xv6/8253pit.c

#include "types.h"
#include "x86.h"
#include "defs.h"
#include "traps.h"

// Register definitions for the Intel
// 8253/8254/82C54 Programmable Interval Timer (PIT).

#define	IO_TIMER1	0x040		/* 8253 Timer #1 */
#define	IO_TIMER2	0x048		/* 8253 Timer #2 (EISA only) */

//
// 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))

#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 */

void
pit8253_timerinit(void)
{
  // initialize 8253 clock to interrupt 100 times/sec
  outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
  outb(IO_TIMER1, TIMER_DIV(100) % 256);
  outb(IO_TIMER1, TIMER_DIV(100) / 256);
  irq_enable(IRQ_TIMER);
}