The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 13
FOR loop by loading FFh into register dpl (the lower byte of dptr), and decrementing dpl
repeatedly until it equals zero.
In assembly language, you also do not have built-in conveniences like BASIC-52’s ability
to terminate a program on CONTROL+C. You have to add these features yourself. In Listing
13-5, after each complete cycle of the sine wave, the program checks the serial port’s receive
flag. If the flag is set, it means that the user has pressed a key, and the program returns to
the BASIC-52 prompt. Otherwise, the program begins another cycle of the sine wave.
To run Listing 13-5, create a source file with your text editor, assemble it, and upload it to
RAM as before. Edit Listing 13-4 by removing lines 150-180 and adding this line:
150 CALL 3100h
Now when you run Listing 13-4, you should again see a sine wave at VOUT, but at a much
higher frequency.
With a 12-Megahertz crystal, the sine wave should be around 350 Hertz, or 2.8 milliseconds
per cycle. You can verify this by consulting the 8052’s data book, which tells the number of
machine cycles required to execute each instruction. At 12 Megahertz, each machine cycle
is 1 microsecond, and one complete cycle requires 11 microseconds multiplied by 255 points
on the wave, plus 6 microseconds to test the serial flag, or 2811 microseconds total.
With different crystal frequencies, the output frequency will vary in direct proportion. For
example, with a 6-Megahertz crystal, the sine wave will be half as fast.
To slow down the sine wave, you can add “do-nothing” instructions to the code. For example
adding a nop (no operation) instruction in the main loop will add 1 microsecond to the time
between points on the wave, for a frequency of 326 Hertz. For long delays, you can insert
a timing loop that executes after each point in the wave.
Listing 13-5 still relies on BASIC-52 to calculate the sine values and store them in RAM.
Although you can also write these parts in assembly language, doing so in BASIC is much
easier, and doesn’t affect the frequency of the sine wave that results. Even if you later decide
to write this part in assembly language, with BASIC-52 you can test each section of the code
as you go along.
When you have your assembly-language routine in the form you want it, you can use Listing
B-2 or an EPROM programmer to store the code in EPROM. If your EPROM has different
addressing than the RAM you used to test the code, you must change the ORG directive in
the source file to match the new location, and reassemble the file before you program it into
the EPROM.
230 The Microcontroller Idea Book