文章目录

• 一、前言
• 二、系列文章
• 三、准备工作
• • 3.1 查找PWM引脚
  • 3.2 原理图：确认引脚位置
  • 3.3 PWM Controller
• 四、查阅PWM bindings
• 五、编写设备树节点
• • 5.1 实现节点：pwm-leds
  • 5.2 测试命令
• 六、后语

一、前言

在完成了基于GPIO的LED设备驱动的文章后，本文继续介绍基于PWM的LED设备驱动。同样的，站在巨人的肩膀上，不写一行代码……，但工资还是要领……

二、系列文章

第1篇 ：不写一行代码(一)：实现安卓基于GPIO的LED设备驱动

第2篇 ：不写一行代码(二)：实现安卓基于PWM的LED设备驱动

第3篇：不写一行代码(三)：实现安卓基于i2c bus的Slaver设备驱动

三、准备工作

3.1 查找PWM引脚

（1）通过查看Datasheet，结合原理图，姑且选择GPIOZ_6作为实验所用的PWM引脚。

（2）GPIOZ_6在复用为Func4时，可作为PWM_D使用

3.2 原理图：确认引脚位置

（1）GPIOZ_6引脚，通过原理图可知，它位于J31插座左边的第7个PIN，即PIN-13

3.3 PWM Controller

（1）通过查阅板子的DTS设备树文件，查找PWM_D的控制器

（2）如下所示，控制PWM_D的控制器名为:pwm_cd， 此控制器同时控制了PWM_C和PWM_D

//文件：common\arch\arm\boot\dts\amlogic\mesontl1.dtsipwm_cd: pwm@1a000 {compatible = "amlogic,tl1-ee-pwm";reg = <0x1a000 0x20>;#pwm-cells = <3>;clocks = <&xtal>,<&xtal>,<&xtal>,<&xtal>;clock-names = "clkin0","clkin1","clkin2","clkin3";status = "disabled";};

四、查阅PWM bindings

想要站在大神的肩膀上，自然要查看大神遗留的Leds-pwm说明文档，以了解什么样的设备才能与大神的leds-pwm驱动绑定

• 文件位置：Documentation/devicetree/bindings/leds/leds-pwmtxt

LED connected to PWMRequired properties:
- compatible : should be "pwm-leds".Each LED is represented as a sub-node of the pwm-leds device.  Each
node's name represents the name of the corresponding LED.LED sub-node properties:
- pwms : PWM property to point to the PWM device (phandle)/port (id) and tospecify the period time to be used: <&phandle id period_ns>;
- pwm-names : (optional) Name to be used by the PWM subsystem for the PWM deviceFor the pwms and pwm-names property please refer to:Documentation/devicetree/bindings/pwm/pwm.txt
- max-brightness : Maximum brightness possible for the LED
- active-low : (optional) For PWMs where the LED is wired to supplyrather than ground.
- label :  (optional)see Documentation/devicetree/bindings/leds/common.txt
- linux,default-trigger :  (optional)see Documentation/devicetree/bindings/leds/common.txtExample:twl_pwm: pwm {/* provides two PWMs (id 0, 1 for PWM1 and PWM2) */compatible = "ti,twl6030-pwm";#pwm-cells = <2>;
};twl_pwmled: pwmled {/* provides one PWM (id 0 for Charing indicator LED) */compatible = "ti,twl6030-pwmled";#pwm-cells = <2>;
};pwmleds {compatible = "pwm-leds";kpad {label = "omap4::keypad";pwms = <&twl_pwm 0 7812500>;max-brightness = <127>; };charging {label = "omap4:green:chrg";pwms = <&twl_pwmled 0 7812500>;max-brightness = <255>;};
};

五、编写设备树节点

5.1 实现节点：pwm-leds

	pwmleds {status = "okay";compatible = "pwm-leds";pwm_cd-led {//向&pwm_cd控制器传送了3个参数，分别是：1.index=6   2.Period=30541  3.极性=0pwms = <&pwm_cd 3 30541 0>;  //GPIOZ-6的index=3，这个具体要查阅datasheet和驱动源码max-brightness = <255>; //设置Levels，最大256级调整};};

5.2 测试命令

（1）进入pwm_cd-led设备目录
:/sys/class/leds/pwm_cd-led # ls
brightness device max_brightness power subsystem trigger uevent
:/sys/class/leds/pwm_cd-led # cat brightness
0
:/sys/class/leds/pwm_cd-led # cat max_brightness
255（2）尝试不同LEV的亮度值，值越大则占空比越大（越亮），反正则越小（越暗）
:/sys/class/leds/pwm_cd-led # echo 200 >  brightness
:/sys/class/leds/pwm_cd-led # echo 100 >  brightness
:/sys/class/leds/pwm_cd-led # echo 255 >  brightness
:/sys/class/leds/pwm_cd-led # echo 1 >  brightness
:/sys/class/leds/pwm_cd-led # echo 20 >  brightness
:/sys/class/leds/pwm_cd-led # echo 80 >  brightness
:/sys/class/leds/pwm_cd-led # echo 200 >  brightness
:/sys/class/leds/pwm_cd-led # echo 250 >  brightness
:/sys/class/leds/pwm_cd-led #

六、后语

继续周末爆肝……