mini2440的时钟

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

转载于:https://www.cnblogs.com/-song/archive/2011/10/21/3331946.html

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