You may have noticed something peculiar with the fuse setting in the program.
The compiler is set up to produce code for a 40MHz crystal, but a 20MHz crys-
tal is actually used. And the fuse settings are for a x4 PLL, which will multiply
the crystal’s frequency by 4. Now the maximum (official) frequency that a
PICmicro can operate is 40MHz, and this arrangement is usually implemented
using a 10MHz crystal (10MHz * 4). See the PICmicro’ data sheet for more in-
formation concerning the x4 PLL fuse setting, or peruse the midrange reference
manual for the 16-bit core devices. Both of these are downloadable, free of
charge, from Microchip’s web site at www.microchip.com. You might think that using a 20MHz crystal with a x4 multiplier would allow
the PICmicro to operate at 80MHz. However, this is unachievable on the cur-
rent breed of PICmicros and it actually settles at a frequency of approx 55MHz.
40MHz is the closest crystal that is implemented by the compiler therefore all delays will be somewhat wrong, which they are anyway because the interrupt
routine is taking most of the PICmicro’s time. This high speed allows higher pitched octaves to be achieved, thus producing a more pleasant sound to the
tune. Not bad for 20MHz crystal ? In tests, no PICmicro chosen failed to oscillate, and because the program is not
dependant on its frequency, it really doesn’t matter if he oscillator is out by a few KHz or even a few MHz. We just need speed! Converting a Midi file to LDATA tables. Being able to play a tune is all well and good, but actually creating the tune to
play is most of the fun with this project. As mentioned earlier, tunes can be
downloaded in the form of .MID files (midi files) for playing on a PC soundcard.
However, some of these are very complex compositions, consisting of many
tracks containing many instruments, so you will need to choose the midi file
carefully. Piano or guitar tunes tend to be more suitable, and classical music is
very good for conversion because they usually do not contain a drum track
(which we cannot recreate). And of course, traditional Christmas tunes are also
more suitable. Once you’ve chosen a midi file that you think may be suitable for conversion,
you will need a piece of software capable of viewing and editing the file. When
creating the project I used a shareware program named MIDINOTATE. A 30-
day fully working demo of this can be downloaded from www.notation.com. SO
this is the program I will use to illustrate conversion. If you’ve built the previous circuit, you will have guessed that the tune was “We
Wish You a Merry Christmas”, so we’ll take a look at how this tune was con-
verted. The MERRY CHRISTMAS.MID file containing the tune can b e found along with the BASIC programs listed in the article at the PROTON Users Page.
