Mixed-signal ASICs are used more and more when developing new industrial control systems and this paper discusses how mixed signal ASICs are changing the face of this technology.
ASICs are often thought to be only applicable to high production volume applications. However by combining expertise in custom IC design with modern foundry processes, they have become a differentiative technology compared to traditional discrete ICs and components in a wide spectrum of products, ranging from consumer electronics to medical and automotive and industrial sensor interfaces. One of the fastest growing application areas is that of industrial control systems.
Whilst industrial controllers cover many industries and come in many forms, they all carry out a similar function: they monitor a variable, compare the measured value with a desired value and adjust a controlled variable to reduce any discrepancy. This usually incorporates some sort of feedback loop with three term (proportional, integral and derivative) control. The signals generated by the sensors that carry out the monitoring function may be digital, but more often are analogue values of temperature, pressure, level or of analytical quality, and these need to be conditioned and digitised to allow them to interface with the control systems.
The nature of the sensors has changed as material science has allowed the development of CMOS sensing elements, which lend themselves to integration either on chip or in the same package. Industrial sensors, which are often located in confined and difficult environments, are typically in operation for many years and mixed signal (analogue and digital) ASICs offer a rugged and compact solution with the additional benefit of obsolescence protection, an important factor for lifetime maintenance and spares. Communication between the sensor and the controller can be one of many standard or proprietary protocols, either RF or wired. Data rates tend to be low, so the most common protocols are the Serial Peripheral interface, (SPI) or the Inter-Integrated Circuit (I2C).
One of the first areas of industrial control to adopt mixed signal ASICs was motor controllers, allowing low cost variable speed drive systems to be developed. They include functionality for current sensing, control of the motor, encoders to sense the position and speed of the motor, and communications interfaces that enable the motor control logic to link to other parts of system. Variable speed drive systems optimise the speed of a motor to the duty required and are widely used to control pumps to give a constant head or constant flow. They have replaced flow control valves in many bulk flow applications like water distribution giving improved efficiency and reducing carbon footprints.
Smart meters provide a remote read-out of consumption information in real time and allow utilities and consumers to better track usage. ASICs are used to measure voltage and current conditions and to interface the meter to the outside environment by a variety of RF communications.
Linear and Rotary Encoders
Many industrial control systems use linear and rotary encoders. A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analogue or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller. The encoder can be either absolute or incremental (relative), and motion can be determined by change in position over time. Linear encoders, which may include optical, magnetic, inductive, capacitive and eddy current technologies, are used in metrology instruments, motion systems and high precision machining tools ranging from digital callipers and coordinate measuring machines to stages, CNC mills, manufacturing gantry tables and semiconductor steppers.
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analogue or digital signal. As with linear, rotary encoders are either absolute or incremental relative. The output of absolute encoders indicates the current position of the shaft, making them angle transducers, whilst the output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance, and position. Rotary encoders are used in many applications that require precise shaft rotation: industrial controls, robotics, special purpose photographic lenses, computer input devices (such as optomechanical mice and trackballs), controlled stress rheometers, and rotating radar platforms.
Today, it is possible to integrate substantial functionality into an ASIC, especially at the smaller CMOS geometries. However, some of these smaller geometries are best suited for predominately digital designs so, for industrial market mixed signal designs, the favoured geometries tend to be in the 180nm to 55nm range. These can still achieve a high level of digital integration, including microprocessors, whilst offering the flexibility for the analogue part of the design. Power devices and high temperature solutions are also possible, opening new application areas for integration onto the ASIC.
Communications & Flexibility
In some cases, the solution is for a platform approach, where one ASIC will be used in different scenarios within a product family, and it will support many sensor interfaces and communication ports. This type of architecture can perform information processing local to the sensor and report back to the main controller. Being generic allows it to interface with many different sensor types, and the controller communications can be wired or wireless. It can also be programmed, so has some flexibility.
ASIC Design Team
Whilst mixed signal ASICs are changing the face of industrial controllers, both in unique performance characteristics and longevity of availability, it is important to note that an ASIC design is not simply a question of implementing an existing PCB design.
Different techniques are needed for the design of the ASIC. Excellent system knowledge from the customer and experienced ASIC design engineers are essential for a successful project. In most cases, this comes from an amalgamation of the ASIC design team and the customer’s system designers, so it is crucial to build a strong relationship between the project teams.
Making the Right Choices
When undertaking any ASIC project, it is critical to make sure that your ASIC partner has all the appropriate skill in house.
This may sound obvious, but it is often overlooked. The final goal of any ASIC project is to have a fully qualified, production released device within the minimum amount of time. So, selecting the right partner will have a major impact on this. A digital design requires different skills to an analogue design, so a partner with the correct skill sets should be selected. Also, it is not just the design that has to be considered at this stage, the production testing of the device is critical and should be examined. A creditable ASIC partner will be able to show you the whole in house ASIC capabilities, from specification through production test to order fulfilment, and not just the design.
The Factors to be Considered
When undertaking an ASIC design, many factors must be considered. Some of these are obvious where as others may be more subtle. For example, careful consideration of ASIC partitioning (i.e. what is going to be implemented onto the ASIC), should be considered in order to provide an optimised custom IC. On chip and off chip functionality is important as it determines the die size and the production test time. It will also affect the yield as a larger and more complex design has a higher probability of wafer imperfections and localised defects, which will cause chip failures at test. So, the effective partitioning of a design is critical to its success. However, there can be substantial savings by incorporating a microprocessor into the ASIC, and such decisions have to be balanced with any licensing fees and flexibility. Look for a mature development processes, that are well documented and examples of previous designs that are representative to your requirements. Included in these should be a detailed ASIC specification and test strategy for qualification and production.
Any third party IP licence fees can add significant costs to the project, (also potentially to the unit price if a royalty based model is employed), but could save design time and reduce BOM costs. When considering using third party IP, the designer needs to consider if the IP has been previously proven in silicon in the selected process. Also any qualification requirements need to be taken into account. Any specialist quality standard requirements need to be thought through (e.g. ISO 26262), at the design and IP selection stage.
There are many wafer fabrication processes available and the selected process should be carefully matched to the design elements and required ASIC performance criteria. As the process geometry decreases, the mask costs rise quickly and certain analogue functionality can become more difficult to achieve. Therefore, the selection of a smaller geometry must be carefully considered from a performance, commercial and availability of IP point of view. It is not always the case that the latest, smallest geometry process is the best choice for your mixed signal application.
Whilst the design of the device is critical, it is not just the functionality that has to be considered, as the device must be manufactured and production tested. Decisions at the design phase will impact the yields and subsequent cost of the development and production devices. Yields can be influenced by the process selection, so again a partner who has detailed knowledge and access to a selection of wafer foundries and processes will be able to offer the most optimised process for your requirement. The product life cycle is another critical factor, in that if the device is to be available for many years, the right process must be selected to enable this, so an obsolescence assurance policy or plan is desirable.
For some industries there are strict constraints on field failure rates, which can be improved further by using wafer probe part average testing techniques, to maintain rates of less than one part per million. This involves statistical analysis of parts at the limits of the specification and rejection of part that have a higher probability of infant mortality, even though they are within the specified design limits. This can be achieved with adjacent failing part reject techniques, and additional stress testing to identify likely field failures and discarding of those parts at final test.
The Major Decisions in ASIC Development
Selecting the right custom IC partner is critical when starting on the path to your ASIC project. Essentially a partner with all the relevant expertise of your ASIC development, production test and order fulfilment is required. This must include logistic support and fast reaction times should a problem arise in production, so tight control over production test and yields is maintained. This will allow you to be able to treat the ASIC as an off the shelf part, confident in the knowledge that should a problem arise it will be addressed in a quick and professional manner.
Working in Partnership
There are many points in an ASIC development and production project which impact on the choice of partner and the competitiveness of the final ASIC. That is why, at Swindon, we have evolved a mutual approach with the aim of working in partnership with our customers to achieve the right balance of decisions and choices in terms of development and production supply.
The Advantages of ASICs
ASIC’s have many benefits and will provide you with significant market advantages, both technically and commercially. Some of these benefits are higher and focussed performance, protection of your IP and supply longevity. Swindon has five decades experience and expertise to call upon to guide you through the many choices you have. With mature development processes, long established wafer foundry relationships, fully qualified automotive capability, and 100% inhouse production test facilities. When it comes to ASICs, Swindon has all the answers.