{"id":2144,"date":"2015-02-04T13:14:31","date_gmt":"2015-02-04T13:14:31","guid":{"rendered":"http:\/\/www.chalk-elec.com\/?p=2144"},"modified":"2015-05-15T16:41:19","modified_gmt":"2015-05-15T16:41:19","slug":"how-to-control-lcd-backlight-usb-hid","status":"publish","type":"post","link":"https:\/\/www.chalk-elec.com\/?p=2144","title":{"rendered":"How to control LCD backlight (USB HID)"},"content":{"rendered":"

Starting from version 2.0<\/strong> all firmwares for our LCD and HDMI converters support LCD backlight control through standard USB HID protocol. <\/p>\n

HID report descriptor<\/strong><\/p>\n

The following HID report descriptor is used:<\/p>\n

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<\/div><\/td>
    0x05<\/span>,<\/span> 0x0C<\/span>,<\/span>             \/\/    Usage Page (Consumer)<\/span>
\n    0x09<\/span>,<\/span> 0x01<\/span>,<\/span>             \/\/    Usage (Consumer Control)<\/span>
\n    0xA1<\/span>,<\/span> 0x01<\/span>,<\/span>             \/\/    Collection (Application)<\/span>
\n    0x15<\/span>,<\/span> 0x00<\/span>,<\/span>             \/\/       Logical Minimum (0)<\/span>
\n    0x25<\/span>,<\/span> 0x01<\/span>,<\/span>             \/\/       Logical Maximum (1)<\/span>
\n    0x75<\/span>,<\/span> 0x01<\/span>,<\/span>             \/\/       Report Size (1)<\/span>
\n    0x95<\/span>,<\/span> 0x07<\/span>,<\/span>             \/\/       Report Count (7)<\/span>
\n    0x19<\/span>,<\/span> 0x6F<\/span>,<\/span>             \/\/       Usage Minimim (0x6F)<\/span>
\n    0x29<\/span>,<\/span> 0x75<\/span>,<\/span>             \/\/       Usage Maximum (0x75)    <\/span>
\n    0x91<\/span>,<\/span> 0x02<\/span>,<\/span>             \/\/       * Output (Data, Var, Abs)<\/span>
\n    0x95<\/span>,<\/span> 0x01<\/span>,<\/span>             \/\/       Report Count (1)<\/span>
\n    0x91<\/span>,<\/span> 0x03<\/span>,<\/span>             \/\/       * Output (Const, Var, Abs)<\/span>
\n    0x25<\/span>,<\/span> 0x23<\/span>,<\/span>             \/\/       Logical Maximum (35)<\/span>
\n    0x75<\/span>,<\/span> 0x08<\/span>,<\/span>             \/\/       Report Size (8)<\/span>
\n    0x09<\/span>,<\/span> 0x71<\/span>,<\/span>             \/\/       Usage (0x71 -  Display Brightness)<\/span>
\n    0x91<\/span>,<\/span> 0x02<\/span>,<\/span>             \/\/       * Output (Data, Var, Abs)<\/span>
\n    0x09<\/span>,<\/span> 0x35<\/span>,<\/span>             \/\/       Usage (0x35 - Illumination)<\/span>
\n    0x26<\/span>,<\/span> 0xFF<\/span>,<\/span> 0x00<\/span>,<\/span>       \/\/       Logical Maximum (255)<\/span>
\n    0x81<\/span>,<\/span> 0x02<\/span>,<\/span>             \/\/       ** Input (Data, Var, Abs)<\/span>
\n    0x05<\/span>,<\/span> 0x20<\/span>,<\/span>             \/\/       Usage Page (Sensors)<\/span>
\n    0x09<\/span>,<\/span> 0x41<\/span>,<\/span>             \/\/       Usage (0x41 -  Ambient Light Sensor)<\/span>
\n    0x81<\/span>,<\/span> 0x02<\/span>,<\/span>             \/\/       ** Input (Data, Var, Abs)<\/span>
\n    0xC0<\/span>                    \/\/    End Collection<\/span><\/div><\/td><\/tr><\/tbody><\/table><\/div>\n

\nReport usages follow recent changes in official USB HID specification proposed by Microsoft in change request #HUTRR41<\/a>. Interface #<\/strong> is 0 for HDMI-dualLVDS converter and FullHD+ LCD bundle, Interface #<\/strong> is 1 for open frame\/black frame 7″ LCD and for new 10″ integrated LCD. Report ID is 0 for all cases.<\/p>\n

Packet format<\/strong><\/p>\n

Firmware accepts 2-bytes packet request from host and reply with 2-bytes packet with information about current backlight mode and ambient light level:
\n\"Backlight<\/a>
\n
\nOnly one bit can be active in command byte of request packet. If bit 5 is active (Set Brightness command), then next byte contains desired brightness level. For others commands second byte is ignored. LCD will reply with current backlight status and ambient light sensor value to any received command. To get current status without performing command you can send request with command byte equal to 0. MAX_BL<\/strong> is 35 for HDMI-dualLVDS converter and FullHD+ LCD bundle, MAX_BL<\/strong> is 18 for open frame\/black frame 7″ LCD and for new 10″ integrated LCD.<\/p>\n

How to send commands<\/strong><\/p>\n

The simplest way to communicate with HID devices is to to use cross-platform library HIDAPI from Signal11<\/a>. With HIDAPI it is just several lines of code to control LCD brightness:<\/p>\n

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<\/div><\/td>
#include <stdio.h><\/span>
\n#include "hidapi.h"<\/span>
\n
\n\/\/ Try to open HID device and return pointer to opened device<\/span>
\nhid_device*<\/span> openHID(<\/span>)<\/span> {<\/span>
\n    struct<\/span> hid_device_info *<\/span>devs,<\/span> *<\/span>cur_dev;<\/span>
\n    hid_device*<\/span> device =<\/span> NULL;<\/span>
\n    int<\/span> iface =<\/span> 1<\/span>;<\/span>  \/\/ We use interface# 1 for 7" and 10", and interface# 0 for dualLVDS\/FullHD+<\/span>
\n
\n    \/\/ VID=0x04D8, PID=0xF724 for 7" and 10" multi-touch firmware, interface# is 1<\/span>
\n    \/\/ VID=0x04D8, PID=0xF723 for 7" and 10" single-touch firmware, interface# is 1<\/span>
\n    \/\/ VID=0x04D8, PID=0x003F for dualLVDS\/FullHD+, interface# is 1<\/span>
\n    devs =<\/span> hid_enumerate(<\/span>0x04D8<\/span>,<\/span> 0xF724<\/span>)<\/span>;<\/span>
\n    if<\/span> (<\/span>devs ==<\/span> NULL)<\/span> {<\/span>
\n        devs =<\/span> hid_enumerate(<\/span>0x04D8<\/span>,<\/span> 0xF723<\/span>)<\/span>;<\/span>
\n        if<\/span> (<\/span>devs ==<\/span> NULL)<\/span> {<\/span>
\n            devs =<\/span> hid_enumerate(<\/span>0x04D8<\/span>,<\/span> 0x003F<\/span>)<\/span>;<\/span>
\n            if<\/span> (<\/span>devs ==<\/span> NULL)<\/span> return<\/span> NULL;<\/span>
\n            else<\/span> iface =<\/span> 0<\/span>;<\/span>
\n        }<\/span>
\n    }<\/span>
\n
\n    \/\/ Walk through all enumerated devices to find one with correct interface number<\/span>
\n    cur_dev =<\/span> devs;<\/span>
\n    while<\/span> (<\/span>cur_dev)<\/span> {<\/span>
\n        if<\/span> (<\/span>(<\/span>cur_dev-><\/span>interface_number)<\/span> ==<\/span> iface)<\/span> {<\/span>
\n            device =<\/span> hid_open_path(<\/span>cur_dev-><\/span>path)<\/span>;<\/span>
\n            printf<\/span>(<\/span>"Found Backlight interface at path: %s"<\/span>,<\/span> cur_dev-><\/span>path)<\/span>;<\/span>
\n            break<\/span>;<\/span>
\n        }<\/span>
\n        cur_dev =<\/span> cur_dev-><\/span>next;<\/span>
\n    }<\/span>
\n    hid_free_enumeration(<\/span>devs)<\/span>;<\/span>
\n
\n    return<\/span> device;<\/span>
\n}<\/span>
\n
\nint<\/span> main(<\/span>int<\/span> argc,<\/span> char<\/span> *<\/span>argv[<\/span>]<\/span>)<\/span> {<\/span>
\n    hid_device*<\/span> device=<\/span>NULL;<\/span>
\n    unsigned<\/span> char<\/span> buf[<\/span>3<\/span>]<\/span>;<\/span>
\n
\n    buf[<\/span>0<\/span>]<\/span> =<\/span> 0x0<\/span>;<\/span>   \/\/ Report ID (0)<\/span>
\n    buf[<\/span>1<\/span>]<\/span> =<\/span> 0x04<\/span>;<\/span>  \/\/ Command (0x20 - Set Backlight)<\/span>
\n    buf[<\/span>2<\/span>]<\/span> =<\/span> 9<\/span>;<\/span>     \/\/ Backlight value for command Set Backlight<\/span>
\n
\n    hid_init(<\/span>)<\/span>;<\/span>
\n
\n    device =<\/span> openHID(<\/span>)<\/span>;<\/span>
\n    if<\/span> (<\/span>device ==<\/span> NULL)<\/span> {<\/span>
\n        puts<\/span>(<\/span>"Can't find USB device with interface for backlight"<\/span>)<\/span>;<\/span>
\n        return<\/span> -<\/span>1<\/span>;<\/span>
\n    }<\/span>
\n
\n    hid_set_nonblocking(<\/span>device,<\/span> true<\/span>)<\/span>;<\/span>
\n
\n
\n    printf<\/span>(<\/span>"Writing bytes: %02X,%02X,%02X\\n<\/span>"<\/span>,<\/span> buf[<\/span>0<\/span>]<\/span>,<\/span> buf[<\/span>1<\/span>]<\/span>,<\/span> buf[<\/span>2<\/span>]<\/span>)<\/span>;<\/span>
\n
\n    if<\/span> (<\/span>hid_write(<\/span>device,<\/span> buf,<\/span> 3<\/span>)<\/span> ==<\/span> -<\/span>1<\/span>)<\/span> {<\/span>
\n        puts<\/span>(<\/span>"Error writing bytes to device!"<\/span>)<\/span>;<\/span>
\n        hid_close(<\/span>device)<\/span>;<\/span>
\n        return<\/span> -<\/span>1<\/span>;<\/span>
\n    }<\/span>
\n
\n    if<\/span> (<\/span>hid_read_timeout(<\/span>device,<\/span> buf,<\/span> 2<\/span>,<\/span> 100<\/span>)<\/span> ==<\/span> -<\/span>1<\/span>)<\/span> {<\/span>
\n        puts<\/span>(<\/span>"Error reading bytes to device!"<\/span>)<\/span>;<\/span>
\n        hid_close(<\/span>device)<\/span>;<\/span>
\n        return<\/span> -<\/span>1<\/span>;<\/span>
\n    }<\/span>
\n
\n    printf<\/span>(<\/span>"Received bytes: %02X,%02X\\n<\/span>"<\/span>,<\/span> buf[<\/span>0<\/span>]<\/span>,<\/span> buf[<\/span>1<\/span>]<\/span>)<\/span>;<\/span>
\n
\n    return<\/span> 0<\/span>;<\/span>
\n}<\/span><\/div><\/td><\/tr><\/tbody><\/table><\/div>\n

\nAbove example is universal and will work with all our products with USB backlight control feature.<\/p>\n

GUI application for LCD control<\/strong><\/p>\n

Cross-platform GUI application to control different LCD settings including brightness is available here:
\nLCD control GUI<\/a>
\nYou will need Qt framework<\/a> to build it from sources.<\/p>\n

See in action<\/strong><\/p>\n

Video with backlight test using our dualLVDS converter FullHD+ LCD is available here: Backlight test video<\/a>
\n<\/p>\n

<\/p>\n

Follow us on Twitter to get instant notification about updates: Follow @ChalkElec<\/a>h
\n