the designer of a real-time system must be more diligent in his work. He must guarantee reliable operation of the software and hardware under all possible conditions. And, to the degree that human lives depend upon the system's proper execution, this guarantee must be backed by engineering calculations and descriptive paperwork.
1.2 Variations on the Theme
By definition all embedded systems contain a processor and software, but what other features do they have in common? Certainly, in order to have software, there must be a place to store the executable code and temporary storage for runtime data manipulation. These take the form of ROM and RAM, respectively; any embedded system will have some of each. If only a small amount of memory is required, it might be contained within the same chip as the processor. Otherwise, one or both types of memory will reside in external memory chips.
all embedded systems also contain some type of inputs and outputs. For example, in a microwave oven the inputs are the buttons on the front panel and a temperature probe, and the outputs are the human-readable display and the microwave radiation. It is almost always the case that the outputs of the embedded system are a function of its inputs and several other factors (elapsed time, current temperature, etc.). The inputs to the system usually take the form of sensors and probes, communication signals, or control knobs and buttons. The outputs are typically displays, communication signals, or changes to the physical world. See Figure 1-1 for a general example of an embedded system.
Figure 1-1. A generic embedded system
With the exception of these few common features, the rest of the embedded hardware is usually unique. This variation is the result of many competing design criteria. Each system must meet a completely different set of requirements, any or all of which can affect the compromises and tradeoffs made during the development of the product. For example, if the system must have a production cost of less than $10, then other things like processing power and system reliability might need to be sacrificed in order to meet that goal.
Of course, production cost is only one of the possible constraints under which embedded hardware designers work. Other common design requirements include the following:
tradeoff between production cost and development cost is affected most by the number of units expected to be produced and sold. For example, it is usually undesirable to develop your own custom hardware components for a low-volume product.
In addition to these general requirements, there are the detailed functional requirements of the system itself. These are the things that give the embedded system its unique identity as a microwave oven, pacemaker, or pager.
Table 1-1 illustrates the range of possible values for each of the previous design requirements. These are only estimates and should not be taken too seriously. In some cases, two or more of the criteria are linked. For example, increases in processing power could lead to increased production costs. Conversely, we might imagine that the same increase in processing power would have the effect of decreasing the development costs by reducing the complexity of the hardware and software design. So the values in a particular column do not necessarily go together.
Table 1-1. Common Design Requirements for Embedded Systems
| Criterion | Low | Medium | High |
|---|---|---|---|
| Processor | 4 or 8-bit | 16-bit | 32 or 64-bit |
| Memory | < 16 KB | 64 KB to 1 MB | > 1 MB |
| Development cost | < $100,000 | $100,000 to $1,000,000 | > $1,000,000 |
| Production cost | < $10 | $10 to $1,000 | > $1,000 |
| Number of units | < 100 | 100-10,000 | > 10,000 |
| Expected lifetime | days, weeks, or months | years | decades |
| Reliability | may occasionally fail | must work reliably | must be fail-proof |
In order to simultaneously demonstrate the variation from one embedded system to the next and the possible effects of these design requirements on the hardware, I will now take some time to describe three embedded systems in some detail. My goal is to put you in the system designer's shoes for a few moments before beginning to narrow our discussion to embedded software development.
1.2.1 Digital Watch
The typical digital watch contains a simple, inexpensive 8-bit processor. Because such small processors cannot address very much memory, this type of processor usually contains its own on-chip ROM. And, if there are sufficient registers available, this application may not require any RAM at all. In fact, all of the electronics-processor, memory, counters and real-time clocks are likely to be stored in a single chip. The only other hardware elements of the watch are the inputs (buttons) and outputs (LCD and speaker).