Archive for June, 2008

Embedded Systems and the Environment

Friday, June 20th, 2008 Nigel Jones

With the recent run up in the price of oil, it seems as if everyone is talking about energy and how to conserve it. For most people, the only impact they can have on the environment is through their own individual actions and choices. Engineers however, are in a different position because at a professional level, the design choices we make can have a profound effect on the environment. If we believe the figures about the number of embedded processors shipped each year (billions) and we make the very conservative estimate that each processor is in a system that consumes 1 WH per day, then the annual energy consumption of new embedded systems runs to at least 1E9 * 1 * 365 = 365 Tera Watt hours, with an average power consumption of around 41 Megawatts. If we assume that the average life of an embedded system is 5 years, then the embedded systems out there are burning about 200 Megawatts. That’s a lot of power folks.

Now here’s the interesting thing. Most embedded projects are for products that are made in the thousands. Individually, these products power consumption is irrelevant. Collectively they are huge. Thus if as an industry we made a concerted effort to reduce the power consumption of our products, the benefits to society would be substantial. So how exactly do we do this? Although a lot of the power consumption comes from the hardware design, the firmware design can also have a dramatic impact on the overall power consumption of the system. In my next posting I’ll look at some of the ways you can design your system firmware so as to minimize power consumption.

Efficient C Tips #1 – Choosing the correct integer size

Sunday, June 15th, 2008 Nigel Jones

From time to time I write articles for Embedded Systems Design magazine. A number of these articles have concentrated on how to write efficient C for an embedded target. Whenever I write these articles I always get emails from people asking me two questions:

1. How did you learn this stuff?
2. Is there somewhere I can go to learn more?

The answer to the first question is a bit long winded and consists of:
1. I read compiler manuals (yes, I do need a life).
2. I experiment.
3. Whenever I see a strange coding construct, I ask the author why they are doing it that way. From time to time I pick up some gems.
4. I think hard about what the compiler has to do in order to satisfy a particular coding construct. It’s really helpful if you know assembly language for this stage.

The answer to the second question is short: No!

To help rectify this, in my copious free time I’ll consider putting together a one day course on how to write efficient C for embedded systems. If this is of interest to you then please contact me .

In the interim, I’d like to offer up my first tip on how to choose the correct integer size.

In my experience in writing programs for both embedded systems and computers, I’d say that greater than 95% of all the integers used by those programs could fit into an 8 bit variable. The question is, what sort of integer should one use in order to make the code the most efficient? Most computer programmers who use C will be puzzled by this question. After all the data type ‘int’ is supposed to be an integer type that is at least 16 bits that represents the natural word length of the target system. Thus, one should simply use the ‘int’ data type.

In the embedded world, however, such a trite answer will quickly get you into trouble – for at least three reasons.
1. For 8 bit microcontrollers, the natural word length is 8 bits. However you can’t represent an ‘int’ data type in 8 bits and remain C99 compliant. Some compiler manufacturer’s eschew C99 compliance and make the ‘int’ type 8 bits (at least one PIC compiler does this), while others simply say we are compliant and if you are stupid enough to use an ‘int’ when another data type makes more sense then that’s your problem.
2. For some processors there is a difference between the natural word length of the CPU and the natural word length of the (external) memory bus. Thus the optimal integer type can actually depend upon where it is stored.
3. The ‘int’ data type is signed. Much, indeed most, of the embedded world is unsigned, and those of us that have worked in it for a long time have found that working with unsigned integers is a lot faster and a lot safer than working with signed integers, or even worse a mix of signed and unsigned integers. (I’ll make this the subject of another blog post).

Thus the bottom line is that using the ‘int’ data type can get you into a world of trouble. Most embedded programmers are aware of this, which is why when you look at embedded code, you’ll see a veritable maelstrom of user defined data types such as UINT8, INT32, WORD, DWORD etc. Although these should ensure that there is no ambiguity about the data type being used for a particular construct, it still doesn’t solve the problem about whether the data type is optimal or not. For example, consider the following simple code fragment for doing something 100 times:


for (i = 0; i < 100; i++)
 // Do something 100 times

Please ignore all other issues other than what data type should the loop variable ‘i’ be?Well evidently, it needs to be at least 8 bits wide and so we would appear to have a choice of 8,16,32 or even 64 bits as our underlying data type. Now if you are writing code for a particular CPU then you should know whether it is an 8, 16, 32 or 64 bit CPU and thus you could make your choice based on this factor alone. However, is a 16 bit integer always the best choice for a particular 16 bit CPU? And what about if you are trying to write portable code that is supposed to be used on a plethora of targets? Finally, what exactly do we mean by ‘optimal’ or ‘efficient’ code?I wrestled with these problems for many years before finally realizing that the C99 standards committee has solved this problem for us. Quite a few people now know that the C99 standard standardized the naming conventions for specific integer types (int8_t, uint8_t, int16_t etc). What isn’t so well known is that they also defined data types which are “minimum width” and also “fastest width”. To see if your compiler is C99 compliant, open up stdint.h. If it is compliant, as well as the uint8_t etc data types, you’ll also see at least two other sections – minimum width types and fastest minimum width types.

An example will help clarify the situation:

Fixed width unsigned 8 bit integer: uint8_t

Minimum width unsigned 8 bit integer: uint_least8_t

Fastest minimum width unsigned 8 bit integer: uint_fast8_t

Thus a uint8_t is guaranteed to be exactly 8 bits wide. A uint_least8_t is the smallest integer guaranteed to be at least 8 bits wide. An uint_fast8_t is the fastest integer guaranteed to be at least 8 bits wide. So we can now finally answer our question. If we are trying to consume the minimum amount of data memory, then our TBD_DATATYPE should be uint_least8_t. If we are trying to make our code run as fast as possible then we should use uint_fast8_t. Thus the bottom line is this. If you want to start writing efficient, portable embedded code, the first step you should take is start using the C99 data types ‘least’ and ‘fast’. If your compiler isn’t C99 compliant then complain until it is – or change vendors. If you make this change I think you’ll be pleasantly surprised at the improvements in code size and speed that you’ll achieve.

Next Tip


Thoughts on the optimal time to test code

Friday, June 6th, 2008 Nigel Jones

Today I’d like to take on one of the sacred cows of the embedded industry, namely the temporal relationship between coding and testing of the aforementioned code. The conventional wisdom seems to be as follows.

“Write a small piece of code. As soon as possible test the code. Repeat until the task is complete”

I know for many of you, me merely having the temerity to suggest this might be sub-optimal will put me firmly into the category of hopeless heretic. Well, before you write me off as a lunatic, let me tell you about an alternative approach, how I stumbled upon it and why I think it has much to commend it.

Being in the consulting business I’m typically working on multiple projects at once. Often a given project will be put on hold for any number of reasons which aren’t germane to this post. As a result, it’s not uncommon for me to write some code, compile it and then not touch it again for several months. I then find myself in the position of having to test / debug code that I wrote months ago. Having now done this many times, I’ve come to the conclusion that rather than this being a problem, it is instead the optimal temporal relationship between coding and testing.

How can this be you ask? Surely after a multi-month hiatus, the code is no longer fresh in your mind and so it must make it that much more difficult to test and debug? Well the answer is of course yes – the code is no longer fresh in my mind, and yes it does make it a little harder to test and debug in the short term. In my emphasis lies the point of my argument.

Why do we write code? Most people would claim we write code in order to make a functional product. I disagree with this assertion. I think we write code so that people coming after us can understand it and modify it. This rather strange claim is based upon those studies that show that companies spend far more money maintaining code than they do writing it. Thus the smart way to write code is to do so in a manner that gives preeminent importance to the long term maintenance of that code. So how does one do this? Well that’s a topic for another post. What I can tell you, is that having to test and debug code that you wrote several months ago is a terrific way for the developer of the code to see the code as someone who’ll be maintaining it will see it. You’ll see the inadequate or plain wrong comments. You’ll see the copy and paste errors. You’ll see where you got tired and took a short cut, and you’ll see those stupid mistakes caused by the telephone ringing at the wrong time.

Indeed because you don’t expect the code to work (after all it’s never been tested) I find you cast a very jaundiced eye over the code – and in the process find a plethora of the mistakes that one typically finds by sitting in front of a debugger. Maybe it’s just me, but I’d rather find bugs via code inspection than by fighting the debug environments common to most embedded systems.

So in a nutshell, I think the optimal way to write and test code is as follows:

1. Write the code. Make sure it compiles and is Lint free.
2. Wait a few months.
3. Reread the code looking for the usual suspects of bad / wrong comments, copy and paste errors, sloppy coding etc.
4. Test it.

The person that maintains your code (quite likely a future version of you) will thank you for doing it this way.