Hardware Reference
In-Depth Information
that 2 bytes need to be replaced in the instruction queue. Each optional fetch indicates that
only 1 byte needs to be replaced. For example, an instruction composed of 5 bytes does two pro-
gram fetches and one optional fetch. If the first byte of the 5-byte instruction was even aligned,
the optional fetch is converted into a free cycle. If the first byte was odd aligned, the optional
fetch is executed as a program fetch.
Two external pins, IPIPE[1:0], provide time-multiplexed information about instruction
execution and data movement in the queue. Decoding and using the IPIPE[1:0] signals are
discussed in Chapter 14.
The content of queue stage 1 advances to stage 2, stage 2 advances to stage 3, and stage 1 is
loaded with a word of program information from the data bus.
The invention of the microprocessor in 1968 resulted in a revolution in the electronics
industry. The first microprocessor, the Intel 4004, incorporated a simplified CPU into an in-
tegrated circuit. Following the introduction of the 4-bit 4004, Intel introduced the 8-bit 8008,
8080, and 8085 over three years. The 8085 was a big success because of its programmabil-
ity. Through this programmability, many products could be designed and constructed. Other
companies joined in the design and manufacturing of microprocessors, Zilog, Motorola, and
Rockwell being among the more successful.
The earliest microprocessors still needed peripheral chips to interface with I/O devices
such as seven-segment displays, printers, timers, and so on. Memory chips were also needed
to hold the application program and dynamic data. Because of this, the products designed with
microprocessors could not be made as small as desired. Then came the introduction of micro-
controllers, which incorporated the CPU, some amount of memory, and peripheral functions
such as parallel I/O ports, timer, and serial interface functions onto one chip. The development
of microcontrollers has had the following impacts:
•
I/O interfacing is greatly simplified.
•
External memory is no longer needed for many applications.
•
System design time is greatly shortened.
A microcontroller is not designed to build a desktop computer. Instead, it is used as the
controller of many products. End users of these products do not care what microcontrollers are
used in their appliances; they only care about the functionality of the product. A product that
uses a certain microcontroller as a controller and has this characteristic is called an
embedded
system
. Cell phones, automobiles, cable modems, HDTVs, and home security systems are well-
known embedded systems.
Over the last 20 years, we have clearly seen that a microcontroller needs to incorporate
some or all of the following peripheral functions in order to be useful:
•
Timer module that incorporates input capture, output compare, real-time interrupt,
and counting capability
•
Pulse-width modulation function for easy waveform generation
•
Analog-to-digital converter
•
Digital-to-analog converter
•
Temperature sensor
•
Direct memory access controller
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