树莓派 -- i2c学习 续（1） DeviceTree Overlay实例化rtc

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上文中讨论了通过sysfs来实例化i2c设备 （rtc ds3231)
https://blog.csdn.net/feiwatson/article/details/81048616

本文继续看看如何通过 DeviceTree Overlays来实例化rtc ds3231

write dts

首先，写一个dts

/dts-v1/;
/plugin/;
/ {
       compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";

       fragment@0 {
          target = <&i2c1>;
          __overlay__ {
                #address-cells = <1>;
                #size-cells = <0>;
                rtc@68 {
                        compatible = "maxim,ds3231";
                        reg = <0x68>;
                        #address-cells = <2>;
                        #size-cells = <1>;
                };      
          };
       };
    };

注：
1. compatible = “maxim,ds3231”, 源码 drivers\rtc\rtc-ds1307.c中定义
2. i2c1在arch\arm\boot\dts\bcm283x.dtsi中定义

保存为文件rasp-rtc.dts

compile

copy dts到树莓派中并编译，

dtc -I dts -O dtb -o /boot/overlays/rasp-rtc.dtbo rasp-rtc.dts

生成的.dtbo到/boot/overlays

update /boot/config.txt

首先要确认i2c是使能的。如果没有使能，通过raspi-config命令来使能。
使能后在config.txt可以看到

dtparam=i2c_arm=on

增加下面内容至config.txt

# Uncomment this to enable the rtc ds3231 
dtoverlay=rasp-rtc

重启

reboot后，查看/dev可以看到rtc

pi@raspberrypi:/dev $ ls -la | grep rtc
lrwxrwxrwx  1 root root           4 Jul 16 13:49 rtc -> rtc0
crw-------  1 root root    253,   0 Jul 16 13:49 rtc0

测试

pi@raspberrypi:/dev $ sudo hwclock --debug
hwclock from util-linux 2.29.2
Using the /dev interface to the clock.
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
/dev/rtc does not have interrupt functions. Waiting in loop for time from /dev/rtc to change
...got clock tick
Time read from Hardware Clock: 2018/07/16 14:13:13
Hw clock time : 2018/07/16 14:13:13 = 1531750393 seconds since 1969
Time since last adjustment is 1531750393 seconds
Calculated Hardware Clock drift is 0.000000 seconds
2018-07-16 14:13:12.176793+0000

rtc driver Code

rtc-ds1307.c中定义compatible

#ifdef CONFIG_OF
static const struct of_device_id ds1307_of_match[] = {
    {
        .compatible = "dallas,ds1307",
        .data = (void *)ds_1307
    },
    {
        .compatible = "dallas,ds1308",
        .data = (void *)ds_1308
    },
    {
        .compatible = "dallas,ds1337",
        .data = (void *)ds_1337
    },
    {
        .compatible = "dallas,ds1338",
        .data = (void *)ds_1338
    },
    {
        .compatible = "dallas,ds1339",
        .data = (void *)ds_1339
    },
    {
        .compatible = "dallas,ds1388",
        .data = (void *)ds_1388
    },
    {
        .compatible = "dallas,ds1340",
        .data = (void *)ds_1340
    },
    {
        .compatible = "dallas,ds1341",
        .data = (void *)ds_1341
    },
    {
        .compatible = "maxim,ds3231",
        .data = (void *)ds_3231
    },
    {
        .compatible = "st,m41t0",
        .data = (void *)m41t00
    },
    {
        .compatible = "st,m41t00",
        .data = (void *)m41t00
    },
    {
        .compatible = "microchip,mcp7940x",
        .data = (void *)mcp794xx
    },
    {
        .compatible = "microchip,mcp7941x",
        .data = (void *)mcp794xx
    },
    {
        .compatible = "pericom,pt7c4338",
        .data = (void *)ds_1307
    },
    {
        .compatible = "epson,rx8025",
        .data = (void *)rx_8025
    },
    {
        .compatible = "isil,isl12057",
        .data = (void *)ds_1337
    },
    { }
};
MODULE_DEVICE_TABLE(of, ds1307_of_match);
#endif