Question

Question 15 (7 marks) Question 15.1 (4 marks) What design metrics should be considered in order to make a good choice between a general purpose processor and a custom single purpose processor? Question 15.2 (3 marks) Are there any advantages of using both single purpose processors and a general purpose processor in a single embedded system? Explain your answer.

Answer 1

15.1) Design Metrics:

• A Design Metric is a measurable feature of the system’s performance, cost, time for implementation and safety etc.
• Most of these are conflicting requirements i.e. optimizing one shall not optimize the other: e.g. a cheaper processor may have a lousy performance as far as speed and throughput is concerned.
• Following metrics are generally taken into account while designing embedded systems
• NRE cost (nonrecurring engineering cost):
• It is one-time cost of designing the system.
• Once the system is designed, any number of units can be manufactured without incurring any additional design cost; hence the term nonrecurring.
• Suppose three technologies are available for use in a particular product. Assume that implementing the product using technology ‘A’ would result in an NRE cost of $2,000 and unit cost of $100, that technology B would have an NRE cost of $30,000 and unit cost of $30, and that technology C would have an NRE cost of $100,000 and unit cost of $2. Ignoring all other design metrics, like time-to-market, the best technology choice will depend on the number of units we plan to produce.
• Unit cost:
• The monetary cost of manufacturing each copy of the system, excluding NRE cost.
• Size:
• The physical space required by the system, often measured in bytes for software, and gates or transistors for hardware.
• Performance:
• The execution time of the system
• Power Consumption:
• It is the amount of power consumed by the system, which may determine the lifetime of a battery, or the cooling requirements of the IC, since more power means more heat.
• Flexibility:
• The ability to change the functionality of the system without incurring heavy NRE cost. Software is typically considered very flexible.
• Time-to-prototype:
• The time needed to build a working version of the system, which may be bigger or more expensive than the final system implementation, but it can be used to verify the system’s usefulness and correctness and to refine the system’s functionality.
• Time-to-market:
• The time required to develop a system to the point that it can be released and sold to customers. The main contributors are design time, manufacturing time, and testing time. This metric has become especially demanding in recent years. Introducing an embedded system to the marketplace early can make a big difference in the system’s profitability.
• Maintainability:
• It is the ability to modify the system after its initial release, especially by designers who did not originally design the system.
• Correctness:
• This is the measure of the confidence that we have implemented the system’s functionality correctly. We can check the functionality throughout the process of designing the system, and we can insert test circuitry to check that manufacturing was correct.
• The Performance Design Metric:
• Performance of a system is a measure of how long the system takes to execute our desired tasks. The two main measures of performance are:
• Latency or response time:
• This is the time between the start of the task’s execution and the end. For example, processing an image may take 0.25 second.
• Throughput:
• This is the number of tasks that can be processed per unit time. For example, a camera may be able to process 4 images per second These are the some of the cost measures for developing an RTES.
• Optimization of the overall cost of design includes each of these factors taken with some multiplying factors depending on their importance.
• And the importance of each of these factors depends on the type of application. For instance in defense related applications while designing an anti-ballistic system the execution time is the deciding factor.
• On the other hand, for de-noising a photograph in an embedded camera in your mobile handset the execution time may be little relaxed if it can bring down the cost and complexity of the embedded Digital Signal Processor.

15.2) by using both single purpose processor and general purpose processor in a single embedded system,

• Advantage:
• Reduce time to design and verify system repeatatively.
• usable for several applications,
• low price,
• short design time
• easy to design and use (only programming the memory)
• reprogrammability (changing functionalities or improving a system becomes easy when you only need to change the program)
• Disadvantage:
• low performance,
• high energy consumption
• large dimensions
• they consume much power