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