11. USB command encoding

As described in Section 8, the Tic’s USB commands each use one of four formats: quick, 7-bit write, 32-bit write, and block read (except for the “set setting” command that uses its own unique format, also described below). This section explains how these four command formats are encoded as USB control transfer requests.

For a reference implementation of this protocol, see our Tic Stepper Motor Controller software source code.

All of the Tic’s USB commands are implemented as vendor-defined control transfers on endpoint 0. The tables below show what data is sent in the SETUP packet and the data phase of the control transfer.

Quick

bmRequestType	bRequest	wValue	wIndex	wLength	Data
0x40	command	0	0	0	(none)

A quick command sends no data and is a request that simply contains the command byte in the bRequest field.

7-bit write

bmRequestType	bRequest	wValue	wIndex	wLength	Data
0x40	command	data	0	0	(none)

A 7-bit write command is a request that contains the command byte in bRequest and a 7-bit value in the wValue field (the upper byte of wValue and the the most-significant bit of the lower byte are always 0).

32-bit write

bmRequestType	bRequest	wValue	wIndex	wLength	Data
0x40	command	data[15:0]	data[31:16]	0	(none)

A 32-bit write command is a request that contains the command byte in bRequest and a 32-bit value split between the wValue field (lower two bytes) and the wIndex field (upper two bytes).

Block read

bmRequestType	bRequest	wValue	wIndex	wLength	Data (in)
0xC0	command	0	offset	length	response data (length bytes)

A block read command reads a block of data from the Tic. The bRequest field holds the command byte, the offset in wIndex specifies the offset within the data that the response should start at, and wLength specifies how many bytes of data the response should include. The length must be between 1 and 128. Any multi-byte values contained in the response are in little-endian order (starting with the least-significant byte).

Set setting

bmRequestType	bRequest	wValue	wIndex	wLength	Data
0x40	command	data	offset	0	(none)

The “set setting” command uses a unique format. This request contains the command byte in bRequest, an 8-bit value in wValue (the upper byte is always 0), and an offset in wIndex.