This document discusses the 8253/54 programmable interval timer. It begins with an acknowledgment and then provides information about the 8253/54 timer, including that the 8254 replaced the 8253 in IBM PCs starting with the AT model. It describes the pinout and functions of the timers, noting they have 3 independent counters that are programmed separately. The document also explains the control word structure and the various operational modes of the timers.

1. EC 304 Microprocessors and
Microcontrollers Lab
Topic – 8253/54 programmable interval timer
Submitted By:
Name: Aayush Dhamani
Roll No. : BTECH/25056/20
Branch: Electronics and Communication
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2. Table of contents
 8253/83 timer
 Pin diagram
 8254 functional description
 8254 programming
 Control word
 Operational modes
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3. Acknowledgement
I would like to express my special thanks of gratitude to
our Professor Mr. Snehanshu Shekhar who gave me the
golden opportunity to do this wonderful project on the
topic 8085 micro processor training kit which also helped
us in understanding the topic and we came to know about
so many new things. I am really thankful to him.
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4. PTI (programmable Interval Timer)
 The 8253 chip was used in the IBM PC/XT, but starting
with the IBM PC/AT, the 8254 replaced the 8253.
 8253 and 8254 have exactly the same pinout.
 8254 is a superset of the 8253.
8253/54 Timer
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5. Programmable Interval Timer - 8254
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8. A0, A1, and CS
 Inside the 8253/54 timer, there
are 3 counters.
 Each timer works
independently and
programmed separately.
 Each counter is assigned an
individual port address.
 The control register common
to all 3 counters and has its
own port.
Pin Description of 8253/54
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12. The figure shows the one-byte control word of the 8253/54.
 D0 chooses between a binary number divisor of 0000 to
FFFFH or a BCD divisor of 0000 to 9999H.
 The highest number is 216 for binary and 104 for BCD.
 To get the highest count, the counter is loaded with zeros.
 D1, D2, and D3 are for mode selection.
Control word
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13. • D4 and D5 are for RL0 and RL1
• RL0 and RL1 are used to indicate the size of the divisor, and have
3 options:
1. Read/write MSB only
2. Read/write LSB only
3. Read/write LSB first followed immediately by the MSB.
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14.  Mode 0 Interrupt on terminal count
 Mode 1 Programmable one-shot
 Mode 2 Rate Generator
 Mode 3 Square wave rate generator
 Mode 4 Software triggered strobe
 Mode 5 Hardware trigger strobe
Operation Modes
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