5. Low-Level Function Reference
This section describes the low level functions of the U3. These are commands sent over USB
directly to the processor on the U3.
The majority of Windows users will use the high-level UD driver rather than these low-level
functions.
5.1 General Protocol
Following is a description of the general U3 low-level communication protocol. There are two
types of commands:
Normal: 1 command word plus 0-7 data words.
Extended: 3 command words plus 0-125 data words.
Normal commands have a smaller packet size and can be faster in some situations. Extended
commands provide more commands, better error detection, and a larger maximum data
payload.
Normal command format:
Byte
0 Checksum8: Includes bytes 1-15.
1 Command Byte: DCCCCWWW
Bit 7: Destination bit:
0 = Local,
Extended command format:
1 = Remote.
Bits 6-3: Normal command number (0-14).
Bits 2-0: Number of data words.
2-15 Data words.
Byte
0 Checksum8: Includes bytes 1-5.
1 Command Byte: D1111CCC
Bit 7: Destination bit:
0 = Local,
1 = Remote.
Bits 6-3: 1111 specifies that this is an extended command.
Bits 2-0: Used with some commands.
2 Number of data words.
3 Extended command number.
4 Checksum16 (LSB)
5 Checksum16 (MSB)
6-255 Data words.
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