Embedded -arm (bare board development) -2

One 、LED drive

1. For example, we need to control a LED On and off , According to this circuit diagram , We can know , Give Way LED1 On or off is the control wire GPIOC12 Low on / High level .

2. And find GPIOC12 and CPU Connected pins , It can be further understood as , control LED1 On or off , In fact, it is control CPU W15 Pin output low / High level .

3. In the data book , We learned that ,W15 If the pin wants to control GPIOC12 Pin , Need to switch to function 1 Can only be  .

4. How to switch functions , You need to check the GPIOxALTFUN0 This table （x On behalf of all kinds of GPIO type , such as A、B、C、D、E）, We can see ,GPIOC12 The occupied bit is 24 And the 25 position , By changing the bits of these two bits , To achieve the effect of switching functions .

5. How can I know this GPIO Is it input or output mode , You need to check the following table , Then do the corresponding bit operation , We see , Each type has 32 position , So we can know , Need to put C Group No 12 Location 1, Because I want you to GPIOC12 Output .

6. After configuring the I / O mode , We also need to specify whether the output is high level or low level , Used to decide LED On and off .

namely ：

  GPIOCALTFN0 0xc001c020

      take [25:24]  Configure to 01, take W15 The pins are configured to function 1 （GPIO）

  GPIOCOUTENB 0xc001c004

      [12] Configure to 1, take W15 The pin is further configured to output mode

  GPIOCOUT    0xc001c000

      [12]      To configure 0/1 representative W15 Low output / High level

Two 、 Code writing

1. First review the pointer

       int i = 0;

      &i: take i The first address of the variable

      *(&i) = 100;// i=100

So the operations on addresses we learned today can be equivalent to ：

       constant ： 0xc001c020

       Address ： (int *)0xc001c020

       visit ： *((int *)0xc001c020)

      

       Standard writing for accessing special function registers ：

             *((volatile unsigned int *)0xc001c020)

2.  Bit operation

      Principles for operating special function registers ： Change only relevant bit, Other bit Keep the original value unchanged

  The wrong sample ：

      // FALSE Change the functions of other pins to functions 0

      *((volatile unsigned int *)0xc001c020) = 0x01000000;

      // Yes BUG If bit25 It turned out to be 1           

      *((volatile unsigned int *)0xc001c020) |= 0x01000000;     

     The correct sample ：

      *((volatile unsigned int *)0xc001c020) &= 0xfcffffff;

      *((volatile unsigned int *)0xc001c020) |= 0x01000000;     

     Standard writing ：

      *((volatile unsigned int *)0xc001c020) &= ~(3<<24);

      *((volatile unsigned int *)0xc001c020) |= (1<<24);

The actual code is as follows ：

But at this time, it is only finished .c Compilation of documents , Links cannot be compiled directly and are used by development boards .

Because in the current PC Next ,x86 Of CPU Next , Compile the program directly , A program that can run （ Or library files ）, It can be directly in the current environment , namely x86 Of CPU Next , The current computer is running . But we will ARM Of CPU Run under , You can't do this directly , Need to be in x86 On the platform ,（ With the cross compiler ） To compile the code you have written , Compile generated （ The executable ） Program , It is put on the target development board ,arm Of CPU Running on . So called ： stay x86 Compile on platform , stay ARM Run on the platform .

3、 ... and 、 Cross compilation

1. Why cross compile

The reason for cross compilation , The main reason is ： There are too few resources in the embedded system . The specific explanation is ： Cross compiled programs , In the target environment to run , Various resources , Are relatively limited , So it is difficult to compile directly locally .

2. Construction of cross compilation environment

because ARM yes 32 position , And I used Ubuntu yes 64 Bit , So install 32 Bit Library .

sudo apt-get install lib32ncurses5 lib32z1 lib32stdc++6

  download ARM-Linux GCC 4.4.3

  I use what is provided by friendly Electronics

Guangzhou Youyi Electronic Technology Co., Ltd (arm9.net)http://www.arm9.net/download.asp

Create your own folder , Used to store downloaded compressed packages , And extract the ：

tar xf arm-cortex_a9-eabi-4.7-eglibc-2.18.tar.gz        

       Add to environment variables ：

vi ~/.bashrc  

Add

export PATH=$PATH:/opt/arm-cortex_a9-eabi-4.7-eglibc-2.18/bin/

Immediate effect environment variable ( You can also close the window , Open it again )     

 source  ~/.bashrc

Check out the version , Check that the installation was successful ：

arm-linux-gcc -v

A similar interface means success ：

3. Compile operation

arm-none-linux-gnueabi-gcc -c -nostdlib  led.c -o led.o 

arm-none-linux-gnueabi-ld -nostdlib -nostartfiles -Ttext=0x48000000 -eled_test led.o -o led

arm-none-linux-gnueabi-objcopy -O binary led led.bin

The above knowledge points involved in the compilation process

         -nostdlib: compile Standard library files are not used in the linking process

         -nostartfiles: Link use without adding standard startup code

         -Ttext=0x48000000： Specify the starting link address of the code segment , The link address and running address should be consistent , Otherwise, the program is likely to run away .

         -e: Specify the entry point .( The entry point found by the default linker is _start function )

        arm-cortex_a9-linux-gnueabi-objcopy： be stripped ELF Additional information in              

Run the code in the target board ：cp led.bin /tftpboot/

Execute... On the development board （ A serial port ）：

     tftp 48000000 led.bin

     go 48000000

After running the command , Of the board LED The light will start flashing ：