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/* Copyright (C) 2004 The KOS Team
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
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,
#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;