Microcontroller VU

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1 Microcontroller VU Kyrill Winkler SS 2014 Featuring Today: Structured C Programming

2 Weekly Training Objective Already done C demo program Floating point operations Interrupts Generating periodic signals This week Interrupt & callback demo Input capture UART receiver UART sender Keypad Next week PWM signals and glitches TWI (I 2 C) Button debouncing LCD 137

3 138 Follow-up from last time: My dear compiler int o = y + (z ^ 255); What does (z ^ 255) do?

4 138 Follow-up from last time: My dear compiler int o = y + (z ^ 255); What does (z ^ 255) do? It depends.

5 138 Follow-up from last time: My dear compiler int o = y + (z ^ 255); What does (z ^ 255) do? It depends. What data-type does z have? Lets assume it is int, then it has... bits!

6 138 Follow-up from last time: My dear compiler int o = y + (z ^ 255); What does (z ^ 255) do? It depends. What data-type does z have? Lets assume it is int, then it has 16 bits!

7 138 Follow-up from last time: My dear compiler int o = y + (z ^ 255); What does (z ^ 255) do? It depends. What data-type does z have? Lets assume it is int, then it has 16 bits! So the snipplet negates the lower 8 bits of z.

8 138 Follow-up from last time: My dear compiler int o = y + (z ^ 255); What does (z ^ 255) do? It depends. What data-type does z have? Lets assume it is int, then it has 16 bits! So the snipplet negates the lower 8 bits of z. The size of int is varies across toolchains/architectures! Therefore be explicit and use uint8 t or uint16 t etc.!

9 139 While on the topic of variable declarations... b o o l t i m e o u t = f a l s e ; ISR (MY TIMER INTERRUPT, ISR NOBLOCK){ t i m e o u t = t r u e ; i n t main ( void ) { i f ( t i m e o u t == t r u e ) { t u r n o n l e d s ( ) ;

10 139 While on the topic of variable declarations... b o o l t i m e o u t = f a l s e ; ISR (MY TIMER INTERRUPT, ISR NOBLOCK){ t i m e o u t = t r u e ; i n t main ( void ) { i f ( t i m e o u t == t r u e ) { t u r n o n l e d s ( ) ; On some versions of the toolchain the if-statement will be interpreted as always false!

11 140 The volatile keyword b o o l v o l a t i l e t i m e o u t = f a l s e ; ISR (MY TIMER INTERRUPT, ISR NOBLOCK){ t i m e o u t = t r u e ; i n t main ( void ) { i f ( t i m e o u t == t r u e ) { t u r n o n l e d s ( ) ; volatile prevents the compiler from optimizing timeout, in particular it won t assume that timeout is never changed.

12 141 The const keyword const is used to declare variables that are written to only once (more on that later)

13 141 The const keyword const is used to declare variables that are written to only once (more on that later) C allows using pointers as output parameters

14 141 The const keyword const is used to declare variables that are written to only once (more on that later) C allows using pointers as output parameters You should declare whether this is your intention or not: myfunc(uint8 t *str) myfunc(const uint8 t *str).

15 142 Side effects Avoid side effects! Never use printf("%i", i++) etc.

16 142 Side effects Avoid side effects! Never use printf("%i", i++) etc. Be aware of short-circuit evaluation: if (a!= 0 && myfunc(b)!= 0)

17 142 Side effects Avoid side effects! Never use printf("%i", i++) etc. Be aware of short-circuit evaluation: if (a!= 0 && myfunc(b)!= 0) Depending on value of a, myfunc(b) may or may not be called!

18 143 Structured C Programming 2 important pradigms: background (BG) tasks vs. interrupt service routines (ISRs) callback functions

19 143 Structured C Programming 2 important pradigms: background (BG) tasks vs. interrupt service routines (ISRs) callback functions Helpful extensions ( -std=c99 ) <util/atomic.h> <stdbool.h> useful macros for interrupts, eeprom, sleepmode, crc, baudrate, watchdog,... detailed documentation: avr-libc manual

20 144 Backgrounding Basic structure of a C-program: i n t main ( void ){ i n i t ( ) ; // c a l l s i n i t i a l i z t i o n s o f used modules f o r ( ; ; ) { background ( ) ; // c a l l s background t a s k s s l e e p ( ) ; Every interrupt executes the background code once.

21 145 Backgrounding Basic structure of a background task: void background ( void ){ i f ( mystate == i n i t ){ b g t a s k p l a y I n t r o M u s i c ( ) ; b g t a s k s h o w I n t r o A n i m a t i o n ( ) ; e l s e i f ( mystate == g o a l ){...

22 145 Backgrounding Basic structure of a background task: void background ( void ){ i f ( mystate == i n i t ){ b g t a s k p l a y I n t r o M u s i c ( ) ; b g t a s k s h o w I n t r o A n i m a t i o n ( ) ; e l s e i f ( mystate == g o a l ){... Beware of monopolizing background tasks! Animation will be played after bgtask playintromusic returned.

23 146 Backgrounding Implementing a basic scheduler: void background ( void ){ w h i l e ( t h e r e i s s o m e t h i n g t o d o ( ) ) { i f ( mystate==i n i t ){ i f (! done music ) done music=b g t a s k p l a y I n t r o M u s i c ( ) ; i f (! done anim ) done anim=b g t a s k s h o w I n t r o A n i m a t i o n ( ) ; e l s e i f ( mystate==g o a l ){...

24 Callback functions 147

25 147 Callback functions 1 v o i d ( m y C a l l b a c k F u n c t i o n ) ( u i n t 8 t ) ; // g l o b a l f u n c t i o n p o i n t e r v a r i a b l e 2 3 ISR (MY TIMER INTERRUPT, ISR NOBLOCK){ 4 / e x e c u t e t h e c a l l b a c k f u n c t i o n / 5 mycallbackfunction ( 1 ) ; 6 7 v o i d s e t c a l l b a c k ( v o i d ( c a l l b a c k ) ( u i n t 8 t )){ 8 m y C a l l b a c k F u n c t i o n = c a l l b a c k ; 9 10 v o i d myfunc ( u i n t 8 t i ){ i n t main ( v o i d ){ 15 s e t c a l l b a c k ( myfunc ) ; // r e g i s t e r myfunc as c a l l b a c k f u n c t i o n 16

26 148 ATMega1280 s memories 128 KB program memory (flash) Biggest memory in ATMega1280 Great for storing constants 8 KB internal RAM RAM size is quite restricted Beware of stack overflow 4 KB EEPROM Useful for storing values that rarely change Limited number of read/write cycles

27 149 Using ATMega1280 s memories in C Memory sections available:.text: program memory, flash.data: RAM and flash.bss: RAM.eeprom The.data memory section contains constants and variable initializations; it occupies both RAM and flash space. RAM is volatile and therfore not programmable (before run-time). Therefore: put constant values into flash alone! No need to occupy twice the required memory! How? See Section 5 of the avr-libc manual.

28 150 The.initN sections (section 4.6 avr-libc).init0 Weakly bound to init(). If user defines init(), it will be jumped into immediately after a reset..init1 Unused. User definable..init2 In C programs, weakly bound to initialize the stack, and to clear zero reg (r1)..init3 Unused. User definable..init4 For devices with > 64 KB of ROM,.init4 defines the code which takes care of copying the contents of.data from the flash to SRAM. For all other devices, this code as well as the code to zero out the.bss section is loaded from libgcc.a..init5 Unused. User definable..init6 Unused for C programs, but used for constructors in C++ programs..init7 Unused. User definable..init8 Unused. User definable..init9 Jumps into main().

29 151 Using.initN sections Applying attributes to functions v o i d m y i n i t p o r t b ( v o i d ) a t t r i b u t e ( ( naked, used, s e c t i o n (. i n i t 3 ) ) ) ; v o i d m y i n i t p o r t b ( v o i d ) { PORTB = 0 x f f ; DDRB = 0 x f f ;

30 152 Benchmarking memory usage After compilation, the static memory usage of your program can be analyzed via avr-size. Analyzing the dynamic memory usage can be done by (1) Code analysis (requires bounding recursion depth and heap usage). (2) Run-time analysis directly on the µc or via simulation (needs a software that monitors memory access).

31 153 Dynamic memory benchmark example v o i d m e m s t a t I n i t ( v o i d ) a t t r i b u t e ( ( naked, used, s e c t i o n (. i n i t 3 ) ) ) ; e x t e r n u i n t 8 t end ; // end o f b s s // n e v e r c a l l t h i s f u n c t i o n! v o i d memstatinit ( ) { r e g i s t e r u i n t 1 6 t i ; f o r ( i =( u i n t 1 6 t )& end ; i<ramend; i ++) { ( u i n t 8 t ) i = ( u i n t 8 t ) i ;

32 154 Dynamic memory benchmark example 1 m em s tat t memstatgetstat ( v o i d ) { 2 s t a t i c m e m s t a t t s t a t ; 3 u i n t 1 6 t i ; 4 u i n t 8 t u n i n i t ; 5 6 s t a t. maxstacksize = 0 x f f f f ; 7 s t a t. maxheapsize = 0 x f f f f ; 8 // search end of stack, s t a r t at SP and go down // search end of heap, s t a r t at bss end and go up 11 u n i n i t = 0 ; 12 f o r ( i =( u i n t 1 6 t )& end ; i<ramend; i ++) { 13 i f ( ( u i n t 8 t ) i == ( u i n t 8 t ) i ) { 14 u n i n i t ++; 15 i f ( u n i n i t == 10) { 16 s t a t. maxheapsize = i ( u i n t 1 6 t )& end 9 ; 17 break ; e l s e { u n i n i t = 0 ; r e t u r n &s t a t ; 22

33 155 Linker scripts (in the lab) located at /usr/avr/lib/ldscripts/ Mapping of virtual memory adresses for the ELF-file Very powerful!

34 156 Bit tricks Memory constraints demand efficient programming; in particular, using a byte to store a single bit (flag) is infeasible. Thus: pack eight bits in a byte using techniques that efficiently pack and upack bytes (union, struct). Other bitwise techniques also come in handy, e.g. swap contents of two registers without using a temp register.

35 157 Struct/union example RTC Config Register: typedef s t r u c t { u i n t 8 t r s : 2 ; u i n t 8 t : 2 ; u i n t 8 t sqwe : 1 ; u i n t 8 t : 2 ; u i n t 8 t out : 1 ; d s s q w c o n f i g t ; typedef union { d s s q w c o n f i g t r e g s ; u i n t 8 t raw ; ds1307 sqw mem t ;

36 158 Summary Put constants into the flash (program) memory. Analyze the static and dynamic memory usage of your programs. Apply bitwise techniques. Read avr-libc manual.

37 Weekly Training Objective Already done C demo program Floating point operations Interrupts Generating periodic signals This week Interrupt & callback demo Input capture UART receiver UART sender Keypad Next week PWM signals and glitches TWI (I 2 C) Button debouncing LCD 159

CprE 288 Introduction to Embedded Systems Exam 1 Review. 1

CprE 288 Introduction to Embedded Systems Exam 1 Review http://class.ece.iastate.edu/cpre288 1 Overview of Today s Lecture Announcements Exam 1 Review http://class.ece.iastate.edu/cpre288 2 Announcements