Nuts and Volts - July/August 2018

Vintage Computing

PRINT ASC “A”
65
PRINT ASC “<backspace>”
 200
 PRINT ASC “<F6>”
 213
 PRINT ASC “<left arrow>”
 192

how it works, but my comments just made things more confusing. Here’s what’s going on, line by line: • Line 10 sets the Propeller I/O pin we’ll be using to 24, which is the left channel of the Amigo audio section.

• Line 15 sets the direction of our audio pin to output, but for now masks the output with a 1. In the Propeller chip, a cog counter output bit is OR’d with the corresponding bit of the output register, and AND’d with the corresponding bit of the direction register.

So, OUTA[p]=1 sets both output and direction registers to 1, creating a masked output on the pin. We’ll use OUTA[p]=0 to unmask the counter output pin in Line The FILE command can be used on either side of the “equals sign” assignment operator to read or write the next byte of the currently open file. The code that follows (available to download) creates the file TEST.DAT on the SD card and then uses FILE to write the alphabet to it. It then reopens the file and uses FILE to read and print bytes until it finds the end of file.

30.

• Line 20 sets the value-to-add register to 24365, which for the Amigo is the FRQx for 130.8 Hz (one octave below middle C). From the Parallax application note (you really should take a look at it!), the NCO frequency is given by: Note that Color BASIC adds a -1 as the end-of-file character on file closure, and that on opening, FILE points to the first record of the file.

fhz = FRQx / 232 x System Frequency for values of FRQx between 0 and $8000_0000. For the Amigo, with its 80 MHz clock, this arithmetic simplifies to FRQx = 186.26 fhz. So, middle C (261.6 Hz) is FRQx =

48730; A4 (440 Hz) is FRQx = 81954; and so forth.

• Line 25 left shifts the value for Single-Ended NCO mode (%00100) into the C TRMODE field of C TRA and then adds 24 into the APIN field. The PLLDIV and BPIN fields are not used in this counter mode of operation.

At this point, the counter is now operating, but the output is being masked by the 1 we placed in the output register in Line 15.

5 REM —- FILE.BAS —-
10 CLS
15 OPEN “test.dat”,W
20 PRINT “Writing alphabet to TEST.DAT...”
25 FOR n=65 TO 90
30 FILE = n
35 NEXT n
40 PRINT “Closing file...”
45 CLOSE
50 DISPLAY 13,13: PRINT “Press a key to
continue...”
55 IF INKEY =0 THEN GOTO 55
60 DISPLAY 13,13: PRINT “Reading data from
TEST.DAT...”
65 OPEN “test.dat”,R

• Line 30 places a 0 in the output register for pin 24, unmasking the APIN output from the counter and applying it to the Amigo left audio channel.

Command Action Performed ASC “n” Returns the ASCII value of the single character enclosed in quotes.

• Line 35 lets the counter run, applying a steady tone to the audio channel until you press a key.

Reads or writes the next byte in the currently open file on the SD card.

• Lines 40-55 adjust FRQA to slide the tone up an octave, then hold it there for one second.

FILE VARIABLE = FILE reads a byte from a file open in the Read mode. FILE =

• Line 60 turns the counter off.

<expression> writes a byte to a file in the Write or Append mode.

With our introduction of the counter commands complete, let’s finish up the remaining Color BASIC commands that were not covered in the initial Mentor’s

WORD [nnnnn]

Friend documentation. These are summarized in Figure 8, and we’ll briefly cover each with an example.

Like the BYTE command, but with a 16-bit value. Reads or writes a 16-bit value to the Amigo main memory address specified, where nnnnn is between 0 and 32767. The command can be used on either side of the “equals sign” assignment operator, and the least significant bit of the address is ignored.

The ASC command is a convenient way to find the ASCII code of a character or key on the keyboard. To demonstrate this, type in the commands below, entering