Editing Embedded Programming Tips and Tricks for Beginners
Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.
The edit can be undone.
Please check the comparison below to verify that this is what you want to do, and then save the changes below to finish undoing the edit.
Latest revision | Your text | ||
Line 22: | Line 22: | ||
However care should be taken when using signed variables.<br> | However care should be taken when using signed variables.<br> | ||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
=Fast Multiplication= | =Fast Multiplication= | ||
Line 47: | Line 30: | ||
or<br> | or<br> | ||
''B = B << 8; (Will result in a multiply by 256)<br>'' | ''B = B << 8; (Will result in a multiply by 256)<br>'' | ||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
=Repeating Code Using Timer Overflow Interrupts= | =Repeating Code Using Timer Overflow Interrupts= | ||
Line 66: | Line 41: | ||
Such a system might look like this | Such a system might look like this | ||
− | |||
===AVR=== | ===AVR=== | ||
<pre> | <pre> | ||
− | #define | + | #define off 0 |
− | #define | + | #define on 1 |
− | volatile uint16_t clk1000ms= | + | volatile uint16_t clk1000ms=1; // initializing them to 1 insures there is an initial delay cycle when |
− | volatile uint16_t clk100ms= | + | volatile uint16_t clk100ms=1; // the code first starts, otherwise they would all happen at |
− | volatile uint8_t clk10ms= | + | volatile uint8_t clk10ms=1; // once during first run. |
ISR (TIMER0_OVF_vect) | ISR (TIMER0_OVF_vect) | ||
{ | { | ||
− | if (clk1000ms != | + | if (clk1000ms!=off) |
{ | { | ||
clk1000ms++; | clk1000ms++; | ||
− | if (clk1000ms >= 10001) clk1000ms= | + | if (clk1000ms>=10001) clk1000ms=off; |
} | } | ||
− | if (clk100ms != | + | if (clk100ms!=off) |
{ | { | ||
clk100ms++; | clk100ms++; | ||
− | if (clk100ms >= 1001) clk100ms= | + | if (clk100ms>=1001) clk100ms=off; |
} | } | ||
− | if (clk10ms != | + | if (clk10ms!=off) |
{ | { | ||
clk10ms++; | clk10ms++; | ||
− | if (clk10ms >= 101) clk10ms= | + | if (clk10ms>=101) clk10ms=off; |
} | } | ||
// Timer clock is 1mhz, timer is 8bit. | // Timer clock is 1mhz, timer is 8bit. | ||
Line 104: | Line 78: | ||
{ | { | ||
− | if (clk1000ms== | + | if (clk1000ms==off) |
{ | { | ||
// put code here to run every second | // put code here to run every second | ||
− | clk1000ms = | + | clk1000ms = on; |
} | } | ||
− | if (clk100ms== | + | if (clk100ms==off) |
{ | { | ||
// put code here to run 10 times a second | // put code here to run 10 times a second | ||
− | clk100ms = | + | clk100ms = on; |
} | } | ||
− | if (clk10ms== | + | if (clk10ms==off) |
{ | { | ||
// put code here to run 100 times a second | // put code here to run 100 times a second | ||
− | clk10ms = | + | clk10ms = on; |
} | } | ||
Line 129: | Line 103: | ||
You may have been told in computer programming courses that 'global variables' are very bad and to never use them. | You may have been told in computer programming courses that 'global variables' are very bad and to never use them. | ||
− | The reason for this is that | + | The reason for this is that in computer programming applications are quite complex and lots of global variables make things very hard to follow. |
This rule, while still true for embedded programming, is much more relaxed, especially with 8/16bit micros. | This rule, while still true for embedded programming, is much more relaxed, especially with 8/16bit micros. | ||
− | In fact it can work | + | In fact it can work in reverse and make your embedded code very unwieldy if you make every variable local and pass them around on almost every function call on a small project. |
Doing this also increases the number of cycles needed to execute your functions as the variables needs to be loaded/unloaded every function call. On a low MHz micro with lots of small functions this can slow your code down. | Doing this also increases the number of cycles needed to execute your functions as the variables needs to be loaded/unloaded every function call. On a low MHz micro with lots of small functions this can slow your code down. | ||
− | + | Therefor, in small-scale embedded programming, is perfectly acceptable to use global variables when the information is something most of your program needs access to. | |
It's a balancing act to decide if a variable is something that should be passed to a function or available to every function as a global variable. Usually however, it is quite obvious. | It's a balancing act to decide if a variable is something that should be passed to a function or available to every function as a global variable. Usually however, it is quite obvious. | ||
Line 144: | Line 118: | ||
<pre> | <pre> | ||
struct iodata { | struct iodata { | ||
− | volatile uint8_t p_dataready; //pin change | + | volatile uint8_t p_dataready; //pin change intterupt |
− | volatile uint8_t p_alarm; //pin change | + | volatile uint8_t p_alarm; //pin change intterupt |
− | volatile uint8_t p_pulsecount; //pin change | + | volatile uint8_t p_pulsecount; //pin change intterupt |
uint8_t s_tempsensor; //spi | uint8_t s_tempsensor; //spi | ||
Line 160: | Line 134: | ||
This global struct can then be used throughout your entire program to store and retrieve i/o data. | This global struct can then be used throughout your entire program to store and retrieve i/o data. | ||
− | In general, the more complex your embedded project is (and faster your clock is) the more you should limit your global variables. But | + | In general, the more complex your embedded project is (and faster your clock is) the more you should limit your global variables. But don't be afraid to use them when you think they make things easier and neater. |
=Common mistakes= | =Common mistakes= | ||
Line 166: | Line 140: | ||
==The Equals Sign== | ==The Equals Sign== | ||
− | One of the most common mistakes | + | One of the most common mistakes new (and old) programmers make in C is the following. |
<pre> | <pre> | ||
− | if (MyVariable=5) | + | if (MyVariable=5) DoMyFunction(); |
− | |||
− | |||
− | |||
</pre> | </pre> | ||
− | The mistake here is the single equals sign. A single equals sign is used for doing an 'assignment' | + | The mistake here is the single equals sign. A single equals sign is used for doing an 'assignment' eg MyVariable=5 <br> |
− | + | In the example above we want to do a 'comparison/relation' to produce a true/false answer for the if statement. | |
− | In the example above we | + | The operator for this is double equals. |
− | + | So the corrected code is | |
<pre> | <pre> | ||
− | if (MyVariable==5) | + | if (MyVariable==5) DoMyFunction(); |
− | |||
− | |||
− | |||
</pre> | </pre> | ||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− |