FreeBSD/Linux Kernel Cross Reference
sys/dev/ic/i8253reg.h

     /*-
      * SPDX-License-Identifier: BSD-3-Clause
      *
      * Copyright (c) 1993 The Regents of the University of California.
      * All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      from: Header: timerreg.h,v 1.2 93/02/28 15:08:58 mccanne Exp
     * $FreeBSD: releng/12.0/sys/dev/ic/i8253reg.h 326022 2017-11-20 19:36:21Z pfg $
     */
    
    /*
     * Register definitions for the Intel 8253 Programmable Interval Timer.
     *
     * This chip has three independent 16-bit down counters that can be
     * read on the fly.  There are three mode registers and three countdown
     * registers.  The countdown registers are addressed directly, via the
     * first three I/O ports.  The three mode registers are accessed via
     * the fourth I/O port, with two bits in the mode byte indicating the
     * register.  (Why are hardware interfaces always so braindead?).
     *
     * To write a value into the countdown register, the mode register
     * is first programmed with a command indicating the which byte of
     * the two byte register is to be modified.  The three possibilities
     * are load msb (TMR_MR_MSB), load lsb (TMR_MR_LSB), or load lsb then
     * msb (TMR_MR_BOTH).
     *
     * To read the current value ("on the fly") from the countdown register,
     * you write a "latch" command into the mode register, then read the stable
     * value from the corresponding I/O port.  For example, you write
     * TMR_MR_LATCH into the corresponding mode register.  Presumably,
     * after doing this, a write operation to the I/O port would result
     * in undefined behavior (but hopefully not fry the chip).
     * Reading in this manner has no side effects.
     */
    
    /*
     * Macros for specifying values to be written into a mode register.
     */
    #define TIMER_REG_CNTR0 0       /* timer 0 counter port */
    #define TIMER_REG_CNTR1 1       /* timer 1 counter port */
    #define TIMER_REG_CNTR2 2       /* timer 2 counter port */
    #define TIMER_REG_MODE  3       /* timer mode port */
    #define         TIMER_SEL0      0x00    /* select counter 0 */
    #define         TIMER_SEL1      0x40    /* select counter 1 */
    #define         TIMER_SEL2      0x80    /* select counter 2 */
    #define         TIMER_INTTC     0x00    /* mode 0, intr on terminal cnt */
    #define         TIMER_ONESHOT   0x02    /* mode 1, one shot */
    #define         TIMER_RATEGEN   0x04    /* mode 2, rate generator */
    #define         TIMER_SQWAVE    0x06    /* mode 3, square wave */
    #define         TIMER_SWSTROBE  0x08    /* mode 4, s/w triggered strobe */
    #define         TIMER_HWSTROBE  0x0a    /* mode 5, h/w triggered strobe */
    #define         TIMER_LATCH     0x00    /* latch counter for reading */
    #define         TIMER_LSB       0x10    /* r/w counter LSB */
    #define         TIMER_MSB       0x20    /* r/w counter MSB */
    #define         TIMER_16BIT     0x30    /* r/w counter 16 bits, LSB first */
    #define         TIMER_BCD       0x01    /* count in BCD */

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