Hệ thống nhúng - Chương 5 pot

Flight plan • In this lesson we will review the basics of string declaration as an excuse to introduce the memory allocation techniques used by the MPLAB C30 compiler.. Memory space al

Hệ thống nhúng

Thạc sĩ Lê Mạnh Hải

Embedded Systems

Lesson 5 : Taking a look under the

hood

Motivation:

• Memory space allocation

• Program space visibility

• Investigating memory allocation

• Looking at the MAP

• Pointers

• The heap

• MPLAB® C30 memory models

Flight plan

• In this lesson we will review the basics of string

declaration as an excuse to introduce the memory

allocation techniques used by the MPLAB C30 compiler.

• The RISC architecture of the PIC24 poses some

interesting challenges and offers innovative solutions.

Preflight checklist

• This lesson will be performed exclusively with software tools including the MPLAB IDE, MPLAB C30 compiler and the MPLAB SIM simulator.

• Use the “New Project Set-up” checklist to create a new project called “Strings” and a new source file similarly called “strings.c”.

The flight

1 char s[5] = { 'H', 'E', 'L', 'L', 'O'};

2 char s[5] = "HELLO";

3 char s[] = "HELLO";

4 char s[6] = { 'H', 'E', 'L', 'L', 'O', '\0' };

The flight

char c; // declare c as an 8-bit signed integer

c = 'a'; // assign to it the value corresponding to 'a' in the ASCII table

c ++; // increment it it will represent the ASCII character 'b' now

Something wrong here

char s[15]; // declare s as a string of 15 characters

s = "Hello!"; // Error! This does not work!

copy the content of a string onto another:

strcpy( s, "HELLO"); // s : "HELLO"

append (or concatenate) two strings:

strcat( s, " WORLD"); // s : "HELLO WORLD"

determine the length of a string:

i = strlen( s); // i : 11

and many more

Memory space allocation

char s[] = "Flying with the PIC24";

const char s[] = "Flying with the PIC24";

strcpy( s, “HELLO”);

The string “HELLO” was implicitly defi ned as of const char type, and similarly assigned to the const section in program memory to

be accessible via the Program Space Visibility window

Program space visibility

• The PIC24 architecture is somewhat different from most other

16-bit microcontroller architectures you might be familiar with

It was designed for maximum effi ciency according to the

Harvard model, as opposed to the more common Von Neumann model

• The big difference between the two is that there are two

completely separated and independent buses available, one for access to the Program Memory (Flash) and one for access to the Data Memory (RAM).

• The PIC24 architecture offers two methods to read data from

program memory: using special table access instructions (tblrd) and through a second mechanism, called the Program Space

Visibility or PSV This is a window of up to 32K bytes of program memory accessible from the data memory bus In other words the PSV is a bridge between the program memory bus and the data memory bus.

Testing

Post-flight briefing

• In this lesson, we have learned not only what data types are available and how much memory is allocated to them, but also how they affect the resulting compiled program— code size and the execution speed

• We used the MPLAB SIM simulator Stopwatch function to measure the number of instruction cycles (and therefore time) required for the execution of a series of code

segments

• Some of the information gathered will, hopefully, be useful

to guide our actions in the future when balancing our

needs for precision and performance in embedded-control applications.

Investigating memory allocation

#include <p24fj128ga010.h>

#include <string.h>

// 1 variable declarations

const char a[] = “Learn to fly with the PIC24”;

char b[100] = “”;

// 2 main program

main()

{

strcpy( b, “MPLAB C30”); // assign new content to b

} //main

12 Watch window

13

Pointers

int *pi; // define a pointer to an integer

int i; // index/counter

int a[10]; // the array of integers

// 1 sequential access using array indexing

for( i=0; i<10; i++)

a[ i] = i;

// 2 sequential access using a pointer

pi = a;

for( i=0; i<10; i++)

{

*pi = i;

pi++;

}

Pointers

// 2 sequential access to array using pointers

for( i=0, p=a; i<10; i++)

*pi++ = i;

The heap

• What is heap?

• The “heap” is the area of data memory reserved for such use, and a set of functions, part of the standard C library

“stdlib.h”, provides the tools to allocate and free the

memory blocks.

• void *malloc(size_t size);

takes a block of memory of requested size from the heap, and returns a pointer to it.

• void free(void *ptr);

Exercises

Develop new string manipulation

functions to perform the following

operations:

1 Search for a string in an array of

strings, sequential.

2 Implement a Binary search.

3 Develop a simple Hash Table

management library.

What is next?

• CHAPTER 7: Communication (pg 89 – pg103)

– Synchronous serial interfaces

– Asynchronous serial interfaces

– Parallel interfaces

– Synchronous communication using the SPI modules

– Testing the Read Status Register command

– Writing to the EEPROM

– Reading the memory contents

– A nonvolatile storage library

– Testing the new NVM library

– UART configuration

– Sending and receiving data

– Testing the serial

– communication routines