Tag Archives: 8253/8254 PIT

8253/8254 PIT – Programmable Interval Time

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Original source: http://heim.ifi.uio.no/~stanisls/helppc/8253.html

Spec documentation:  8254 PROGRAMMABLE INTERVAL TIMER

Port 40h, 8253 Counter 0 Time of Day Clock (normally mode 3)
Port 41h, 8253 Counter 1 RAM Refresh Counter (normally mode 2)
Port 42h, 8253 Counter 2 Cassette and Speaker Functions
Port 43h, 8253 Mode Control Register, data format:
	|7|6|5|4|3|2|1|0|  Mode Control Register
	 | | | | | | | `---- 0=16 binary counter, 1=4 decade BCD counter
	 | | | | `--------- counter mode bits
	 | | `------------ read/write/latch format bits
	 `--------------- counter select bits (also 8254 read back command)
Bits
76 Counter Select Bits
00 select counter 0
01 select counter 1
10 select counter 2
11 read back command (8254 only, illegal on 8253, see below)
Bits
54 Read/Write/Latch Format Bits
00 latch present counter value
01 read/write of MSB only
10 read/write of LSB only
11 read/write LSB, followed by write of MSB
Bits
321 Counter Mode Bits
000 mode 0, interrupt on terminal count; countdown, interrupt,

then wait for a new mode or count; loading a new count in the
middle of a count stops the countdown

001 mode 1, programmable one-shot; countdown with optional

restart; reloading the counter will not affect the countdown
until after the following trigger

010 mode 2, rate generator; generate one pulse after ‘count’ CLK

cycles; output remains high until after the new countdown has
begun; reloading the count mid-period does not take affect
until after the period

011 mode 3, square wave rate generator; generate one pulse after

‘count’ CLK cycles; output remains high until 1/2 of the next
countdown; it does this by decrementing by 2 until zero, at
which time it lowers the output signal, reloads the counter
and counts down again until interrupting at 0; reloading the
count mid-period does not take affect until after the period

100 mode 4, software triggered strobe; countdown with output high

until counter zero; at zero output goes low for one CLK
period; countdown is triggered by loading counter; reloading
counter takes effect on next CLK pulse

101 mode 5, hardware triggered strobe; countdown after triggering

with output high until counter zero; at zero output goes low
for one CLK period
Read Back Command Format (8254 only)
	|7|6|5|4|3|2|1|0| Read Back Command (written to Mode Control Reg)
	 | | | | | | | `--- must be zero
	 | | | | | | `---- select counter 0
	 | | | | | `----- select counter 1
	 | | | | `------ select counter 2
	 | | | `------- 0 = latch status of selected counters
	 | | `-------- 0 = latch count of selected counters
	 `----------- 11 = read back command
Read Back Command Status (8254 only, read from counter register)
	|7|6|5|4|3|2|1|0|  Read Back Command Status
	 | | | | | | | `--- 0=16 binary counter, 1=4 decade BCD counter
	 | | | | `-------- counter mode bits (see Mode Control Reg above)
	 | | `----------- read/write/latch format (see Mode Control Reg)
	 | `------------ 1=null count (no count set), 0=count available
	 `------------- state of OUT pin (1=high, 0=low)
  • the 8253 is used on the PC & XT, while the 8254 is used on the AT+
  • all counters are decrementing and fully independent
  • the PIT is tied to 3 clock lines all generating 1.19318 MHz.
  • the value of 1.19318MHz is derived from (4.77/4 MHz) and has it’s roots based on NTSC frequencies
  • counters are 16 bit quantities which are decremented and then tested against zero. Valid range is (0-65535). To get a value
    of 65536 clocks you must specify 0 as the default count since
    65536 is a 17 bit value.
  • reading by latching the count doesn’t disturb the countdown but reading the port directly does; except when using the 8254 Read
    Back Command
  • counter 0 is the time of day interrupt and is generated approximately 18.2 times per sec. The value 18.2 is derived from
    the frequency 1.10318/65536 (the normal default count).
  • counter 1 is normally set to 18 (dec.) and signals the 8237 to do a RAM refresh approximately every 15æs
  • counter 2 is normally used to generate tones from the speaker but can be used as a regular counter when used in conjunction
    with the 8255
  • newly loaded counters don’t take effect until after a an output pulse or input CLK cycle depending on the mode
  • the 8253 has a max input clock rate of 2.6MHz, the 8254 has max input clock rate of 10MHz

Programming considerations:

1. load Mode Control Register
2. let bus settle (jmp $+2)
3. write counter value
4. if counter 0 is modified, an INT 8 handler must be written to

call the original INT 8 handler every 18.2 seconds. When it
does call the original INT 8 handler it must NOT send and EOI
to the 8259 for the timer interrupt, since the original INT 8
handler will send the EOI also.

Example code:

	countdown  equ	8000h ; approx 36 interrupts per second
	   cli
	   mov	al,00110110b  ; bit 7,6 = (00) timer counter 0
				  ; bit 5,4 = (11) write LSB then MSB
				  ; bit 3-1 = (011) generate square wave
				  ; bit 0 = (0) binary counter
	   out	43h,al	      ; prep PIT, counter 0, square wave&init count
	   jmp	$+2
	   mov	cx,countdown  ; default is 0x0000 (65536) (18.2 per sec)
				  ; interrupts when counter decrements to 0
	   mov	al,cl	      ; send LSB of timer count
	   out	40h,al
	   jmp	$+2
	   mov	al,ch	      ; send MSB of timer count
	   out	40h,al
	   jmp	$+2