whether it be from a CMOS clock (which, as noted, you
          should not touch, lest you damage an expensive system board) or
          an XT timer board, is so weak, that the input of an ordinary
          frequency counter may swamp the oscillator and make it quit.
          
               You can, however, make an impedance converter to allow you
          to read the oscillator with the counter. Go to Radio Shack and
          purchase a VMOS power FET. Connect the source to ground through a
          1,000 ohm resistor. The drain is connected to plus nine volts
          (ordinary nine-volt battery), through a 10,000 ohm resistor.
          Connect a 2 meg resistor from the gate to the ground, and a 10
          meg resistor from the gate to the plus nine volt line. Connect a
          .1 ufd capacitor to the gate; the signal from the clock
          oscillator is picked up from the hot side of the clock crystal
          and fed into the converter through the capacitor. Connect another
          .1 ufd capacitor from the FET drain to the input of your
          frequency counter. 
          
          This setup will let you observe the output from the clock on your
          counter, However, the signal from the clock is so tiny that you
          may still have to fiddle with the bias some (the 10 meg
          resistor), to get a good, clean output.
          
               The clocks all use 32,768 HZ crystals, so you should adjust
          the trimmer for a 32,768 count. If you are really finicky, you'll
          want to use a frequency divider; and divide the output down to
          the point where you can use the "period" mode on your counter, to
          get a super-accurate reading.
          
               All of this may, however, be a waste of time. Recently, I
          bought a couple of clock boards by mail. They had a place for a
          trimmer, but no trimmer was installed. I figured the manufacturer
          was just trying to save money, so I installed the missing
          trimmers. Then , I hooked up the frequency counter, and began
          adjusting the trimmers, to see how much I could "rubber" the
          crystals. To my surprise, twisting the trimmers did nothing at
          all! The oscillators just kept ticking away at 32,768, no matter
          what I did. If I added too much capacitance, the oscillators just
          quit!
          
               Finally, I realized that the manufacturer had eliminated the
          trimmers because they were not needed. Apparently, the boards
          have an improved type of clock crystal, which won't allow itself
          to be "rubbered". As a matter of fact, the boards seem to keep
          nearly perfect time, but I still miss the ability to fiddle
          withthe trimmer.
          
               Finally, a few words should be said about the clock
          oscillators themselves. Some use low powered 7400 series IC's.
          Others are constructed from varicap diodes, and sometimes, you
          will see the oscillator built into the clock chip. In all cases,
          however, the best place to pick up the signal for testing is
          right at the crystal itself. Use a micro-clip lead from Radio
          Shack, and always isolate the lead from the frequency counter,
          scope, or other test instrument, with an isolation capacitor.
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