為OpenWrt添加實時鐘(RTC)--以MT7620為例

前言

實時鐘,英文名RTC(Real Time Clock),在PC裡面很常見,在OpenWrt裡面卻鮮有提及,手頭上有一個DS1307的TinyRTC實時鐘模塊,經過一番折騰,將其融合到了OpenWrt系統,遂將操作過程記錄成文.

背景知識

DS1307在最新的OpenWrt中已經提供支持,卻沒有整合進ramips中,因此,本文的一個重點便是如何為ramips系統配置ds1307.另外,為ds1307編寫合適的dts節點也是本文的一個重點.

實現過程

1.為ramips配置ds1307支持

系統在./scripts/medatata.pl中判斷並處理RTC_SUPPORT開關,分析之後,原來是在 target/linux/ramips/mt7620/target.mk中,將

原始的內容:

FEATURES+=usb

修改為:

FEATURES+=usb rtc

即可打開mt7620對rtc的支持.

此時,make kernel_menuconfig進入配置菜單.同時應注意,由於ds1307是i2c接口的模塊,因此,在device中需要配置i2c的支持.選中i2c之後,便可以在

Device Drivers -> Real Time Clock中看到ds1307

 .config - Linux/mips 3.14.28 Kernel Configuration
 > Device Drivers > Real Time Clock ─────────────────────────────────────
  ┌──── Dallas/Maxim DS1307/37/38/39/40, ST M41T00, EPSON RX-8025 ────┐
  │ CONFIG_RTC_DRV_DS1307:                                            │  
  │                                                                   │  
  │ If you say yes here you get support for various compatible RTC    │  
  │ chips (often with battery backup) connected with I2C. This driver │  
  │ should handle DS1307, DS1337, DS1338, DS1339, DS1340, ST M41T00,  │  
  │ EPSON RX-8025 and probably other chips. In some cases the RTC     │  
  │ must already have been initialized (by manufacturing or a         │  
  │ bootloader).                                                      │  
  │                                                                   │  
  │ The first seven registers on these chips hold an RTC, and other   │  
  │ registers may add features such as NVRAM, a trickle charger for   │  
  │ the RTC/NVRAM backup power, and alarms. NVRAM is visible in       │  
  │ sysfs, but other chip features may not be available.              │  
  │                                                                   │  
  │ This driver can also be built as a module. If so, the module      │  
  │ will be called rtc-ds1307.                                        │  
  │                                                                   │  
  │ Symbol: RTC_DRV_DS1307 [=y]                                       │  
  │ Type  : tristate                                                  │  
  │ Prompt: Dallas/Maxim DS1307/37/38/39/40, ST M41T00, EPSON RX-8025 │  
  │   Location:                                                       │  
  │     -> Device Drivers                                             │  
  │       -> Real Time Clock (RTC_CLASS [=y])                         │  
  │   Defined at drivers/rtc/Kconfig:166                              │  
  │   Depends on: RTC_CLASS [=y] && I2C [=y]                          │  
  │                                                                   │  
  │                                                                   │  
  │                                                                   │  
  ├───────────────────────────────────────────────────────────(100%)──┤  
  │                             < Exit >                              │  
  └───────────────────────────────────────────────────────────────────┘  

2.為ds1307建立dts節點

配置好ds1307的編譯開關後,接下來的工作就要在dts裡面添加ds1307的設備節點.查閱ds1307的資料,其配置的i2c地址為0x68,因此,dts中可以添加如下內容:

	i2c@0 { 
		compatible = "i2c-gpio";
		gpios = <&gpio2 0 0 /* sda = wan_led*/ 
				&gpio3 0 0 /* scl = wlan_led*/ >; 
		i2c-gpio,delay-us = <10>;	/* ~20 kHz */
		#address-cells = <1>;
		#size-cells = <0>;
		
		rtc@68 { 
			compatible = "dallas,ds1307";
			reg = <0x68>; 
		};
		 
	}; 

完成這兩步後,就可以make出支持ds1307的固件.

系統啟動時,可以在TTL中看到如下內容:

[    0.130000] i2c-gpio i2c.4: using pins 40 (SDA) and 72 (SCL)
...
[    0.790000] rtc-ds1307 0-0068: rtc core: registered ds1307 as rtc0
[    0.810000] rtc-ds1307 0-0068: 56 bytes nvram
...
[    0.810000] rtc-ds1307 0-0068: setting system clock to 2015-01-26 22:31:15 UTC (1422311475)

說明RTC已經成功的整合到了mt7620的系統中.

ps:系統自帶的hwclock命令可以很方便的對RTC進行相應的操作.

後記

kernel 3.14.28中的i2c-ralink驅動已經能夠正常驅動7620的硬件i2c,只是在用i2cdetect的時候,需要加上-r參數,這樣才能正確probe到設備.

OpenWrt：Ubuntu 12.04搭建OpenWrt編譯環境 http://www.linuxidc.com/Linux/2016-07/133764.htm