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<hr><p align="left"><small>发信人: beary (京酱肉丝), 信区: Embedded <br>
标 题: 嵌入式系统的开发 <br>
发信站: BBS 水木清华站 (Fri May 19 17:59:09 2000) <br>
<br>
1. Introduction <br>
Approximately 3 billion embedded CPUs are sold each year, with smaller (4-, <br>
8-, and 16-bit) CPUs dominating by quantity and aggregate dollar amount [1]. <br>
Yet, most research and tool development seems to be focussed on the needs o <br>
f high-end desktop and military/aerospace embedded computing. This paper see <br>
ks to expand the area of discussion to encompass a wide range of embedded sy <br>
stems. <br>
The extreme diversity of embedded applications makes generalizations difficu <br>
lt. Nonetheless, there is emerging interest in the entire range of embedded <br>
systems (e.g., [2], [3], [4], [5], [6]) and the related field of hardware/so <br>
ftware codesign (e.g., [7]). <br>
This paper and the accompanying tutorial seek to identify significant areas <br>
in which embedded computer design differs from more traditional desktop comp <br>
uter design. They also present "design challenges" encountered in the course <br>
of designing several real systems. These challenges are both opportunities <br>
to improve methodology and tool support as well as impediments to deploying <br>
such support to embedded system design teams. In some cases research and dev <br>
elopment has already begun in these areas -- and in other cases it has not. <br>
The observations in this paper come from the author's experience with commer <br>
cial as well as military applications, development methodologies, and life-c <br>
ycle support. All characterizations are implicitly qualified to indicate a t <br>
ypical, representative, or perhaps simply an anecdotal case rather than a de <br>
finitive statement about all embedded systems. While it is understood that e <br>
ach embedded system has its own set of unique requirements, it is hoped that <br>
the generalizations and examples presented here will provide a broad-brush <br>
basis for discussion and evolution of CAD tools and design methodologies. <br>
2. Example Embedded Systems <br>
Figure 1 shows one possible organization for an embedded system. <br>
Figure 1. An embedded system encompasses the CPU as well as many other resou <br>
rces. <br>
In addition to the CPU and memory hierarchy, there are a variety of interfac <br>
es that enable the system to measure, manipulate, and otherwise interact wit <br>
h the external environment. Some differences with desktop computing may be: <br>
· The human interface may be as simple as a flashing light or as complicate <br>
d as real-time robotic vision. <br>
· The diagnostic port may be used for diagnosing the system that is being c <br>
ontrolled -- not just for diagnosing the computer. <br>
· Special-purpose field programmable (FPGA), application specific (ASIC), o <br>
r even non-digital hardware may be used to increase performance or safety. <br>
· Software often has a fixed function, and is specific to the application. <br>
In addition to the emphasis on interaction with the external world, embedded <br>
systems also provide functionality specific to their applications. Instead <br>
of executing spreadsheets, word processing and engineering analysis, embedde <br>
d systems typically execute control laws, finite state machines, and signal <br>
processing algorithms. They must often detect and react to faults in both th <br>
e computing and surrounding electromechanical systems, and must manipulate a <br>
pplication-specific user interface devices. <br>
Table 1. Four example embedded systems with approximate attributes. <br>
In order to make the discussion more concrete, we shall discuss four example <br>
systems (Table 1). Each example portrays a real system in current productio <br>
n, but has been slightly genericized to represent a broader cross-section of <br>
applications as well as protect proprietary interests. The four examples ar <br>
e a Signal Processing system, a Mission Critical control system, a Distribut <br>
ed control system, and a Small consumer electronic system. The Signal Proces <br>
sing and Mission Critical systems are representative of traditional military <br>
/aerospace embedded systems, but in fact are becoming more applicable to gen <br>
eral commercial applications over time. <br>
Using these four examples to illustrate points, the following sections descr <br>
ibe the different areas of concern for embedded system design: computer desi <br>
gn, system-level design, life-cycle support, business model support, and des <br>
ign culture adaptation. <br>
Desktop computing design methodology and tool support is to a large degree c <br>
oncerned with initial design of the digital system itself. To be sure, exper <br>
ienced designers are cognizant of other aspects, but with the recent emphasi <br>
s on quantitative design (e.g., [8]) life-cycle issues that aren't readily q <br>
uantified could be left out of the optimization process. However, such an ap <br>
proach is insufficient to create embedded systems that can effectively compe <br>
te in the marketplace. This is because in many cases the issue is not whethe <br>
r design of an immensely complex system is feasible, but rather whether a re <br>
latively modest system can be highly optimized for life-cycle cost and effec <br>
tiveness. <br>
While traditional digital design CAD tools can make a computer designer more <br>
efficient, they may not deal with the central issue -- embedded design is a <br>
bout the system, not about the computer. In desktop computing, design often <br>
focuses on building the fastest CPU, then supporting it as required for maxi <br>
mum computing speed. In embedded systems the combination of the external int <br>
erfaces (sensors, actuators) and the control or sequencing algorithms is or <br>
primary importance. The CPU simply exists as a way to implement those functi <br>
ons. The following experiment should serve to illustrate this point: ask a r <br>
oomful of people what kind of CPU is in the personal computer or workstation <br>
they use. Then ask the same people which CPU is used for the engine control <br>
ler in their car (and whether the CPU type influenced the purchasing decisio <br>
n). <br>
In high-end embedded systems, the tools used for desktop computer design are <br>
invaluable. However, many embedded systems both large and small must meet a <br>
dditional requirements that are beyond the scope of what is typically handle <br>
d by design automation. These additional needs fall into the categories of s <br>
pecial computer design requirements, system-level requirements, life-cycle s <br>
upport issues, business model compatibility, and design culture issues. <br>
3. Computer Design Requirements <br>
Embedded computers typically have tight constraints on both functionality an <br>
d implementation. In particular, they must guarantee real time operation rea <br>
ctive to external events, conform to size and weight limits, budget power an <br>
d cooling consumption, satisfy safety and reliability requirements, and meet <br>
tight cost targets. <br>
3.1. Real time/reactive operation <br>
Real time system operation means that the correctness of a computation depen <br>
ds, in part, on the time at which it is delivered. In many cases the system <br>
design must take into account worst case performance. Predicting the worst c <br>
ase may be difficult on complicated architectures, leading to overly pessimi <br>
stic estimates erring on the side of caution. The Signal Processing and Miss <br>
ion Critical example systems have a significant requirement for real time op <br>
eration in order to meet external I/O and control stability requirements. <br>
Reactive computation means that the software executes in response to externa <br>
l events. These events may be periodic, in which case scheduling of events t <br>
o guarantee performance may be possible. On the other hand, many events may <br>
be aperiodic, in which case the maximum event arrival rate must be estimated <br>
in order to accommodate worst case situations. Most embedded systems have a <br>
significant reactive component. <br>
Design challenge: <br>
· Worst case design analyses without undue pessimism in the face of hardwar <br>
e with statistical performance characteristics (e.g., cache memory [9]). <br>
3.2. Small size, low weight <br>
Many embedded computers are physically located within some larger artifact. <br>
Therefore, their form factor may be dictated by aesthetics, form factors exi <br>
sting in pre-electronic versions, or having to fit into interstices among me <br>
chanical components. In transportation and portable systems, weight may be c <br>
ritical for fuel economy or human endurance. Among the examples, the Mission <br>
Critical system has much more stringent size and weight requirements than t <br>
he others because of its use in a flight vehicle, although all examples have <br>
restrictions of this type. <br>
Design challenges: <br>
· Non-rectangular, non-planar geometries. <br>
· Packaging and integration of digital, analog, and power circuits to reduc <br>
e size. <br>
3.3. Safe and reliable <br>
Some systems have obvious risks associated with failure. In mission-critical <br>
applications such as aircraft flight control, severe personal injury or equ <br>
ipment damage could result from a failure of the embedded computer. Traditio <br>
nally, such systems have employed multiply-redundant computers or distribute <br>
d consensus protocols in order to ensure continued operation after an equipm <br>
ent failure (e.g., [10], [11]) <br>
However, many embedded systems that could cause personal or property damage <br>
cannot tolerate the added cost of redundancy in hardware or processing capac <br>
ity needed for traditional fault tolerance techniques. This vulnerability is <br>
often resolved at the system level as discussed later. <br>
Design challenge: <br>
· Low-cost reliability with minimal redundancy. <br>
3.4. Harsh environment <br>
Many embedded systems do not operate in a controlled environment. Excessive <br>
heat is often a problem, especially in applications involving combustion (e. <br>
g., many transportation applications). Additional problems can be caused for <br>
embedded computing by a need for protection from vibration, shock, lightnin <br>
g, power supply fluctuations, water, corrosion, fire, and general physical a <br>
buse. For example, in the Mission Critical example application the computer <br>
must function for a guaranteed, but brief, period of time even under non-sur <br>
vivable fire conditions. <br>
Design challenges: <br>
· Accurate thermal modelling. <br>
· De-rating components differently for each design, depending on operating <br>
environment. <br>
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