sensor: add image sensor doc

add image sensor (camera) introduction and usage to sensor docs.
more details will be updated later.

Signed-off-by: Bao Li <baoli@aiit.org.cn>

docs/.vuepress/sidebar.js

@@ -48,7 +48,8 @@ const sidebar = {
     '/doc/sensor/magneto_sensor',
     '/doc/sensor/photoelec_sensor',
     '/doc/sensor/pressure_sensor',
-    '/doc/sensor/voice_sensor'
+    '/doc/sensor/voice_sensor',
+    '/doc/sensor/image_sensor'
   ],
   'appdev': [
     '/doc/appdev/env',

docs/doc/sensor/README.md

@@ -18,3 +18,5 @@
 
 - [声音传感器](/doc/sensor/voice_sensor.md)
 
+- [图像传感器](/doc/sensor/image_sensor.md)
+

docs/doc/sensor/image_sensor.md

@@ -0,0 +1,41 @@
+# 图像传感器
+
+## 传感器介绍
+
+| 传感器信号 | 传感器说明 | 驱动支持 | 传感器外形 |
+| --- | --- | --- | --- |
+| OV7670 | 最大支持 40x480，30万像素，不支持 JPEG 输出，不支持自动对焦 | 已支持 | ![ov7670](/images/sensor-ov7670.png) |
+| OV2640 | 最大支持 1600x1200，200万像素，支持 JPEG 输出，不支持自动对焦 | 待支持 | ![ov7670](/images/sensor-ov2640.png) |
+
+## 使用说明
+
+本系统支持 dev fs，camera 设备会在 /dev 下面注册。
+
+相关数据结构：
+
+```c
+enum CameraOutputFormat {
+    RGB565,
+    JPEG,
+};
+struct CameraInfo {
+    int width;
+    int height;
+    CameraOutputFormat format;
+};
+```
+
+基本使用说明：
+
+```c
+// open camera device
+int fd = open(“/dev/cam1”);
+
+// get camera info
+struct CameraInfo cam_info;
+int ret = ioctl(fd, GET_INFO, &cam_info);
+
+// read camera data to buf
+void *buf = malloc(cam_info.width * cam_info.height * 2); // assume the format is RGB565
+int ret = read(fd, buf, size);
+```