General Components of Embedded Systems

Introduction

You are now almost familiar with the various components of an embedded system. In this chapter we shall discuss some of the general components such as

• Processors
• Memory
• Input/Out Devices


Processors

The central processing unit is the most important component in an embedded system. It exists in an integrated manner along with memory and other peripherals. Depending on the type of applications the processors are broadly classified into 3 major categories

1. General Purpose Microprocessors
2. Microcontrollers
3. Digital Signal Processors

For more specific applications customized processors can also be designed. Unless the demand is high the design and manufacturing cost of such processors will be high. Therefore, in most of the applications the design is carried out using already available processors in the market. However, the Field Programmable Gate Arrays (FPGA) can be used to implement simple customized processors easily. An FPGA is a type of logic chip that can be programmed. They support thousands of gates which can be connected and disconnected like an EPROM (Erasable Programmable Read Only Memory). They are especially popular for prototyping integrated circuit designs. Once the design is set, hardwired chips are produced for faster performance.

General Purpose Processors

A general purpose processor is designed to solve problems in a large variety of applications as diverse as communications, automotive and industrial embedded systems. These processors are generally cheap because of the manufacturing of large number of units. The NRE (Non-recurring Engineering Cost: Lesson I) is spread over a large number of units. Being cheaper the manufacturer can invest more for improving the VLSI design with advanced optimized architectural features. Thus the performance, size and power consumption can be improved. Most cases, for such processors the design tools are provided by the manufacturer. Also the supporting hardware is cheap and easily available. However, only a part of the processor capability may be needed for a specific design and hence the over all embedded system will not be as optimized as it should have been as far as the space, power and reliability is concerned.






Pentium IV is such a general purpose processor with most advanced architectural features. Compared to its overall performance the cost is also low.
A general purpose processor consists of a data path, a control unit tightly linked with the memory. (Fig. 4.1)

The Data Path consists of a circuitry for transforming data and storing temporary data. It contains an arithmetic-logic-unit(ALU) capable of transforming data through operations such as addition, subtraction, logical AND, logical OR, inverting, shifting etc. The data-path also contains registers capable of storing temporary data generated out of ALU or related operations. The internal data-bus carries data within the data path while the external data bus carries data to and from the data memory. The size of the data path indicates the bit-size of the CPU. An 8-bit data path means an 8-bit CPU such as 8085 etc.

The Control Unit consists of circuitry for retrieving program instructions and for moving data to, from, and through the data-path according to those instructions. It has a program counter(PC) to hold the address of the next program instruction to fetch and an Instruction register(IR) to hold the fetched instruction. It also has a timing unit in the form of state registers and control logic. The controller sequences through the states and generates the control signals necessary to read instructions into the IR and control the flow of data in the data path. Generally the address size is specified by the control unit as it is responsible to communicate with the memory. For each instruction the controller typically sequences through several stages, such as fetching the instruction from memory, decoding it, fetching the operands, executing the instruction in the data path and storing the results. Each stage takes few clock cycles.


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