Source to arch/i386/isa/timerreg.h
* Copyright (c) 1993 The Regents of the University of California.
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* timerreg.h,v 1.3 1993/06/15 21:37:19 mycroft Exp
* 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.
* The outputs of the three timers are connected as follows:
* timer 0 -> irq 0
* timer 1 -> dma chan 0 (for dram refresh)
* timer 2 -> speaker (via keyboard controller)
* Timer 0 is used to call hardclock.
* Timer 2 is used to generate console beeps.
* Frequency of all three count-down timers; (TIMER_FREQ/freq) is the
* appropriate count to generate a frequency of freq hz.
#define TIMER_FREQ 1193182
#define TIMER_DIV(x) ((TIMER_FREQ+(x)/2)/(x))
* Macros for specifying values to be written into a mode register.
#define TIMER_CNTR0 (IO_TIMER1 + 0) /* timer 0 counter port */
#define TIMER_CNTR1 (IO_TIMER1 + 1) /* timer 1 counter port */
#define TIMER_CNTR2 (IO_TIMER1 + 2) /* timer 2 counter port */
#define TIMER_MODE (IO_TIMER1 + 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 */