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Spinpoint M8U-Internal Product Manual REV 3.4
53
INSTALLATION
The STALL handshake is used by a device in one of two distinct occasions. The first case, known as
“functional stall,” is when the Halt feature associated with the endpoint is set. A special case of the functional
stall is the “commanded stall.” Commanded stall occurs when the host explicitly sets the endpoint’s Halt
feature. Once a function’s endpoint is halted, the function must continue returning STALL until the
condition causing the halt has been cleared through host intervention.
The second case, known as “protocol stall,” Protocol stall is unique to control pipes. Protocol stall differs
from functional stall in meaning and duration. A protocol STALL is returned during the Data or Status stage
of a control transfer, and the STALL condition terminates at the beginning of the next control transfer (Setup).
The remainder of this section refers to the general case of a functional stall.
NYET is a high-speed only handshake that is returned in two circumstances. It is returned by a high speed
endpoint as part of the PING protocol described later in this chapter. NYET may also be returned by a hub in
response to a split-transaction when the full-/low-speed transaction has not yet been completed or the hub is
otherwise not able to handle the split-transaction.
ERR is a high-speed only handshake that is returned to allow a high-speed hub to report an error on a
full-/low-speed bus. It is only returned by a high-speed hub as part of the split transaction protocol.
6.3.3.4 Start-of-Frame Packets
Start-of-Frame (SOF) packets are issued by the host at a nominal rate of once every 1.00 ms ±0.0005 ms for
a full-speed bus and 125 µs ±0.0625 µs for a high-speed bus. SOF packets consist of a PID indicating
packet type followed by an 11-bit frame number field as illustrated in Figure 6-21.
Figure 6-21: SOF Packet
The SOF token comprises the token-only transaction that distributes an SOF marker and accompanying
frame number at precisely timed intervals corresponding to the start of each frame. All high-speed and full
speed functions, including hubs, receive the SOF packet. The SOF token does not cause any receiving
function to generate a return packet; therefore, SOF delivery to any given function cannot be guaranteed.
The SOF packet delivers two pieces of timing information. A function is informed that an SOF has
occurred when it detects the SOF PID. Frame timing sensitive functions, that do not need to keep track of
frame number
(e.g., a full-speed operating hub), need only decode the SOF PID; they can ignore the frame
number and its CRC. If a function needs to track frame number, it must comprehend both the PID and the
time stamp. Full-speed devices that have no particular need for bus timing information may ignore the SOF
packet.
6.3.4 Pipes
While the device sends and receives data on a series of endpoints, the client software transfers data through
pipes. A pipe is a logical connection between the host and endpoint(s). Pipes will also have a set of
parameters associated with them such as how much bandwidth is allocated to it, what transfer type (Control,
Bulk, Iso or Interrupt) it uses, a direction of data flow and maximum packet/buffer sizes. For example the
default pipe is a bi-directional pipe made up of endpoint zero in and endpoint zero out with a control transfer
type.
USB defines two types of pipes
• Stream Pipes have no defined USB format, that is you can send any type of data down a stream pipe
and can retrieve the data out the other end. Data flows sequentially and has a pre-defined direction,
either in or out. Stream pipes will support bulk, isochronous and interrupt transfer types. Stream pipes
can either be controlled by the host or device.