The document discusses interfacing with timers and counters using the Intel 8253/8254 Programmable Interval Timer chip. It provides information on the timer modes, programming the chip, and examples of using counters 0 and 1 to generate pulses and square waves. Specifically, it shows: 1) The 8254 timer chip has three 16-bit counters that can be programmed to operate in six different modes to generate time delays, pulses, and digital waveforms. 2) Examples are given to initialize counter 2 to count down from 50,000 in mode 0 and generate a 50us pulse from counter 0 in mode 2. 3) A third example shows how to program counter 1 to generate a 1kHz

1. Interfacing Timer
Topics to be covered
• Timer IC 8253/54
Programmable Interval
Timer (PIT)
• Serial I/O 8251
• Parallel I/O 8255
• A/D Converters
• D/A Converter
• Arithmetic Coprocessors
• System level interfacing
design
Definitions
• The programmable interval timer
generates accurate time delays
and can be used for applications
such as RTC, event counter, digital
one shot, square-wave generator
and complex waveform generator
• 8254 is upgraded version of 8253
and they are pin compatible
• It has three identical 16-bit
counters that can operate
independently in any one of the
six modes
1

2. Block Diagram of 8254
2

3. • Selection of CWR
and Counters:
• Control Word
Format
3
A1 A0 Selection
0 0 Counter 0
0 1 Counter 1
1 0 Counter 2
1 1 Control Register
D7 D6 D5 D4 D3 D2 D1 D0
SC1 SC0 RW1 RW0 M2 M1 M0 BCD
SC1 SC0 Selection
0 0 Counter 0
0 1 Counter 1
1 0 Counter 2
1 1 Read-Back
Command
RW1 RW0 Action
0 0 Counter latch command
0 1 R/W LSB only
1 0 R/W MSB only
1 1 R/W LSB first, then MSB

4. M2 M1 M0 Mode
0 0 0 Mode 0
0 0 1 Mode 1
X 1 0 Mode 2
X 1 1 Mode 3
1 0 0 Mode 4
1 0 1 Mode 5
4
0 Binary Counter 16-Bits
1 BCD Counter
MODE: BCD:

5. Modes
MODE 0:
INTERRUPT
ON TC
Initially OUT is low. Once a count is loaded, the
counter is decremented every cycle and when the
count reaches zero, out goes high which can be
used as interrupt
MODE1:
HW-
RETRIGG-
ERABLE
ONE-SHOT
OUT is initially high. When the GATE is triggered,
the out goes low and at the end of the count, the
out goes high again, thus generating a one-shot
pulse
MODE 2:
RATE
GENERATO
R
This mode is used to generate a pulse equal to the
clock period at a given interval. When a count is
loaded, the out stays high until the count reaches 1,
and then the out low for one clock period. The
count is reloaded automatically and the pulse is
generated continuously 5

6. 6
MODE 3: SQUARE-
WAVE
GENERATOR
When a count is loaded, OUT is high. The count is
decremented by two at every clock cycle, and
when it reached zero, the OUT goes low and the
count is reloaded again. This is repeated
continuously, thus a cont. square wave is
generated
MODE 4:
SOFTWARE-
TRIGGERED
STROBE
OUT is initially high; it goes low for one clock
period at the end of the count. The count must be
reloaded for subsequent outputs.
MODE 5:
HARDWARE-
TRIGGERED
STROBE
Similar to Mode 4 except that it is triggered by the
rising pulse at the gate. Initially OUT is low and
when the gate pulse is triggered from low to high,
the count begins. At the end of the count, the OUT
goes low for one clock period

7. Programming the 8254
Write Operation
• Write a CW into the CR
• Load the low-order byte of a count in the counter
register
• Load the high-order byte of a count in the
counter register
With a clock and appropriate gate signal the above
steps should start the counter
Read Operation
• Reading after stopping counter
• Reading counter on fly
7

8. Programming of 8254
As a Counter: Write a subroutine to Initialize counter 2 in mode 0 with a count
50,000d. Read the count on fly. When the count reaches 0 it should return to
main program.
• Control Word: 10110000 = B0H
• Subroutine:
MVI A, B0H; CW to initialize counter 2
OUT 83H; write in CR
MVI A, LOBYTE; LO Byte of 50,000
OUT 82H; Load Counter 2 with LO Byte
MVI A, HIBYTE; HO Byte of 50,000
OUT 82H; Load Counter 2 with HO Byte
READ: MVI A, 80H; CW to Latch a count
OUT 83H; Write in CR
IN 82H; Read LO Byte
MOV D,A; Save LO Byte in D
IN 82H; Read HO Byte
ORA D; OR LO and HO Bytes to set Z flag
JNZ READ
RET
8

9. Write instructios to generate a pulse every 50us
from counter0.
1. Mode : Mode-2
2. CW:
3. Count = 50 x 10E-6/0.5 x 10E-6 = 64H
(pulse width)/(clock duration)
9
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 1 0 1 0 0
Counter0 Load 8-bit
count
Mode-2 Binary
count

10. 4. Program
PULSE: MVI A, 14H; control word
OUT 83H; write in control register
MVI A, 64H; LO Byte of the count
OUT 80H; load Counter0 with LO Byte
HALT
10

11. Square-Wave Generator
Write instructions to generate a 1kHz square-
wave from Counter1.
1. CW:
2. Count: 10E-3/0.5E-6 = 2000 = 07D0H
11
D7 D6 D5 D4 D3 D2 D1 D0
0 1 1 1 0 1 1 0
Select
counter1
Load 16-bit
count
Mode-3 Binary
count

12. Program:
SQWAVE: MVI A, 76H; control word
OUT 83H; write in control register
MVI A, D0H; LO byte of the count
OUT 81H; load counter 1 with LO byte
MVI A, 07H; HO byte of the count
OUT 81H; load counter1 with LO byte
HALT
12