C in Embedded Systems and the Microcontroller World
Contents
1. C in Embedded Systems and the Microcontroller World ABSTRACT Cis becoming the higher level language of choice for micro controller programming Traditional usage of C depends on assembly language for the intimate interface to the hard ware A few extensions to ANSI C allow embedded systems to connect directly and simply using a single language and avoiding detailed knowledge of the compiler and hardware connections HIGHER LEVEL LANGUAGE USAGE The desires leading to the greater use of higher level lan guages in microcontrollers include increased programmer productivity more reliable programs and portability across hardware Few such languages have served well when re quired to manipulate hardware intimately because most have been for mathematical computation The C language has always been close to machine level Indeed Kernighan and Ritchiel1 refer to it as not really a higher level lan guage one view of C is as a higher level syntax expressing PDP 11 assembly language C has gained a great deal of its reputation and popularity associated with its use for operating systems specifically UNIX 2 and similar systems Many languages will do well enough for the application and utility programs of such a system but being appropriate for the kernel indicates C can probably do the job of hardware control in an effective man ner The needs of an embedded system however are not identi cal to the environment from which C ha2. Direct Access to Hardware Addresses Access to hardware addresses is improper in computation programs is unusual in utility programs is infrequent in op erating systems and is the raison d etre of microcontrollers The normal means of accessing hardware addresses in C is via constant pointers This is adequate if not great when the accesses are minimal For example struct HDLC_registers j define HDLC_1 struct HDLC_registers 0x01a0 allows reference to a structure of HDLC device registers at address 0x01a0 but never actually creates the entity of such a structure If a debugger were asked about HDLC_1 it would not recognize the reference If many registers and devices are involved it becomes a problem to be handled by the programmer not his tools If the debugger tries to read the source for preprocessor statements it adds signifi cant complexity Another way of doing it is struct HDLC_registers E extern struct HDLC registers HDLC_1 and providing an external file defining the address of HDLC__1 written in assembly language This is clean and does create the actual entity of a structure at the address but has required an escape to assembly language for the system although only at the system definition level This was the first choice at National and retains merit because the use of macros in the definition file allows the simple creation of a table exactly like the table in the hardware manual
3. pro duction quantities are measured in hundreds and thou sands so price is a price performance issue Embedded systems end up tucked away in some of the strangest and tiniest places so size can be a success or failure issue quantities are often tens of thousands to millions of units so additional chips or costs are multiplied ferociously and be come a bottom line issue The computer systems for which C was originally developed were relatively small and not especially sophisticated How ever as systems have grown C and its implementation has grown right along with them Most computer systems for which C is used now involve high speed processors with large memory caches to huge memory spaces backed by virtual memory Many have large register sets Such linear memory with heuristic accelerators allow for very large pro grams and fast execution A major effort in optimization is in the allocation and usage of the registers which tend to be general purpose and orthogonally accessible Such sys tems processor chips and compilers compete almost ex clusively in the field of speed Embedded systems and most especially microcontrollers have a different nature While some applications may add external devices and memories to the controller many are meant to be fully self contained on one chip or have at most a few I O chips Microcontroller systems are small are of ten required to fit in a physically small space and are usual ly f
4. either signed or unsigned floating point are offered as float of double both using 32 bit IEEE format All data types storage classes and modifiers are supported All operators are supported and anachronisms have been eliminated as per the standard Structure assignment structure arguments and structure functions are supported Forward reference functions and argument type checking are supported Assembly code may be embedded within C programs be tween special delimiters See Table CCHPC SPECIFICATIONS TABLE Note Extensions are boldface Name length 32 letters 2 cases Numbers Integer Signed and Unsigned 16 32 Bits Short and Long 16 bits and 32 bits Floating Single and Double 32 bits and 32 bits Data Types Arrays Strings Pointers Structures Preprocessor include define define undef if ifdef ifndef if defined else elif endif Declarations auto register const volatile BASEPAGE static static global static function NOLOCAL INTERRUPTn ACTIVE extern extern global extern function char short int long signed unsigned float double void struct union bitfield enum pointer to array of function returning type cast typedef initialization Statments See e expression assignment structure assignments while do while for 3 33 3 loop 5 3 if else switch 3 case default switchf return break continue goto 3 srst Operators
5. primary function array Struct_union struct_pointer gt unary amp sizeof typecast arithmetic A lt lt gt gt relational gt SS PSs Se lS boolean k amp amp assignment t gt gt lt lt BH s misc PE Functions arguments Numbers Pointers Structures return values Numbers Pointers Structures forward reference argument checking Library Definition Limited Freestanding environment Embedded Assembly Code C in Embedded Systems and the Microcontroller World AN 587 CONCLUSIONS With the right extensions the right implementations and the right development environment National is providing its customers with a C compiler tool which allows effective higher level language work within the restrictive require ments of embedded microcontrollers Productivity increases do not have to come at the expense of larger programs and more memory chips No strangeness has been added to the language to cause reliability problems Portability has been retained Assembly language code has been eliminated as the chewing gum and baling wire trying to hold it all togeth er further increasing reliability and portability LIFE SUPPORT POLICY FOOTNOTES 1 Kernighan Brian W and Ritchie Dennis M The C Pro gramming Language Prentice Hall 1978 Pages ix and 1 2 UNIX is a registered trademark of AT amp T 3 Produced by Bit Slice Software Waterloo Onta
6. which has led to some exploration of these issues The needs can be sum marized as Compatibility Direct Access to Hardware Addresses Direct Connection to Interrupts Optimization Considerations Development Environment Re Entrancy Compatibility The first consideration for any such adaptation MUST be compatibility Any attempt to create a different language or another dialect of C will create more problems than using C will solve Dialects create problems in portability mainte nance productivity and possibly reliability A programmer used to working in C will be tripped up by every little gotcha in a dialect everyone will be tripped up by a different lan guage Providing extensions to the language while maintaining compatibility and not creating a new dialect is accom plished by using the C Pre Processor By carefully choosing the extensions and their syntax the use of the preproces sor s macro capability allows them to be discarded for nor mal C operation with non extended compilers By carefully choosing their semantics the elimination of the extensions does not render the program invalid just less effective Within these considerations there should be no unneces sary additions An extension should not be made to avoid the optimizer s having to work hard An extension should be made only to give the user an ability he would not have without it or to tell the compiler something it cannot figure out by itself
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8. What is desirable so that the user can do his own defini tions without resorting to two languages is a means to cre ate the entities and define the addresses of those entities a simple means of saying that this variable or constant is at a specific absolute address The syntax struct HDLC_registers HDLC_1 0x0la0 would be excellent as an official enhancement to the lan guage since the parses like the for an initialization and the program shouldn t initialize a hardware register this way like a variable However this violates the compatibility rule for an extension since the preprocessor cannot throw away the address following the character Therefore struct HDLC_registers HDLC_1 At 0x01a0 is a much more practical form as an extension and can be made to expand to the previous or any other form if it is ever added as an enhancement to the language The result ing forms volatile struct HDLC_registers HDLC_1 At 0x0la0 volatile struct HDLC_registers HDLC_2 At Ox01bO volatile const int Input_Capture_3 At 0x0182 are straightforward simple readable and intuitively under standable and provide the data item definitions as desired Direct Connection to Interrupts Operating systems attach to interrupts in one centralized controlled location and manage them all in that module Em bedded systems attach to varied interrupts for a variety of purposes and frequently the different interrupt routine
9. ed small amounts of power Even when the system is externally expanded the memories provided on chip are significantly faster than the external memories because of buss driving The total addressing space is usually very limit ed 32k 64k with expansion not linear The registers in microcontrollers are usually a limited number of special pur 1995 National Semiconductor Corporation TL DD10348 RRD B30M75 Printed in U S A POM 49 04 U0D0191 N BU pue SWUIa SAS peppequg Ul 9 Z8S NV pose registers thus eliminating orthogonal usage Speed is only one of many considerations in the microcontroller com petition Cost package size power consumption memory size number of timers and I O count are very important considerations Embedded Systems Higher level languages will achieve the goals of program mer productivity program reliability and application portabil ity only if they fit the target environment well If not produc tivity will disappear into work arounds and maintenance reli ability will be lost to kludges and portability will not exist DESIRED TRAITS IN C FOR MICROCONTROLLERS The environment in which C has developed is not the same as the embedded microcontroller world What changes or extensions or implementations of C will provide the means to adapt the language National Semiconductor Microcon troller Division has a compilerl3 developed for the 16 bit High Performance Controller HPCT I4
10. es The National compiler provides an invocation line switch to indicate that all string literals but not string variables can be kept in ROM rather than being copied to RAM on system start up Such strings can be merged in the ROM space to eliminate duplication of strings An extension to the language to identify functions which will not be used recursively is NOLOCAL straight_forward_function which causes all local variables to be converted to static variables which are easier and faster to access and use If the function has no arguments the compiler can even elimi nate the use and creation of the Frame Pointer for the func tion saving additional code and time The particular processor the HPC has a special form of subroutine call Since the optimizer cannot guess across modules which functions should be called with the special form the extension ACTIVE specially_called_function arg was added This may or may not be appropriate for other processors but is a good example of why the language needs careful extensions to take advantage of different processors One command extension was added to the language be cause it allows the programmer to guarantee something the optimizer cannot usually determine The form switchf value provides for a switch case statement without a default case When speed and size become critical the extra code required to validate the control value and process the de fault is highl
11. n board for the PC running UNIX V 3 and some other versions of UNIX The demand has been for the PC version the PC is a very good workstation environment for microcontrollers Other environments may be desirable but the PC is first Re Entrancy Even with all these other considerations handled there is a time bomb lurking in C on microcontrollers C is a single thread synchronous language as it is usually implemented Since most utilities are strictly single thread and the UNIX kernel forces itself into a single thread this is not a big prob lem for them Embedded systems involving controllers are inherently asynchronous the language in which they are im plemented must be multi thread without special rules and exception cases The passing of arguments on the stack and the returning of values in registers allow for complete re entrancy and thus asynchronous multi threading but this breaks down when structures are returned Most implementations of C use a static structure to contain the returned value and actually return a pointer to it the compiler generates the code to access the returned structure value as required This cannot be used in a microcontroller environment because if an in terrupt occurs during the time the static structure is being used it cannot re enter the function On an operating sys tem level such conflicts can be managed with gates sema phores flags or the like but that solution is completely in appro
12. priate on the language level Turning the interrupts off is similarly not a language level concept and is impossible on a system with a NonMaskable Interrupt Telling users not to get themselves into that situation is crippling at best im possible to enforce and extremely difficult to track down and correct The solution should be at the language level and should allow the return of a structure without hindering re entrancy The author s solution developed with National has been to have the code calling the function provide the address of a structure in which to build the return value Since this is frequently on the caller s stack and is never invisibly static the program has no hidden re entrancy flaws The HPC C Compiler The HPC C Compiler CCHPC is a full and complete imple mentation of ANSI Draft Standard C Feb 1986 for free standing environment Certain additions take advantage of special features of the HPC for the specific needs of micro controllers The extensions include the support of two non standard statement types loop and switchf non standard storage class modifiers and the ability to include assembly code in line The compiler supports enumerated types passing of structures by value functions returning struc tures function prototyping and argument checking Symbol Names both internal and external are 32 charac ters Numerics are 16 bit for short or int 32 bit for long and 8 bit for char all as
13. rio Cana da ADDITIONAL INFORMATION Datasheet HPC Software Support Package User s Manual HPC C Compiler Users Manual 424410883 001 Lit 100587 NATIONAL S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which a are intended for surgical implant into the body or b support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness National Semiconductor Japan Ltd Sumitomo Chemical Engineering Center National Semiconductor GmbH Livry Gargan Str 10 D 82256 Furstenfeldbruck National Semiconductor Corporation 2900 Semiconductor Drive P O Box 58090 N Santa Clara CA 95052 8090 Germany Bldg 7F Tel 1 800 272 9959 Tel 81 41 35 0 1 7 1 Nakase Mihama Ku TWX 910 339 9240 Telex 527649 Chiba City Ciba Prefecture 261 Tel 043 299 2300 Fax 043 299 2500 Fax 81 41 35 1 National Semiconductor Hong
14. s are in different modules with associated routines for each pur pose It is possible to do this with another escape to assem bly language but this requires that the system be main tained and enhanced in two languages The solution chosen for the National compiler is to provide an identifier for functions which are to service interrupts These functions obviously take no arguments and return no values so they are worth considering as special The syntax chosen was simply INTERRUPT2 timer_interrupt although a more desirable form as an official enhancement would be INTERRUPT type interrupt_service_routine because the chosen syntax can be preprocessed into what ever might be the final form The semantics of the interrupt function were more difficult to guarantee for the future should an interrupt function be callable by the other func tions Prohibiting it allows eventually permitting it if neces sary for improved efficiency the National compiler does not allow an interrupt function to serve as anything other than an interrupt service routine although one function can be attached to several interrupts Because the functions are special purpose the function en try and exit code can be dedicated to interrupt entry and exit rather than having to hide it in a separate library mod ule The National compiler actually generates the interrupt vector to point directly to the interrupt function the function
15. s come This war rants looking at C as it is and comparing it to the needs of C for the microcontroller world Operating Systems vs Embedded Systems In most non embedded programs it is the processing which is important and the Input Output is only to get the data and report the results In embedded or realtime applica tions it is the Input Output which is vital and the processing serves only to connect inputs with outputs Operating systems are actually not as closely tied to the hardware as they might appear initially and those portions which are close are not very portable Operating systems manipulate hardware registers primarily for memory man agement to map tasks task process switching to activate tasks interrupt response to field requests and device drivers to service requests Because memory manage ment hardware is so different between systems because task process changing is so contingent on processor opera tions and compiler implementations because interrupt sys tem behavior is so varied and because device control is so dependent on architecture and busses these particular as pects of the operating system are not concerned with porta bility As a result they are generally kept separate use a less convenient form of C depending on constants and fre quently are implemented in assembly language This is not a major problem since they comprise only a small portion of the total system and have to change any
16. saves and restores the registers which it may destroy La tency is minimized Interrupt response speed latency and interrupt system per formance are important characteristics of a microcontroller It is one thing inconvenient or embarrassing for a multi MIPS machine to choke on long 9600 baud transmissions and drop a character or two because of inefficient interrupt response It is another thing entirely lethal a total failure for an embedded system s interrupt response to be so poor as to miss even one critical interrupt Optimization Considerations Computer systems compete on speed or at least MIPS rat ings compilers for them must be speed demons Microcon trollers compete on size and costs compilers for them must be frugal Embedded systems are limited in their memory and different memories frequently have significantly differ ent behavior The major concern of optimization comes down to code size In most controller systems as generated code size decreases speed usually increases The effort in the code generation and optimization should be directed towards re ducing code size Claims for exactly how close the generat ed code gets to hand written assembly code depend on specific benchmarks and coding techniques An acceptance criterion for the National HPC compiler was code size com parison on a set of test programs A level slightly below 1 4 times larger than assembly was reached In addition to the implemen
17. tation of the optimization other concerns of microcontrollers affect the way code can be generated An example is the different forms of memory Many controllers have memories which can be accessed by faster or shorter code Certain variables should be placed in these memories without all the variables of a module going there which is a linker process There is no possible way for the optimizer to guess which variables should go there especially in a multiple module program so it must be told The syntax used is Static BASEPAGE int important_variable because the special memory in National s HPC is the first page of RAM memory Several other possibilities offer themselves including using for an official enhancement Static register int important_variable because currently static register variables are specifically prohibited This cannot be an extension because the regis ter word could not be redefined to the preprocessor If some variables need to be accessed by fast code and some need to be accessed by short code and if the two were mutually exclusive it would be desirable to have two separate exten sion words Since such hardware is unlikely the single word BASEPAGE is probably sufficient Additional savings can be achieved by reconsidering string literals The ANSI C requires that each string literal is a sep arate variable but in actual usage they are usually con stants and therefore need not be separate nor variabl
18. way each time the system is ported HPC is a trademark of National Semiconductor Corporation National Semiconductor Application Note 587 David LaVerne March 1989 Embedded systems by their very nature are closely tied to the hardware throughout the system The system consists of manipulating the hardware registers with varying amounts of calculation and data transformations inter spersed with the manipulations As the system gets larger the calculations may get more complex and may become a larger share of the program but it is still the hardware oper ations which are the purpose of the system Because the system in which these hardware pieces reside consists mostly of these hardware pieces it is reasonable to hope for portability across processors or controllers for an appli cation or product Attempting to isolate all of the hardware operations is often impractical using inconvenient forms of C is troublesome throughout the system and throughout its life cycle and implementing them in assembly language de feats the advantages of higher level language usage and eliminates portability for those and related portions For embedded systems conveniently accessing hardware reg isters while doing calculations is essential Computer Systems vs Embedded Systems Computational systems generally can be down the cable and thus down the hall from where they are used and can be whatever size is necessary to get the performance
19. y undesirable when the user s code has already guaranteed a good value The National compiler has one extension which violates the issues stated under compatibility It remains for historical reasons It is a command loop number which produces a shortened form of the for loop without an accessible index This does not provide the user with any new ability it merely allows the compiler optimizer to know without figuring out that the index is not used inside nor outside the loop and can therefore be a special counted form The preprocessor cannot produce an exact semantic equivalent for the statement This is a perfect example of a poor extension and will eventually be eliminated Development Environment Languages developed for large or expensive systems can usually depend on large systems for development support either self hosted or with a large system host providing cross development tools Microcontrollers are often price sensitive are frequently in the laboratory or the field and are not always supported by a large system as a develop ment host Personal computers provide an excellent plat form for the entire suite of development tools National Semiconductor currently provides its compiler and associated cross development programs on the IBM PC and clone type of computer The software is all very porta ble and can be run under VAX VMS VAX Ultrix or VAX BSD4 2 and on the NSC 32000 based Opus add i
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