Application Specific Integrated Circuits (ASICs)

Introduction

The abbreviation ASIC stands for Application Specific Integrated Circuits. Compared with standard circuits an ASIC is designed and manufactured according to specifications resulting from its application. The ASIC user is able to specify the circuit function at design level and/or by means of configuration options. If the function of an IC is only controlled by software then it is not called an ASIC [16.7].

Figure 16.1 shows a possible classification of monolithically integrated circuits. Examples of standard ICs are operational amplifiers (standard analog), TTL and CMOS Logic ICs (standard logic), microprocessors, RAM and ROM. Examples of ASICs are Field Programmable (FPGAs), Gate Arrays, Macro Cell, Standard Cell ICs and Full Custom ICs [16.5].

In some cases the classification standard IC/ASIC is difficult. If an ASIC design is so successful that it sells in large numbers it finally becomes a standard IC. Also the classification ROM/FPGA is not always clear.

Technological Characteristics of ASICs

Some technological characteristics show the dynamic progress in the field of ASICs.

The so called feature size of a certain technology is a measure of the length of the smallest MOS transistor, sometimes also for the minimal allowable distance between two connection lines on the chips. The feature size λ is in the order of the length of a transistor. With decreasing feature size the area needed is reduced and the transistor performance improves, at the same time the manufacturing will be more demanding.

The silicon chip of the IC, the so called die varies in its area from some mm2 to several cm2. The bigger the die and the smaller λ the more transistors can be integrated into one IC.

As a measure of the complexity of an IC one can consider the number of gate equivalents. A gate equivalent is equal to a NAND gate with two inputs.

Application Specific Integrated Circuits (ASICs)-0000

The Semiconductor Industry Association (SIA), an organization supported by all major semiconductor manufacturers, forecasts the progress of the semiconductor technology by means of so called road maps and specifies the needed processes and installations [16.9]. Table 16.1 shows the road map for microprocessors from which it can be derived that the integration density and the speed will allow one to build ASICs with more complex functionality and lower prices. The SIA considers the ASICs as one of the motors of technology with respect to integration density, performance, and package. For this compare chapter 1 and 2.

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Design Goals for ASICs

Compared to system realizations with standard ICs ASICs show several advantages. The most impor- tant design goals for a system are [16.7]:

• cost effective solution;

• low design effort with respect to time and cost;

• high performance, i.e., functionality, speed;

• low power dissipation;

• small volume and weight;

• high reliability;

• low cost for mounting and test.

In order to reach the goals with ASICs certain conditions have to be fulfilled. The methods of design and realization of ASICs the so called design styles have to be chosen optimally with respect to the system environment. The preferred design style is dependent on the expected number of parts, on the design time provided, and on the performance desired.

With respect to the most important goal of cost effectiveness the cost of design and manufacturing have to be considered. A related analysis will be carried out in section 16.3.

There are trade offs between different goals, e.g., higher performance generally demands a higher design effort and also results in a higher power dissipation.

Case Study: CD Player, ASIC as Key Component

Figure 16.2 shows a block diagram of a CD player without showing the operating and display unit [16.12]. The CD player consists of a ‘mechadeck’ comprising mechanical and optical components, of an ASIC for signal processing and control tasks, and of several standard ICs. The player is able to communicate via a bus interface with the proces- sor of the operating and display unit. A further interface to the outer world is formed by the stereo audio signal.

The CD rotates within the ‘mechadeck’. The ‘pickup’ to be found there also comprises the laser and six photo-diodes for reading the CD. The position of the pickup has to be controlled with respect to track and z-coordinate (focus).

The core of the CD player is formed by the ASIC ‘CD one Chip’, which takes over all important functions with respect to signal processing and position control. Embedded into the ASIC are:

• a standard microprocessor;

• a signal processor (DSP);

• memory (SRAM);

• a digital analog converter (Audio-DAC);

• a preamplifier for the read signals of the photo diodes.

The rather complicated decoding and error-correction of the read signal by means of the Reed- Solomon algorithm is realized in a hard wired form. The DSP is partly needed for the complicated calculation of the control signals for track and focus from the read signals. The ASIC contains mechanisms for automatic adjustment which enables the construction of a nearly adjustment- free CD player.

The CD player comprises as standard ICs an operational amplifier and some driver ICs. This poses the question of, why those functions realized with standard ICs are not integrated into the ASIC ‘CD one Chip’. On one hand, the technology used in the ASIC is not suitable for some functions, on the other hand, the used standard ICs are so cheap that a further integration does not make sense.

The ASIC ‘CD one Chip’ meets many of the de- sign goals discussed in the previous paragraph. The IC was designed in full custom style, which

Application Specific Integrated Circuits (ASICs)-0002

meant a high design effort, but led to a very com- pact layout. In a 0.5 μm technology only 30 mm2 chip area was needed to realize the complex functions. Taking a sales volume of the ‘CD one Chip’ in the order of several millions into account this approach led to a cost effective solution.

Compared with a solution with standard ICs the advantages are:

• lower number of components;

• lower power dissipation;

• smaller volume and weight;

• higher reliability;

• lower cost for mounting and test.

In addition, by using an ASIC a certain protection against copying the design by a competitor is given.

Compared with a predecessor model with standard ICs the discussed CD player needs 30 % less PCB area, the overall cost savings are 20 %.

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