/* Copyright (C) 2004  The KOS Team   Copyright (C) 1999  Free Software Foundation, Inc.   This program is free software; you can redistribute it and/or   modify it under the terms of the GNU General Public License   as published by the Free Software Foundation; either version 2   of the License, or (at your option) any later version.      This program is distributed in the hope that it will be useful,   but WITHOUT ANY WARRANTY; without even the implied warranty of   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the   GNU General Public License for more details.      You should have received a copy of the GNU General Public License   along with this program; if not, write to the Free Software   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,   USA. */#include <hwcore/ioports.h>#include "i8254.h"/** 82c54 clock frequency */#define I8254_MAX_FREQ 1193180/* Ports to communicate with the 82c54 */#define I8254_TIMER0  0x40#define I8254_TIMER1  0x41#define I8254_TIMER2  0x42#define I8254_CONTROL 0x43/** * Configure the first timer of the 82c54 chip as a rate generator, * which will raise an IRQ0 on a regular periodic basis, as given by * the freq parameter. Second (RAM refresh) and third (speaker) timers * are left unchanged. Maximum frequency is that of the 8254 clock, ie * 1193180 Hz. * * Ahhh PC systems are nice toys: this maximum "strange" frequency * equals that of the NTSC clock (14.31818 MHz) divided by 12. In * turn, the famous 4.77 MHz cpu clock frequency of the first IBM PC * is this same NTSC frequency divided by 3. Why the NTSC frequency as * a base "standard" ? Because the 14.31818 MHz quartz were cheap at * that time, and because it allows to simply drive altogether the * cpu, the "time of day" timer, and the video signal generators. */sos_ret_t sos_i8254_set_frequency(unsigned int freq){  unsigned int nb_tick;  if (freq <= 0)    return -SOS_EINVAL;  /* Compute counter value */  nb_tick = I8254_MAX_FREQ / freq;  /* Counter must be between 1 and 65536 */  if (nb_tick > 65536)    return -SOS_EINVAL;  if (nb_tick <= 0)    return -SOS_EINVAL;  /* The i8254 interprets 0 to mean counter == 65536, because 65536     cannot be coded on 16bits */  if (nb_tick == 65536)    nb_tick = 0;    /* We want to configure timer0, we want to send both LSB+MSB to set     timer0 freq (-> 0x30), and we configure timer0 in mode 2, ie as a     rate generator (-> 0x4) ==> 0x34 */  outb(0x34, I8254_CONTROL);  /* Send LSB of counter first */  outb((nb_tick & 0xFF), I8254_TIMER0);  /* Send MSB of counter */  outb((nb_tick >> 8) & 0xFF, I8254_TIMER0);  return SOS_OK;}