The document discusses the Intel 8254 programmable interval timer chip. It consists of three independent 16-bit counters that can count in binary or BCD. Each counter has a CLK input, gate input, and OUT output. The 8254 interfaces with the CPU via data, read, write, chip select, and address lines. It is commonly found in PCs and is decoded at ports 40H-43H, with timer 0 generating an 18.2 Hz interrupt, timer 1 refreshing DRAM, and timer 2 generating audio tones. The 8254 has 3 ports that can be configured by software for inputs/outputs and divided into smaller ports.

1. System Connection Of 8254
&
Its Port Selection

2. Supervised to
Mohd. Manjur Alam
Lecturer , Computer Science & Engineering
Bgc Trust University Bangladesh , Chittagong.
Presented by
Md Irfanul Alam
Department of Computer science & engineering
Roll:1728042
Bgc Trust University Bangladesh , Chittagong.

3.  About 8254
 Functional Description
 System Connection Diagram & Description
 Port Selection
 Reference
Contents

4. The 8254 programmable interval timer consists of three independent 16-bit programmable
counters (timers). Each counter is capable of counting in binary or binary-coded decimal (BCD).
The maximum allowable input frequency to any counter is 10 MHz. This device is useful
wherever the microprocessor must control real-time events. Some examples of usage include
real-time clock and an events counter, and for motor speed and direction control.
This timer also appears in the personal computer decoded at ports 40H–43H to do the
following:
 Generate a basic timer interrupt that occurs at approximately 18.2 Hz.
 Cause the DRAM memory system to be refreshed.
 Provide a timing source to the internal speaker and other devices. The timer in the personal
computer is an 8253 instead of an 8254.
About 8254

5. The pin-out of the 8254, which is a higher-speed version of the 8253, and a diagram of one of
the three counters. Each timer contains a CLK input, a gate input, and an output (OUT)
connection. The CLK input provides the basic operating frequency to the timer, the gate pin
controls the timer in some modes, and the OUT pin is where we obtain the output of the timer.
The signals that connect to the microprocessor are the data bus pins (D7–D0), RD , WR, CS and
address inputs A1 and A0. The address inputs are present to select any of the four internal
registers used for programming, reading, or writing to a counter. The personal computer
contains an 8253 timer or its equivalent, decoded at I/O ports 40H–43H. Timer zero is
programmed to generate an 18.2 Hz signal that interrupts the microprocessor at interrupt
vector 8 for a clock tick. The tick is often used to time programs and events in DOS. Timer 1 is
programmed for 15 μs, which is used on the personal computer to request a DMA action used
to refresh the dynamic RAM. Timer 2 is programmed to generate a tone on the personal
computer speaker.
Functional Description

6. The 8254 is a component of the Intel
Microcomputer Systems and interfaces in the
same manner as all other peripherals of the
family. It is treated by the system’s software as an
array of peripheral I/O ports. three are counters
and the fourth is a control register for MODE
programming.
Basically, the select inputs A0,A1 connect to the
A0,A1 address bus signals of the CPU. The CS can
be derived directly from the address bus using a
linear select method. Or it can be connected to
the output of a decoder, such as an Intel 8205 for
larger systems.
System Connection Diagram &
Description
Control Word Format : A1,A0=11 , CS=0 , RD=1 , WR=0.
Figure : 8254 System Connection Diagram

7. Figure : 8254 Port Selection
The 8254 contains three 8-bit ports (A , B, and C). All can be
configured in a wide variety of functional characteristics by the
system software but each has its own special features or
personally to further enhance the power and flexibility of the
8255A.
Port A. One 8 bit data output latch/buffer and one 8-bit data
input latch.
Port B. One 8-bit data output latch/buffer and one 8-bit data
input buffer.
Port C. One 8-bit data output latch/buffer and one 8-bit data
input buffer (no latch for input). This port can be divided into two
4-bit ports under the mode control. Each 4-bit port contains a 4-
bit latch and it can be used for the controls signal outputs and
status signal inputs in conjunction with ports A and B.
Port Selection

8.  https://www.wikipedia.org
https://books.google.com.bd
Reference

9. Thank
You