Jan Axelson, USB Complete
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Chapter 2
56
Bulk and interrupt endpoints increment the Sequence Number for every trans-
action, resetting to zero on rollover or after completing a Set Configuration, Set
Interface, or Clear Feature(ENDPOINT_HALT) request. In isochronous
transfers, the Sequence Number resets to zero at the start of a service interval
and increments on each additional Data Packet within the service interval. The
endpoint descriptor specifies the length of a service interval and the maximum
number of Data Packets per service interval.
On detecting an error in a received Data Packet, the host or device sends an
ACK Transaction Packet with the Retry bit set and the Sequence Number of
the packet that contained the error. The sender of the Data Packet must then
resend all sent Data Packets beginning with that Sequence Number.
Table 2-8: The Data Packet Header provides the Data Packet’s length and other
information. (Reserved fields not shown.)
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Type 5 Data Packet Header (01000
b
)
Route String or reserved 20 In downstream communications, used by hubs to route
a packet to the correct port. Otherwise reserved.
Device Address 7 The device that is the source or receiver of the Data
Packet.
Sequence Number 5 Identifies the Data Packet.
End of Burst (EOB)
(non-isochronous) or Last
Packet Flag (LPF)
(isochronous)
1 For non-isochronous IN endpoints, identifies the last
packet in a burst. For non-isochronous OUT endpoints,
zero. For isochronous endpoints, identifies the last
packet in a service interval.
Direction 1 0 = host to device; 1 = device to host.
Endpoint Number 4 The endpoint that is the source or receiver of the Data
Packet.
Setup 1 Set by the host when the Data Packet is a Setup packet.
Data Length 16 The number of data bytes in the Data Packet Payload.
Stream ID or reserved 16 For bulk endpoints, can identify a stream.
Packets Pending 1 Set by the host when it has another packet scheduled for
the target endpoint.
CRC-16 16 For error checking.
Link Control Word 16 For link management.
Inside USB Transfers
57
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SuperSpeed bulk and interrupt endpoints can support burst transactions, where
the host or device sends multiple Data Packets without waiting for ACK Trans-
action Packets to acknowledge previous received data. Every data payload in a
burst except the last must equal the endpoint’s maximum packet size.
The NumP field in an ACK Transaction Packet sets the number of Data Pack-
ets a device or host can receive in a burst. Valid values are zero or any value from
one less than the value in the previous ACK packet to bMaxBurst + 1 in the
endpoint companion descriptor. Note that bMaxBurst is zero-based, with zero
Table 2-9: Hosts and devices use Transaction Packets to send status and control
information.
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ACK Host Requests data from an IN endpoint and acknowledges a
previously received data packet.
Device Acknowledges data received on an OUT endpoint and
specifies how many data packet buffers are available after
receiving this packet.
NRDY Device On receiving a Data Packet on an OUT endpoint, informs
the host that the device has no buffer space to accept the
data. On receiving an ACK Transaction Packet on an IN
endpoint, informs the host that the device can’t return a data
packet. Valid for non-isochronous endpoints.
ERDY Device An endpoint is ready to send or receive Data Packets. Valid
for non-isochronous endpoints.
STATUS Host The host has initiated the Status stage of a control transfer.
Valid for control endpoints.
STALL Device The endpoint is halted or a requested control transfer is
invalid or unsupported.
DEV_NOTIFICATION Device A change in a device or interface state has occurred. The
high nibble is the type of change:
0h reserved
1h function wake
2h latency tolerance message
3h bus interval adjustment message
4h–Fh reserved
PING Host Before initiating an isochronous transfer when a link is in a
low-power state, requests all paths between the host and the
isochronous endpoint to transition to the active state.
PING_RESPONSE Device Response to PING.
Chapter 2
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indicating a maximum burst of 1 packet, while NumP indicates the actual
number of packets a receiver can accept (which may be zero).
A Set Configuration, Set Interface, or Clear Feature(ENDPOINT_HALT)
request resets the burst size of the associated endpoint(s).
Isochronous endpoints can support isochronous burst transactions, which con-
sist of multiple Data Packets transferred in a service interval with each packet
except the last required to be the endpoint’s maximum packet size. Isochronous
transactions never use ACK.
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Devices and hosts must respond quickly to received Data Packets and ACK
Transaction Packets that request data. On receiving an ACK Transaction Packet
with NumP > 0, a device must begin to return a Data Packet or NRDY Trans-
action Packet within 250 ns. On receiving a Data Packet, a device must begin
Table 2-10: An ACK Transaction Packet can acknowledge received data and
request new data. (Reserved fields not shown.)
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20 Route String Used by hubs in routing packets downstream.
7 Device Address The address assigned during enumeration.
4SubType ACK
1 Retry Data Packet If set, the host or device requests a resend due to not
receiving a packet or receiving a corrupted packet.
1 Direction The direction of the endpoint sending or receiving
the data: 0 = host to device; 1 = device to host.
4 Endpoint Number The endpoint sending or receiving the data.
1 Host Error For host-to-device ACK Transaction Packets,
indicates that host was unable to accept a valid data
packet.
5 Number of Packets
(NumP)
The number of Data Packets the receiver can accept
in a burst.
5 Sequence Number
(Seq Num)
The sequence number of the next expected Data
Packet.
16 Stream ID For bulk endpoints, can identify a stream.
1 Packets Pending Indicates whether the host has another packet for the
endpoint.
Inside USB Transfers
59
to return an ACK or NRDY Transaction Packet within 250 ns. On receiving a
Data Packet, a host must begin to return an ACK or NRDY Transaction Packet
within 3
µs. The maximum interval between Data Packets in a burst is 100 ns.
The device hardware thus handles responding to received packets.
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To conserve bandwidth and to enable inactive links to transition to low-power
states, USB 3.0 hosts stop requesting to send or receive data from SuperSpeed
endpoints that are in the flow control condition. This condition indicates that
the endpoint temporarily can’t send or receive data. To request to resume com-
munications, the endpoint sends an ERDY Transaction Packet. A device can
send the ERDY at any time without waiting for the host to request a packet.
On receiving the ERDY, the host resumes communications with the endpoint.
An IN endpoint is in the flow control condition after responding to an ACK
Transaction Packet with either of the following:
A NRDY Transaction Packet.
A Data Packet with the End of Burst (EOB) field set to 1, indicating that
the packet is the last in a burst. The device sets EOB if the data payload is
equal to the endpoint’s maximum packet size and the endpoint is returning
fewer than the number of packets requested in the previous ACK Transac-
tion Packet.
An OUT endpoint is in the flow control condition on responding to a Data
Packet with either of the following:
A NRDY Transaction Packet.
An ACK Transaction Packet with the NumP field set to zero, indicating that
the endpoint can’t accept any Data Packets.
Hosts retain the option to attempt communications with bulk endpoints in the
flow-control condition before receiving ERDY.
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Link Management Packets have these subtypes:
• Set Link Function defines a bit for use in testing.
• U2 Inactivity Timeout specifies the timeout for transitioning between
low-power states.
• Vendor Device Test provides a mechanism for vendor-specific tests.
• Port Capabilities indicates if the port can be configured as an upstream-fac-
Chapter 2
60
ing port, a downstream-facing port, or both. The ports in a link exchange
this packet after initializing the link. For situations where both ports in a
link support both port types, a tiebreaker field and protocol determines
which port is upstream-facing and which is downstream-facing.
• Port Configuration contains a bit to specify a speed of 5 Gbps. A down-
stream-facing port sends this packet to its link partner.
• Port Configuration Response accepts or rejects a received Port Configura-
tion LMP.
61
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This chapter takes a closer look at USB’s four transfer types: control, bulk,
interrupt, and isochronous. Each type has features that make it suitable for spe-
cific purposes.
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Control transfers have two uses. For all devices, control transfers carry the stan-
dard requests that the host uses to learn about and configure devices. Control
transfers can also carry requests defined by a class or vendor for any purpose.
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Every device must support control transfers over the default pipe at endpoint
zero. A device may also have additional pipes for control transfers, but in reality
there’s no need for more than one. Even if a device needs to send a lot of control
requests, hosts allocate bandwidth for control transfers according to the number
and size of requests, not by the number of control endpoints, so additional con-
trol endpoints offer no advantage.
Chapter 3
62
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Chapter 2 introduced control transfers and their Setup, Data, and Status stages.
Each stage consists of one or more transactions.
Every control transfer must have a Setup stage and a Status stage. Not all trans-
fers have Data stages, though specific requests can require them. Because every
control transfer requires transferring information in both directions, the control
transfer’s message pipe uses both the IN and OUT endpoint addresses.
In a control write transfer, the data in the Data stage travels from the host to the
device. Control transfers that have no Data stage are also considered to be con-
trol write transfers. In a control read transfer, the data in the Data stage travels
from the device to the host. Figure 3-1 and Figure 3-2 show the stages of con-
trol read and control write transfers at low and full speeds on a low/full-speed
bus. There are differences, described later in this chapter, for some high-speed
transfers, low- and full-speed transfers with USB 2.0 hubs on high-speed buses,
and SuperSpeed transfers.
In the Setup stage, the host begins a Setup transaction by sending information
about the request. The token packet’s SETUP PID identifies the transaction as
a Setup transaction that begins a control transfer. The data packet contains
eight bytes of information about the request, including the request number,
whether or not the transfer has a Data stage, and if so, in which direction the
data will travel.
The USB 2.0 and USB 3.0 specifications define standard requests. Successful
enumeration requires specific responses to some requests, such as the request
that sets a device’s address. For other requests, a device can return STALL to
indicate that the request isn’t supported. A STALL ends the transfer. A class
may require a device to support class-specific requests, and devices can support
requests defined by a vendor-specific driver.
When present, the Data stage consists of one or more transactions. Depending
on the request, the host or peripheral may be the source of the data in these
transactions, but all data packets in this stage are in the same direction.
The Status stage consists of one IN or OUT transaction where the device
reports the success or failure of the transfer. The source of the Status stage’s data
packet is the receiver of the data in the Data stage. When there is no Data stage,
the device sends the Status stage’s data packet. On completing or abandoning
the current transfer, the host can begin a new control transfer.
A Transfer Type for Every Purpose
63
Figure 3-1. A USB 2.0 control write transfer contains a Setup transaction, zero or
more Data transactions, and a Status transaction. Not shown are the PING
protocol used in some high-speed transfers with multiple data packets and the
split transactions used with low- and full-speed devices on a high-speed bus.
Chapter 3
64
Figure 3-2. A USB 2.0 control read transfer contains a Setup transaction, one or
more data transactions, and a status transaction. Not shown are the split
transactions used with low- and full-speed devices on a high-speed bus.
A Transfer Type for Every Purpose
65
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As described in Chapter 2, if a high-speed control write transfer has more than
one data packet in the Data stage and the device returns NYET after receiving a
data packet, the host may use the PING protocol before sending the next data
packet.
If a host is performing a control transfer with a low- or full-speed device on a
high-speed bus, the host uses split transactions for all of the transfer’s transac-
tions. To the device, the transaction is no different than a transaction with a
USB 1.x host. The USB 2.0 or USB 3.0 hub nearest the device initiates transac-
tions with the device and returns data and status information to the host.
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On a SuperSpeed bus, the Setup stage’s Setup Data Packet contains the eight
bytes of Setup data. The Data Packet Header uses the following values:
Sequence Number = 0
Data Length = 8
Setup = 1
Figure 3-3 shows the structure of a SuperSpeed control write transfer. The host
begins the transfer with a Setup Data Packet, and on receiving the packet with-
out error, the device responds with an ACK Transaction Packet. If the transfer
has a Data stage, the host sends one or more Data Packets, and the device
responds to each with an ACK Transaction Packet. If the transfer has multiple
Data packets, the Sequence Numbers in the Data and ACK packets increment
for each Data packet. In the Status stage, the host sends a STATUS Transaction
Packet, and the device returns ACK.
Figure 3-4 shows the structure of a SuperSpeed control read transfer., which is
identical to a control write transfer except for the Data stage. In the Data stage,
the host sends one or more ACK Transaction Packets, and the device responds
to each with a Data Packet.
A device can control the flow of a control transfer by responding to the Setup
Data Packet with an ACK Transaction Packet with NumP = 0 and Sequence
Number = 0. The device then requests to start the Data and Status stages by
sending an ERDY Transaction Packet.
In the Data or Status stage, an endpoint can return a STALL or NRDY Trans-
action Packet instead of ACK. A STALL ends the transfer. NRDY halts the
transfer until the device returns ERDY.