mini2440上为了降低电磁干扰配备了12MHz的晶振，如果直接给当做cpu的时钟，那这个s3c2440a就屈才了。幸好cpu内部自带了MPLL将晶振时钟倍频使得cpu工作在FCLk及AHB总线上的外设工作在HCLK和APB总线上的外设工作在PCLK。当然还配有一个UPLL来产生恒定的48MHZ以支持usb2.0.
①.时钟源选择
在系统复位时检测引脚OM3:OM2，若是0:0，则主时钟源选择外部晶振，usb时钟源选择外部晶振
②.MPLLCON  main pll control  主时钟寄存器控制
用于设定FCLK和Fin的倍数。

Mpll(FCLK) = ( 2 × m × Fin ) / ( p × 2^s )
其中m=MDIV+8, p=PDIV+2, s=SDIV，Fin晶振频率

比如
Fin=12MHz
MDIV=0x7F=127，m=135
PDIV=2，p=4
SDIV=1，s=1
则FCLK=405MHz

注意：系统复位时，必须写一次MPLLCON   UPLLCON ,这样系统才能正常工作。即使不改变其值，即使复位后MPLL UPLL都是使能的，也要写一次，另外还有如下   

③.CLKDIVN clock divider control 时钟分频控制寄存器 
用于设置 FCLK  HCLK PCLK三者的比例

而CAMDIVN如下

比如
CAMDIVN[8]=0
CAMDIVN[9]=0
HDIVN=2，则HCLK=FCLK / 4
PDIVN=1,，则PCLK=HCLK / 2

有以下示例，摘自嵌入式linux开发完全手册
 

/*********************************************************************************************************************************/
2011-12-18
2440的片内外设时钟使能控制


linux下将各个片内外设时钟统一管理,组成时钟队列。
arch/arm/plat-s3c24xx/s3c2410-clock.c

/* standard clock definitions */static struct clk init_clocks_disable[] = {{.name        = "nand",.id     = -1,.parent       = &clk_h,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_NAND,}, {.name        = "sdi",.id      = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_SDI,}, {.name     = "adc",.id      = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_ADC,}, {.name     = "i2c",.id      = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_IIC,}, {.name     = "iis",.id      = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_IIS,}, {.name     = "spi",.id      = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_SPI,}
};static struct clk init_clocks[] = {{.name        = "lcd",.id      = -1,.parent       = &clk_h,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_LCDC,}, {.name        = "gpio",.id     = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_GPIO,}, {.name        = "usb-host",.id     = -1,.parent       = &clk_h,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_USBH,}, {.name        = "usb-device",.id       = -1,.parent       = &clk_h,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_USBD,}, {.name        = "timers",.id       = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_PWMT,}, {.name        = "uart",.id     = 0,.parent        = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_UART0,}, {.name       = "uart",.id     = 1,.parent        = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_UART1,}, {.name       = "uart",.id     = 2,.parent        = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_UART2,}, {.name       = "rtc",.id      = -1,.parent       = &clk_p,.enable       = s3c2410_clkcon_enable,.ctrlbit   = S3C2410_CLKCON_RTC,}, {.name     = "watchdog",.id     = -1,.parent       = &clk_p,.ctrlbit  = 0,}, {.name      = "usb-bus-host",.id     = -1,.parent       = &clk_usb_bus,}, {.name       = "usb-bus-gadget",.id       = -1,.parent       = &clk_usb_bus,},
};

//在需要操作各个外设的时钟时，就调用用内核提供的各个函数即可。如摘自mini2440_adc.c
static struct clk    *adc_clock;
adc_clock = clk_get(NULL, "adc");//获取时钟
if (!adc_clock) {
printk(KERN_ERR "failed to get adc clock source\n");
return -ENOENT;
}
clk_enable(adc_clock);//使能时钟//在不需要时，禁止掉
if (adc_clock) {
clk_disable(adc_clock);//禁止时钟
clk_put(adc_clock);
adc_clock = NULL;//源码下是一个空操作,可能是预留
}

此文的adc不错哦，记一下网址
http://www.cnblogs.com/hanyan225/archive/2011/03/29/1998667.html