Wednesday, 12 December 2012

What man's mind can create, man's character can control.

I have a project that required me to programmatically control power to several devices. I have done this before using a Velleman vm8090 board which is relatively easy to control. However they are relatively expensive.

I turned to ebay and found a similar module at a substantially reduced cost. Upon receipt however I discovered that instead of being a simple serial USB interface it presented USB HID and the Debian system I was running it on has loaded the hiddev driver for me but it did not implement any of the standard HID Usage Pages leaving me with no way to control the device.
I did the obligatory
sudo lsusb -d 12bf:ff03 -vvv

Bus 003 Device 019: ID 12bf:ff03  
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0         8
  idVendor           0x12bf 
  idProduct          0xff03 
  bcdDevice            1.00
  iManufacturer           1 Matrix Multimedia Ltd.
  iProduct                2 Flowcode USB HID
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           41
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0x80
      (Bus Powered)
    MaxPower               50mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              0 
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.10
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      54
         Report Descriptors: 
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval               5
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval               5
Device Status:     0x0980
  (Bus Powered)
This simply showed me what I already knew and surprised me that lsusb did not dump HID report descriptor items. Some searching revealed that teh device had to be unbound so lussb could access the descriptor.

Thus a simple
echo -n 3-1.1.4:1.0 | sudo dd of=/sys/bus/usb/drivers/usbhid/unbind
resulted in lussb dumping the descriptor items:

Item(Global): Usage Page, data= [ 0xa0 0xff ] 65440
                (null)
Item(Local ): Usage, data= [ 0x01 ] 1
                (null)
Item(Main  ): Collection, data= [ 0x01 ] 1
                Application
Item(Local ): Usage, data= [ 0x02 ] 2
                (null)
Item(Main  ): Collection, data= [ 0x00 ] 0
                Physical
Item(Global): Usage Page, data= [ 0xa1 0xff ] 65441
                (null)
Item(Local ): Usage, data= [ 0x03 ] 3
                (null)
Item(Local ): Usage, data= [ 0x04 ] 4
                (null)
Item(Global): Logical Minimum, data= [ 0x00 ] 0
Item(Global): Logical Maximum, data= [ 0xff 0x00 ] 255
Item(Global): Physical Minimum, data= [ 0x00 ] 0
Item(Global): Physical Maximum, data= [ 0xff ] 255
Item(Global): Report Size, data= [ 0x08 ] 8
Item(Global): Report Count, data= [ 0x08 ] 8
Item(Main  ): Input, data= [ 0x02 ] 2
                Data Variable Absolute No_Wrap Linear
                Preferred_State No_Null_Position Non_Volatile Bitfield
Item(Local ): Usage, data= [ 0x05 ] 5
                (null)
Item(Local ): Usage, data= [ 0x06 ] 6
                (null)
Item(Global): Logical Minimum, data= [ 0x00 ] 0
Item(Global): Logical Maximum, data= [ 0xff 0x00 ] 255
Item(Global): Physical Minimum, data= [ 0x00 ] 0
Item(Global): Physical Maximum, data= [ 0xff ] 255
Item(Global): Report Size, data= [ 0x08 ] 8
Item(Global): Report Count, data= [ 0x08 ] 8
Item(Main  ): Output, data= [ 0x02 ] 2
                Data Variable Absolute No_Wrap Linear
                Preferred_State No_Null_Position Non_Volatile Bitfield
Item(Main  ): End Collection, data=none
Item(Main  ): End Collection, data=none

By consulting the device class definitions document I determined the device was using the "Vendor defined" Usage page (0xff00 to 0xffff) so I would definitely have to write a program to control the device.

Linux provides a really easy interface to deal with HID devices called hiddev (gosh, such adventurous naming) which I already had to unbind to get my descriptors decoded so I am fairly sure it works ;-)

The kernel documentation and header for hiddev provide the absolute basic mechanics of the interface but no example code or guidance. The obligatory web search turned up very little and even that had to be retrieved from the internet archive. So It seems I would be forced to work it through myself.

It seems the hiddev interface is orientated around HID devices generating reports which the program is expected to read. Numerous ioctl() are provided so the program can obtain the descriptor information necessary to control and process the received reports.

However in this case we need to be able to send reports to the device, all the descriptor information revealed was that there were eight (Item Report Count = 8) values with eight bits each (Item Report Size = 8) with logical and physical values representing the whole range of the octets.

Fortunately the seller provided a website with some control programs and even source. After some time rummaging through the Visual Basic program I finally found (in FrmMain.vb:2989) that the eight bytes were largely unused and the first was simply a bitmask of the eight relays coil status, set for energised clear for off. With bit 0 controlling relay labelled 1 through to bit 7 for relay 8.

To send a report to a HID device the hiddev interface uses the HIDIOCSREPORT ioctl where the report data is first set using HIDIOCSUSAGE .

The HIDIOCSUSAGE ioctl is passed a hiddev_usage_ref structure which must be initialised with information about the report descriptor identifier (constructed from the Usage Page and Usage as set by the items in the descriptor), the index of the item (named usage) we wish to set in the report (in this case the first which is 0) and the value we actually want to set.

After a great deal of debugging the final program is very short indeed but does the job, my main problem now is that if I switch too many (more than one) relays at once the whole device resets. The scope says the supply rails are behaving very badly when this happens, looks like I need to add a load of capacitance to the power well to stabilise it during the switching events.

Oh and add in the fact Relay 1 LED doest work unless you push on it and I do wonder about the wisdom of the economy in this case. Though yet again Linux makes the software side easy.

No comments:

Post a Comment