/* parda.c: A very simple linux user space program for the simple DA-converter
            as a function generator via the Parallel Port and 8 330 Ohm resitors
            between every data pin and the output (= 3 1/2 -bit DAC, short-circuit-proof).

Parameter: I/O-Address (usually 888 for standard parallel port),
           Byte of the data register (pin pattern on output),
           duration of a high/low period in µs.
           Example: ./parda 888 255 12345.

The bit patter of the output determins the amplitude: 8 bits (255) means 100 %; 0 means
0 %, 4 bits (e. g. 15) means 50 % etc.:
0xff           100 %
0x7f, 0xf7, ... 87,5 %
0x3f, 0xf3, ... 75 %
0x1f, 0xf1, ... 62,5 %
0x0f, 0xf0, ... 50 %
0x07, 0x70, ... 37,5 %
0x03, 0x30, ... 25 %
0x01, 0x10, ... 12,5 %
0x00             0 %

Measured frequency with Linux version 2.6.8-24.11-smp (SuSE 9.2) and the onboard port:
delay     frequency/Hz (onboard port)
10000000  0.05
1000000   0.4988
248000    2.00
100000    4.9
10000     41,6
1000      166
1 ... 100 249,9
0         499,9
0 without udelay approx. 200 kHz

The same was measured with a parallel port PCI card.

For higher frequencies you should use a kernel module which should have a limit of
approx. 500 kHz instead of 250 Hz.
For a really constant frequency (no jitter) you need RTAI.

Todo: Check of the parameters + help, a function for triangle wave output with the
output sequence 0 <delay> 1 <delay> 3 <delay> 7 <delay> 15 <delay> 31 <delay> 63
<delay> 127 <delay> 255 <delay> 254 <delay> 252 <delay> 248 <delay> 240 <delay> 224
<delay> 192 <delay> 128 <delay> 0 ..., a function for sine wave output, ...,
get address from /proc/bus/pci from first device with vendor id 0x9710 and device id 0x9805.

Rolf Freitag Feb. 2005, nobodyo@web.de

 */

#include <stdio.h>
#include <sys/io.h>                               // or asm/io.h; for inb, ioperm, iopl  ...
#include <stdlib.h>                               // atoi
#include <unistd.h>                               // getuid(), usleep
#include <errno.h>                                // EPERM
#include <iso646.h>                               // bitand etc.

int
main (int argc, char *argv[])                     // Caution: A wrong parameter value can cause serious damage!
{
  int base;                                       // base address of the parallel port
  int value;                                      // bit pattern for the data pins (0 = off, 255 = all high)
  int delay;                                      // duration of a pulse/leap in µs
  int i_tmp=0;

  if (4 != argc)
  {
    printf ("Usage: %s <parallel port base> <byte for output via the data port> <duration of a pulse/leap in µs>\n", argv[0]);
    exit (-1);
  }
  base = strtol (argv[1],  (char **)NULL, 0);     // base address of the parallel port
  value = strtol (argv[2],  (char **)NULL, 0);    // bit pattern for the data pins (0 = off, 255 = all high)
  delay = strtol (argv[3],  (char **)NULL, 0);    // duration of a pulse/leap in µs
  if (geteuid () != 0)
  {
    printf ("\a\n\nError: $EUID==%d!=0 (you are not a superuser, port access would be denied from the kernel), exiting.\n\n", getuid ());
    exit (-EPERM);
  }
  iopl (3);                                       // unlimited I/O access permission, nessesary above the 0x3ff limit e. g. at 0x9800=38912

  i_tmp = inb(base +2);                           // cache the controll register
//  outb (0x0b, base +2);                          // write mode
  i_tmp or_eq 0x20;                                // set bit for write mode
  outb (i_tmp, base +2);              // write mode
  if (delay > 0)
    for (;;)                                      // endless loop with < 1 % CPU load
  {
    outb (value, base);
    usleep (delay);
    outb (0, base);
    usleep (delay);
  }
  else
    for (;;)                                      // 99 % CPU load
  {
    outb (value, base);
    outb (0, base);
  }
  iopl (0);                                       // no I/O permission
  exit (0);
};