Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!zephyr.ens.tek.com!tektronix!sequent!norsk
From: norsk@sequent.UUCP (Doug Thompson)
Newsgroups: comp.sys.ibm.pc
Subject: .84 usec Timer resolution on PC
Message-ID: <25235@sequent.UUCP>
Date: 21 Nov 89 16:53:44 GMT
Reply-To: norsk@sequent.UUCP (Doug Thompson)
Distribution: usa
Organization: Sequent Computer Systems, Inc.
Lines: 65



PC Timing Resolution of .84 microseconds are possible on a PC machine.

In the April 1984 issue of the late PC TECH JOURNAL, there was an article
titled "Life in the Fast Lane", by Bob Smith and Tom Puckett, that 
discribe the mechanism for obtaining very good timing resolution from the
PC 8253-5 Programmable Interval Timer (PIT).

The clock interrupt is generated by Timer 0 of the PIT. The input to the
timer is 1.193182 Mhz with a period of 0.838095 microseconds or .84 us.
By altering the programming mode of Timer 0 of the PIT from Mode 3 (which is
a square wave output where the count down counter decrements by 2 during
the first half of the cycle, then again during the last half)  to Mode 2
(which has the output normally high, but when the count down reaches 0,
the output goes low for 1 timer-clock cycle then goes high again.)

In this mode, a query via a Timer command of the current count down register
value is possible. The command causes the PIT to latch the current value
and make it available for query. Then this value is the count of timer-clock
cycles that have occurred since the last Timer 0 interrupt. Coupled with
the interrupt count in the BIOS data area, one can measure very in fine
timing the length of short events.


To program Timer 0 to Mode 2 do the following:

	mov	al, 34h		; Timer 0 - Mode 2
	out	43h, al
	xor	al, al		; Get ZERO to give full count of 65536
	jmp 	$+2
	out	40h, al		; load low order byte
	jmp	$+2
	out	40h, al		; hi order byte

To get a current timer value dot he following:

	...
	mov	ax, 40h		; BIOS data segment
	mov	es, ax
	mov	al, 00h		; Latch Timer 0 counter cmd
	cli			; critical section hold off ints
	out	43h, al		; issue latch cmd
	mov	bx, es:[6ch]	; get BIOS timer low word
	mov	cx, es:[6eh]	; get BIOS timer hi word
	in	al, 40h		; fetch latched counter
	mov	ah, al
	jmp	$+1
	in	al, 40h		; fetch hi order latched counter
	sti
	xchg	ah, al		; get in proper byte order
	neg	ax		; get up count from down count
	xchg	ax, cx		; return in common register order

	AX has MSWord of timer interrupt count
	BX has LSWord of timer interrupt count
	CX has count of timer-clock cycles since last timer interrupt (.84us)


	
-- 
Douglas Thompson		UUCP: ..{tektronix,ogcvax,uunet}!sequent!norsk
Sequent Computer Systems	Phone: (503) 526-5727
15450 SW Koll Parkway	!"The scientist builds to learn;the engineer learns in
Beaverton OR 97006	!order to build."  Fred Brooks