Intel 8253 - Programmable Interval Timer

The Intel 8253 and 8254 are Programmable Interval Timers (PTIs) designed for microprocessors to perform timing and counting functions using three 16-bit registers. Each counter has 2 input pins, i.e. Clock & Gate, and 1 pin for “OUT” output. To operate a counter, a 16-bit count is loaded in its register. On command, it begins to decrement the count until it reaches 0, then it generates a pulse that can be used to interrupt the CPU.

Difference between 8253 and 8254

The following table differentiates the features of 8253 and 8254 −

8253	8254
Its operating frequency is 0 - 2.6 MHz	Its operating frequency is 0 - 10 MHz
It uses N-MOS technology	It uses H-MOS technology
Read-Back command is not available	Read-Back command is available
Reads and writes of the same counter cannot be interleaved.	Reads and writes of the same counter can be interleaved.

Features of 8253 / 54

The most prominent features of 8253/54 are as follows −

It has three independent 16-bit down counters.
It can handle inputs from DC to 10 MHz.
These three counters can be programmed for either binary or BCD count.
It is compatible with almost all microprocessors.
8254 has a powerful command called READ BACK command, which allows the user to check the count value, the programmed mode, the current mode, and the current status of the counter.

8254 Architecture

The architecture of 8254 looks as follows −

8254 Pin Description

Here is the pin diagram of 8254 −

In the above figure, there are three counters, a data bus buffer, Read/Write control logic, and a control register. Each counter has two input signals - CLOCK & GATE, and one output signal - OUT.

Data Bus Buffer

It is a tri-state, bi-directional, 8-bit buffer, which is used to interface the 8253/54 to the system data bus. It has three basic functions −

Programming the modes of 8253/54.
Loading the count registers.
Reading the count values.

Read/Write Logic

It includes 5 signals, i.e. RD, WR, CS, and the address lines A₀ & A₁. In the peripheral I/O mode, the RD and WR signals are connected to IOR and IOW, respectively. In the memorymapped I/O mode, these are connected to MEMR and MEMW.

Address lines A₀ & A₁ of the CPU are connected to lines A₀ and A₁ of the 8253/54, and CS is tied to a decoded address. The control word register and counters are selected according to the signals on lines A₀ & A₁.

A₁	A₀	Result
0	0	Counter 0
0	1	Counter 1
1	0	Counter 2
1	1	Control Word Register
X	X	No Selection

Control Word Register

This register is accessed when lines A₀ & A₁ are at logic 1. It is used to write a command word, which specifies the counter to be used, its mode, and either a read or write operation. Following table shows the result for various control inputs.

A₁	A₀	RD	WR	CS	Result
0	0	1	0	0	Write Counter 0
0	1	1	0	0	Write Counter 1
1	0	1	0	0	Write Counter 2
1	1	1	0	0	Write Control Word
0	0	0	1	0	Read Counter 0
0	1	0	1	0	Read Counter 1
1	0	0	1	0	Read Counter 2
1	1	0	1	0	No operation
X	X	1	1	0	No operation
X	X	X	X	1	No operation

Counters

Each counter consists of a single, 16 bit-down counter, which can be operated in either binary or BCD. Its input and output is configured by the selection of modes stored in the control word register. The programmer can read the contents of any of the three counters without disturbing the actual count in process.