操作 memory mapping register I/O

在寫 driver 時，最基本的動作通常是練習控制簡單的 I/O 動作讓某個裝置起作用。
在此便解釋如何控制，以及一些簡單的範例，取自：

• 現代嵌入式系統開發專案實務:菜鳥成長日誌與專案經理的私房菜
• linux-ldp-v1.2

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操作 memory mapping register I/O

• Memory Mapping Register: 寫驅動程式時，要控制接在 CPU 外部的晶片勢必是透過 CPU 的 PIN 腳，可能只是單純的將 GPIO pin 設為 high/low 來控制外部的 chip，同理外部的設備也可以透過 GPIO pin 將訊號傳給 CPU；或者驅動程式可以透過某種通訊協定來控制 I/C，例如 SPI 或 I2C等。但無論是何種 protocol，CPU 和 外部 chip 之間的資料與命令傳遞還是得透過 CPU 的 pin 腳。
• 程式寫法

  *(volatile unsigned char *)(0x302CF)=0x00;
  

  • register 其實是一個位址，使用指標就可以對其設值或取值，要注意的是一定要用 volatile 關鍵字，否則 compiler 的最佳化功能很可能會把某些程式當作是沒意義的。

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範例

• 以下為一 LED driver example

  /* drv_LED.c
  - 假設 CPU 用 P1 這樣 pin 與 LED 相接
  - 當 P1 為 high/low 時， LED 會 on/off
  */
  
  void drv_init_LED(void)
  {
     //設定 P1 為 output pin
     *(volatile unsigned char*)0x200023 |= 0x02; //0000 0010
  
     //default: P1 is low, LED off
     *(volatile unsigned char*)0x200022 &= ~0x02; //1111 1101
  }
  
  void drv_LED_off()
  {
     //set P1 is low, LED off
     *(volatile unsigned char*)0x200022 &= ~0x02; //1111 1101
  }
  
  void drv_LED_on()
  {
     //set P1 is low, LED off
     *(volatile unsigned char*)0x200022 |= 0x02; //0000 0010
  }
  

• 解釋 *(volatile unsigned char*)0x200022 |= 0x02;

  void drv_LED_on(void)
  {
     volatile unsigned char data;    // value of register context
     volatile unsigned char* reg;    // the point to register
  
     reg = (volatile unsigned char*) 0x200022;
     data = *reg;                    // get the value of the register in 0x200022
     data = data | 0x2;              // set 1 to 2th bit
     *reg = data;                    // set the new data to register
  }
  

• linux-ldp-v1.2 例子

  • omap_writel((omap_readl(OMAP2_CONTROL_PBIAS_1) | 0x6) & ~0x1, OMAP2_CONTROL_PBIAS_1);
    

  • /*
    * Functions to access the OMAP IO region
    *
    * NOTE: - Use omap_read/write[bwl] for physical register addresses
    *  - Use __raw_read/write[bwl]() for virtual register addresses
    *  - Use IO_ADDRESS(phys_addr) to convert registers to virtual addresses
    *  - DO NOT use hardcoded virtual addresses to allow changing the
    *    IO address space again if needed
    */
    #define omap_readb(a)  (*(volatile unsigned char  *)IO_ADDRESS(a))
    #define omap_readw(a)  (*(volatile unsigned short *)IO_ADDRESS(a))
    #define omap_readl(a)  (*(volatile unsigned int   *)IO_ADDRESS(a))
    
    #define omap_writeb(v,a) (*(volatile unsigned char  *)IO_ADDRESS(a) = (v))
    #define omap_writew(v,a) (*(volatile unsigned short *)IO_ADDRESS(a) = (v))
    #define omap_writel(v,a) (*(volatile unsigned int   *)IO_ADDRESS(a) = (v))
    

  • #define IO_OFFSET 0x90000000
    #define IO_ADDRESS(pa) ((pa) + IO_OFFSET) /* Works for L3 and L4 */