More USB stuff

Useful USB debug knowledge:

You can test the USB mass storage gadget's behavior with a Linux host.

To send a Set-Config request with value 0, to de-configure the gadget

        echo 0 >/sys/bus/usb/devices/.../bConfigurationValue

To send a Set-Config request with value 1, to re-configure the gadget

        echo 1 >/sys/bus/usb/devices/.../bConfigurationValue

where the "..." part is replaced with the gadget's device path.

Set-Config has only two stages, Setup and Status stage; there is no Data stage.

Host                        Device
----- Setup Packet --->         |
----- Data0 Packet --->         |== Setup stage
<---- Ack Packet ------         |

----- In Packet ------>         |
<---- Data1 Packet ----         |== Status stage
----- Ack Packet ----->         |


The Data1 packet above contains no data, as below.


      PID   !PID  CRC

USB software layering:

Client SW:

It decides what transfers are to be made with the USB device function.

USB Driver (USBD):

The software interface between the USB System SW and the Client SW. USBD provides the device drivers that the operating systems use to access USB devices.

USB System SW:

It is usually supplied with the operating systems. It can be seemed to include USBD and HCD.

Host Controller Driver (HCD):

The software interface between Host Controller and USB System SW. This interface allows a USBD to support different Host Controllers. HCD is an abstraction of the Host Controller hardware.

Host Controller:

It manages USB protocol level interface. It ensures USB bus access rules defined by the protocol are obeyed, such as inter packet timings, timeouts.

USB Host and Device Partitioning

Control Transfer:

It is started with setup transaction from Host to Device. The Setup transaction is followed by zero or more Data transactions. A Status Transaction completes the control transfer.

Setup Transaction consists of:
setup packet
data0 packet - the request
Ack packet

Data transaction
In packet
data1 packet - the data that is returned by device
Ack packet

Status transaction
out packet
data1 packet - has no data
Ack packet

USB Device Request:

The USB host uses USB requests to get info from USB devices. These request are made using control transfer, from host to device default control pipe.The standard requests are:

• Get Status
• Clear Feature
• Set Feature
• Set Address
• Get Descriptor
• Set Descriptor
• Get Configuration
• Set Configuration
• Get Interface
• Set Interface
• Synch Frame

An example of USB request sequence in USB Command Verifier test:

80 06 00 01 00 00 12 00  -  Get device descriptor Data(12 01 00 02 00 00 00 40 25 05 A5 A4 33 03 01 02  00 01)  - Return device descriptor

80 06 00 02 00 00 09 00 – get config descriptor Data(09 02 20 00 01 01 04 C0 01) – return config descriptor

00 09 01 00 00 00 00 00 – set configuration 0 byte data

80 06 00 02 00 00 09 00 – get config descriptor 09 02 20 00 01 01 04 C0 01 – return config desc

80 06 00 02 00 00 20 00 – get config desc 0 byte data

80 06 00 01 00 00 12 00 – get device desc Data(12 01 00 02 00 00 00 40 25 05 A5 A4 33 03 01 02  00 01) – return device desc

80 06 00 06 00 00 0A 00 – get device qualifier Data(0A 06 00 02 00 00 00 40 01 00) – return device qualifier

80 06 00 02 00 00 09 00 – get config desc Data(09 02 20 00 01 01 04 C0 01) – return config desc

80 08 00 00 00 00 01 00 – get configuration Data(01) – return

USB Descriptor:

USB devices report their attributes using descriptors. A descriptor is a defined data structure. Standard USB descriptors are:

• Device descriptor
• Device Qualifier descriptor
• Configuration descriptor
• Interface descriptor
• Endpoint descriptor
• String descriptor