Functional Description, General, Oscillator

SCHOTTKY BIPOLAR 8224

FUNCTIONAL DESCRIPTION

General

The 8224 is a single chip Clock Generator/Driver for the S080A CPU. It contains a crystal-controlled oscillator, a "divide by nine" counter, two high-level drivers and several auxiliary logic functions.

Oscillator

The oscillator circuit derives its basic operating frequency from an external, series resonant, fundamental mode crystal. Two inputs are provided for the crystal connections (XTAL 1, XTAL2).

The selection of the external crystal frequency depends mainly on the speed at which the 8080A is to be run at. Basically, the oscillator operates at 9 times the desired pro- cessor speed.

A simple formula to guide the crystal selection is:

Crystal Frequency = -'-times 9 tCY

Example' : (500ns tCY)

2mHz times 9 = 18mHz*

Example 2: (800ns tCY)

'.25mHz times 9 = '1.25mHz

The waveforms generated by the decode gating follow a simple 2-5-2 digital pattern. See Figure 2. The clocks gen- erated; phase 1 and phase 2, can best be thought of as con- sisting of "units" based on the oscillator frequency. Assume that one "unit" equals the period of the oscillator frequency. By multiplying the number of "units" that are contained in a pulse width or delay, times the period of the oscillator fre- quency, the approximate time in nanoseconds can be derived.

The outputs of the clock generator are connected to two high level drivers for direct interface to the 80S0A CPU. A TTL level phase 2 is also brought out ct>2 (TTL) for external timing purposes. It is' especially useful in DMA dependant activities. This signal is used to gate the requesting device on- to the bus once the 8080A CPU issues the Hold Ack- nowledgement (H LOA).

Several other signals are also generated internally so that optimum timing of the auxiliary flip-flops and status strobe (STSTB) is achieved.

Another input to the oscillator is TANK. This input allows the use overtone mode crystals. Th is type of crystal gen- erally has much lower IIgain" than the fundamental type so an external LC network is necessary to provide the additional "gain" for proper oscillator operation. The external LC net- work is connected to the TANK input and is AC coupled to ground. See Figure 4.

The formula for the LC network is:

F= l __

21TVLC

The output of the oscillator is buffered and brought out on ElSC (pin 12) so that other system timing signals can be derived from this stable, crystal-controlled source.

*When using crystals above 10mHz a small amount of frequency "trimming" may be necessary to produce the exact desired fre- quency. The addition of a small selected capacitance (3pF - 10pF) in series with the crystal will accomplish this function.

Clock Generator

The Clock Generator consists of a synchronous IIdivide by nine" counter and the associated decode gating to create the waveforms of the two 8080A clocks and auxiliary timing signals.

1 UNIT = o~c.

FREQ.

4>,

EXAMPLE: (8080 t CY =500ns)

OSC = 18mHz/55ns 4>1 = 110ns (2 x 55ns) cP2 = 275ns (5 x 55ns) 4>2-4>,= 110ns (2 x 55ns)

5-2

8080 specifications

The Intel 8085 and 8080 microprocessors were groundbreaking innovations in the world of computing, paving the way for future microprocessor development and personal computing.

The Intel 8080, introduced in 1974, was an 8-bit microprocessor that played a fundamental role in the early days of personal computing. With a 16-bit address bus, it had the capability to address 64 KB of memory. Running at clock speeds of 2 MHz, the 8080 was notable for its instruction set, which included 78 instructions and 246 opcodes. It supported a range of addressing modes including direct, indirect, and register addressing. The 8080 was compatible with a variety of peripherals and played a crucial role in the development of many early computers.

The microprocessor's architecture was based on a simple and efficient design, making it accessible for hobbyists and engineers alike. It included an 8-bit accumulator, which allowed for data manipulation and storage during processing. Additionally, the 8080 featured registers like the program counter and stack pointer, which facilitated program flow control and data management. Its ability to handle interrupts also made it suitable for multitasking applications.

The Intel 8085, introduced in 1976, was an enhancement of the 8080 microprocessor. It maintained a similar architecture but included several key improvements. Notably, the 8085 had a built-in clock oscillator, simplifying system design by eliminating the need for external clock circuitry. It also featured a 5-bit control signal for status line management, which allowed for more flexible interfacing with peripheral devices. The 8085 was capable of running at speeds of up to 3 MHz and had an extended instruction set with 74 instructions.

One of the standout features of the 8085 was its support for 5 extra instructions for stack manipulation and I/O operations, which optimized the programming process. Additionally, it supported serial communication, making it suitable for interfacing with external devices. Its 16-bit address bus retained the 64 KB memory addressing capability of its predecessor.

Both the 8080 and 8085 microprocessors laid the groundwork for more advanced microprocessors in the years that followed. They demonstrated the potential of integrated circuits in computing and influenced the design and architecture of subsequent Intel microprocessors. Their legacy endures in the way they revolutionized computing, making technology accessible to a broader audience, and their influence is still felt in the design and architecture of modern microprocessors today.

Intel 8080 manual Functional Description, General, Oscillator, Clock Generator

Models: 8080