Silicon Laboratories Stepper Machine Specifications download pdf (Page 33)

Standard Device Request Format

 The host initializes a device through a series of device requests via control transfers to

Endpoint 0

 These are defined by the USB spec and have specific control transfer formats

Number of bytes to transfer if there is a Data

stage

Count2wLength6

Word-sized field that varies according to

request; typically used to pass an index or

Offset

Index or

Offset

2wIndex4

Word-sized field that varies according to

Request

Value2wValue2

Specific request

Value1bRequest1

Characteristics of request:

D7: Data transfer direction

0 = Host-to-device

1 = Device-to-host

D6...5: Type

0 = Standard

1 = Class

2 = Vendor

3 = Reserved

D4...0: Recipient

0 = Device

1 = Interface

2 = Endpoint

3 = Other

4...31 = Reserved

Bitmap1bmRequestType0

DescriptionValueSizeFieldOffset

Data format of

the 8 bytes

transferred

during the

Setup stage of

the control

transfer

Now we are getting into what is termed the Standard Requests or Chapter 9 of the

USB spec. Remember those 8 data bytes that was sent with the Setup token for

control transfers? The data that was sent during that request conformed to this

table. The first section defines the direction of the data flow with respect to the host

and what the target is, endpoint for example. On the next slide we will see the

values that can be used to fill the bRequest field as well as the others. Essentially

how these fields are set determine what the host is asking for and it is up to the

firmware to parse these fields and make decisions based on the values received.

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Silicon Laboratories Stepper Machine Specifications Page 33

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