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Spinpoint M8U-Internal Product Manual REV 3.4
55
INSTALLATION
6.3.5.1 Control Transaction
Control transfers are typically used for command and status operations. They are essential to set up a USB
device with all enumeration functions being performed using control transfers. They are typically bursty,
random packets which are initiated by the host and use best effort delivery. The packet length of control
transfers in low speed devices must be 8 bytes, high speed devices allow a packet size of 8, 16, 32 or 64 bytes
and full speed devices must have a packet size of 64 bytes.
A control transfer can have up to three stages, and each stage made up of three phases.
Figure 6-22 shows control transaction concept.
Figure 6-22: Control Transaction Model
The Setup Stage (Fiqure 6-23) is where the request is sent. This consists of three packets. The setup token is
sent first which contains the address and endpoint number. The data packet is sent next and always has a PID
type of data0 and includes a setup packet which details the type of request. We detail the setup packet later.
The last packet is a handshake used for acknowledging successful receipt or to indicate an error. If the
function successfully receives the setup data (CRC and PID etc OK) it responds with ACK, otherwise it
ignores the data and doesn’t send a handshake packet. Functions cannot issue a STALL or NAK packet in
response to
a setup packet.
Figure 6-23: Setup Stage
The optional Data Stage (Fiqure 6-24) consists of one or multiple IN or OUT transfers. The setup request
indicates the amount of data to be transmitted in this stage. If it exceeds the maximum packet size, data will
be sent in multiple transfers each being the maximum packet length except for the last packet. The data stage
has two different scenarios depending upon the direction of data transfer.
• IN: When the host is ready to receive control data it issues an IN Token. If the function receives the IN token
with an error e.g. the PID doesn't match the inverted PID bits, then it ignores the packet. If the token was
received correctly, the device can either reply with a DATA packet containing the control data to be sent, a
stall packet indicating the endpoint has had a error or a NAK packet indicating to the host that the endpoint