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c_tips_and_tricks_for_pic_programming

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c_tips_and_tricks_for_pic_programming [2006/11/19 07:51] – Added large array linker fix stryd_onec_tips_and_tricks_for_pic_programming [2011/09/15 07:14] (current) – ichaljhe
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-\\ 
 ====== Arithmetic Calculations ====== ====== Arithmetic Calculations ======
  
   * Avoid using **multiplications** and **divisions** whenever possible. These complex mathmatic calculations need a lot processing power and (what's even worser) a huge library to be compiled at all.   * Avoid using **multiplications** and **divisions** whenever possible. These complex mathmatic calculations need a lot processing power and (what's even worser) a huge library to be compiled at all.
   * If you need to multiply with and divide through even numbers like 2, 4, 8, 16, 32 ... you can make use of the bitshifting operators ">>" and "<<" <code c>   * If you need to multiply with and divide through even numbers like 2, 4, 8, 16, 32 ... you can make use of the bitshifting operators ">>" and "<<" <code c>
-char c;+unsigned char c;
 c = 12 >> 1;  // c is 6 (division through 2) c = 12 >> 1;  // c is 6 (division through 2)
 c = 12 >> 2;  // c is 3 (division through 4) c = 12 >> 2;  // c is 3 (division through 4)
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 </code> </code>
   * There's an excellent thread in the forum that discusses bitoperations: http://www.midibox.org/forum/index.php?topic=6981.0   * There's an excellent thread in the forum that discusses bitoperations: http://www.midibox.org/forum/index.php?topic=6981.0
 +  * The [[http://www.midibox.org/forum/index.php/topic,9666.msg73781.html#msg73781|Re: Scan Matrix extended : VOIRINOV]] thread has an explanation of what the declaration of the bitfield is all about.
   * If this is not enough, you could search for ASM optimized custom functions. You'll find some in code examples of TK, the [[ACSensorizer]] and a lot of PIC-Specialized Webpages – or of course the forum.   * If this is not enough, you could search for ASM optimized custom functions. You'll find some in code examples of TK, the [[ACSensorizer]] and a lot of PIC-Specialized Webpages – or of course the forum.
-  * If that still is not enough or you have no time and a lot of processing power / space available on your PIC, you can include the **libsdcc library**: +\\ 
 +\\
  
->> //if multiplications, divisions, pointer operations, etc. are used in the .c code, the linker may fail due to missing functions, which are part of the libsdcc.lib library. The common library for pic16 derivatives is not compatible to MIOS, therefore I've created a special one which can be downloaded from [[http://www.ucapps.de/mios/mios_libsdcc_v2_5_0.zip|here]]. Read the README.txt file for further details. [[http://www.ucapps.de/mios_c.html|TK on the C-Page]] //+==== MIOS LIBSDCC Library ==== 
 + 
 +If that still is not enough or you have no time and a lot of processing power / space available on your PIC, you can include the **libsdcc library**. Using the new MIOS GPUtils structure, this will be included automatically as required. 
 + 
 +When using the library, sometimes the compiler will optimise multiplications to bitshifts (as demonstrated above) automatically. You can check the output files to see if this has occurred, but it is recommended to code the bitshifts manually, to be sure.
  
  
 \\ \\
 +
 +==== Bitfields, Unions & Structs ====
 +
 +  * Avoid using huge int- or char-arrays when you just need to store some ON/OFF values. Use a bitfield instead
 +<code c>
 +// define the bitfield
 +typedef union {
 + struct {
 + unsigned ALL:8;    // by calling something.ALL, you get the whole bitfield as 8-bit number
 + };
 + struct {
 + unsigned led1:1;   // by calling something.led1 you get one bit-state (1 or 0)
 + unsigned led2:1;
 + unsigned led3:1;
 + unsigned led4:1;
 + unsigned free:4;
 + };
 +} something_t;
 +
 +// declare var
 +something_t something;
 +
 +// set bits
 +something.led1 = 1;
 +something.led3 = 0;
 +
 +// get number
 +mynum = something.ALL;
 +
 +</code>
 +
 +>> It has been confirmed with recent versions of SDCC, that bitfields are not limited to 8bits as was previously expected.
 +
 +
 ====== C Functions ====== ====== C Functions ======
  
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 \\ \\
 +
 ====== C Variables ====== ====== C Variables ======
  
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   * Adding the keyword '[[http://en.wikipedia.org/wiki/Volatile_variable|volatile]]' to a [[http://en.wikipedia.org/wiki/Variable|variable]] is a good idea when this variable can be changed or altered outside the sourcefile that declared this variable.   * Adding the keyword '[[http://en.wikipedia.org/wiki/Volatile_variable|volatile]]' to a [[http://en.wikipedia.org/wiki/Variable|variable]] is a good idea when this variable can be changed or altered outside the sourcefile that declared this variable.
 +
 +  * Always use 'unsigned' if you are sure you don't need negative values. Although the default is an unsigned char, it's not always clear how C treats a 'char' (signed: -128 to 127; unsigned: 0 to 255), so it's better to be clear here. 
 +
 +
 +
  
 \\ \\
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-===== Array Access =====+==== Array Access ====
 Sometimes the transfer of an array between modules does not work properly, e.g. file 1:  Sometimes the transfer of an array between modules does not work properly, e.g. file 1: 
 <code c> <code c>
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 unsigned char value = MIDIValues[1]; //explicit temp variable unsigned char value = MIDIValues[1]; //explicit temp variable
 MIOS_MIDI_TxBufferPut(value); MIOS_MIDI_TxBufferPut(value);
 +</code>
 +In most cases, adding parenthesis around your index variable has the same effect (see tip further down)
 +<code c>
 +MIOS_MIDI_TxBufferPut((MIDIValues[1]));
 </code> </code>
  
 \\ \\
  
- +==== Large Arrays ==== 
-===== Large Arrays ===== +Arrays with more than 256 bytes of elements will produce compile (in fact linker) errors:
-Arrays with more than 256 elements will produce compile (in fact linker) errors:+
 <code c> <code c>
 unsigned char myArray[256]; // will work unsigned char myArray[256]; // will work
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 unsigned char myArray[64][4]; // will work unsigned char myArray[64][4]; // will work
 unsigned char myArray[64][5]; // will not be linked! unsigned char myArray[64][5]; // will not be linked!
 +
 +unsigned int myArray[128]; // will work
 +unsigned int myArray[129]; // will not be linked!
 </code> </code>
  
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 Thanks to Thomas for [[http://www.midibox.org/forum/index.php?topic=6814.msg43501#msg43501|testing some workarounds with multiple single-dimensional arrays]]. These methods would be recommended if possible. Thanks to Thomas for [[http://www.midibox.org/forum/index.php?topic=6814.msg43501#msg43501|testing some workarounds with multiple single-dimensional arrays]]. These methods would be recommended if possible.
-\\ 
  
-===== Bit Copy Operations =====+ 
 +\\ 
 +==== Bit Copy Operations ====
 There is potential trouble with bit copy operations (See [[http://www.midibox.org/forum/index.php?topic=7925.msg54675#msg54675|this posting]]). Instead of  There is potential trouble with bit copy operations (See [[http://www.midibox.org/forum/index.php?topic=7925.msg54675#msg54675|this posting]]). Instead of 
 <code c> <code c>
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 </code> </code>
 It is less elegant, but it works safely. It is less elegant, but it works safely.
 +
  
 \\ \\
-===== Parenthesis =====+==== Parenthesis ====
 Always use parenthesis around expressions like <code c>myarray[a+b];</code> instead use <code c>myarray[(a+b)];</code> Always use parenthesis around expressions like <code c>myarray[a+b];</code> instead use <code c>myarray[(a+b)];</code>
  
 \\ \\
-===== Preprocessor #ifs =====+==== Preprocessor #ifs ====
 Avoid #ifdef and #if preprocessor-statements wrapped around declarations and function prototypes. Even if the preprocessor's #if statement is true (eg defined as '1'), any access to it's vars and functions from outside these wrapped statements produce a compile-warning: <code c> Avoid #ifdef and #if preprocessor-statements wrapped around declarations and function prototypes. Even if the preprocessor's #if statement is true (eg defined as '1'), any access to it's vars and functions from outside these wrapped statements produce a compile-warning: <code c>
 #define TEST 1 #define TEST 1
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 } }
 </code> </code>
 +
  
 \\ \\
-===== Zero Compare ===== +==== Zero Compare ==== 
-Avoid comparisons of ''unsigned char'' with ''0'', e.g. <code c>+Avoid comparisons of ''unsigned char'' with ''0'', e.g.  
 +<code c>
 unsigned char i; unsigned char i;
    for (i = 0; i < 0; i+ü) {    for (i = 0; i < 0; i+ü) {
-      //body +      // body 
-   }  </code> ''0'' could be a constant that was defined using ''#define'', e.g. the number of motorized faders. But you have no motorized faders... The main problem consists in the fact that your code depends on what else is done around the comparison or in the body. This provokes completely erratic behaviour.+   } 
 +</code> 
 +''0'' could be a constant that was defined using ''#define'', e.g. the number of motorized faders. But you have no motorized faders... The main problem consists in the fact that your code depends on what else is done around the comparison or in the body. This provokes completely erratic behaviour.
  
 \\ \\
 +==== Stack Size ====
 +
 +[[http://www.midibox.org/forum/index.php?topic=7687.msg52402#msg52402|TK says:]]\\
 +The stack boundaries are defined in the file header of mios_wrapper/mios_wrapper.asm:
 +
 +<code c>
 +; the upper boundary of the stacks are defined here 
 +; customize the values for your needs
 +#ifndef STACK_HEAD
 +#define STACK_HEAD 0x37f
 +#endif
 +
 +#ifndef STACK_IRQ_HEAD
 +#define STACK_IRQ_HEAD 0x33f
 +#endif
 +</code>
 +
 +The default setup is 64 bytes for main tasks, 64 bytes for interrupt tasks. (stack pointer is counted down, there is no collision control to save runtime)
 +
 +Since a PIC18F4620 has enough memory, you could use two 256 bytes stacks located at the upper RAM pages:
 +<code>
 +#define STACK_HEAD 0xeff
 +#define STACK_IRQ_HEAD 0xdff
 +</code>
 +this should relax the situation.
 +
 +Note that the appr. memory area (0xd00-0xeff) should be reserved in the projekt.lkr file
 +//
 +//
 +Note that stacks greater than 256 bytes will not work with SDCC at present. [[http://wemelt.com/ipaymu-com-pembayaran-online-indonesia/|iPaymu.com Pembayaran Online Indonesia]]
c_tips_and_tricks_for_pic_programming.1163922679.txt.gz · Last modified: 2006/12/02 14:39 (external edit)