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Protocol in Depth – USB

Protocol in Depth – USB
by Luigi Filho on 08-30-2020 at 10:00 am

Protocol in Depth USB

The USB protocol is a very complex protocol, so there is no way i can explain every detail in a post, but i can let much more easy to understand what happens in a bit level.

There isn’t much good material for easy understand about USB, so i made some assumptions for make easier explain everything. In this post i’ll explain how it goes.

The USB protocol doesn’t have a very well-defined layers, so i’ll divide in 3 layers: physical, protocol and framework. In this series each post will be about one of theses layers, how it’s shown in the figure below.

No alt text provided for this image

Of course this is how i choose to represent the USB layers, in the next posts:

  • In the next section I’ll talk about the physical layer.
  • After that I’ll talk about protocol layer.
  • In 4th section I’ll talk about the software in the framework.

Protocol in depth – USB – Physical Layer

In this layer I’ll explain about the Transceiver and Serial Interface Engine (SIE) but let me be clear, there some important concepts that I’ll not talk about like Endpoint, Host, Device, Hub.etc, and I’ll talk a little about the Low-Speed, Full-Speed and High-Speed, as I’m using the USB 2.0 specification I’ll not talk about SuperSpeed. There is no way that I can cover all topics in such small posts.


At each end of the data link between host and device is a transceiver circuit. The transceivers are similar, differing mainly in the associated resistors.

Figure 1. Upstream End Transceiver
Figure 2. Downstream End Transceiver (Full Speed)

A typical upstream end transceiver is shown in first figure with high speed components omitted for clarity. By upstream, we mean the end nearer to the host. The upstream end has two 15K pull-down resistors.

Each line can be driven low individually, or a differential data signal can be applied. The maximum ‘high’ level is 3.3V.

The equivalent downstream end transceiver, as found in a device, is shown in second figure.

When receiving, individual receivers on each line are able to detect single ended signals, so that the so-called Single Ended Zero (SE0) condition, where both lines are low, can be detected. There is also a differential receiver for reliable reception of data.

Some signaling that the transceiver need to be aware is described below:

  • Speed identification

At the device end of the link a 1.5 kohm resistor pulls one of the lines up to a 3.3V supply derived from VBUS.

This is on D- for a low speed device, and on D+ for a full speed device.

(A high speed device will initially present itself as a full speed device with the pull-up resistor on D+.)

  • Line States

Given that there are just 2 data lines to use, it is surprising just how many different conditions are signaled using them:

  • Detached

When no device is plugged in, the host will see both data lines low, as its 15 kohm resistors are pulling each data line low.

  • Attached

When the device is plugged in to the host, the host will see either D+ or D- go to a ‘1’ level, and will know that a device has been plugged in.

The ‘1’ level will be on D- for a low speed device, and D+ for a full (or high) speed device.

  • Idle

The state of the data lines when the pulled up line is high, and the other line is low, is called the idle state. This is the state of the lines before and after a packet is sent.

  • J, K and SEO States

To make it easier to talk about the states of the data lines, some special terminology is used. The ‘J State’ is the same polarity as the idle state (the line with the pull-up resistor is high, and the other line is low), but is being driven to that state by either host or device.

The K state is just the opposite polarity to the J state.

The Single Ended Zero (SE0) is when both lines are being pulled low.

The J and K terms are used because for Full Speed and Low Speed links they are actually of opposite polarity.

All details is shown in the figure below:

usb protocol
  • Single Ended One (SE1)

This is the illegal condition where both lines are high. It should never occur on a properly functioning link.

  • Reset

When the host wants to start communicating with a device it will start by applying a ‘Reset’ condition which sets the device to its default unconfigured state.

The Reset condition involves the host pulling down both data lines to low levels (SE0) for at least 10 ms. The device may recognise the reset condition after 2.5 us.

  • EOP signal

The End of Packet (EOP) is an SE0 state for 2 bit times, followed by a J state for 1 bit time.

  • Suspend

One of the features of USB which is an essential part of today’s emphasis of ‘green’ products is its ability to power down an unused device. It does this by suspending the device, which is achieved by not sending anything to the device for 3 ms.

Normally a SOF packet (at full speed) or a Keep Alive signal (at low speed) is sent by the host every 1 ms, and this is what keeps the device awake.

A suspended device may draw no more than 0.5 mA from Vbus.

A suspended device must recognise the resume signal, and also the reset signal.

  • Resume

When the host wants to wake the device up after a suspend, it does so by reversing the polarity of the signal on the data lines for at least 20ms. The signal is completed with a low speed end of packet signal.

  • Keep Alive Signal

This is represented by a Low speed EOP. It is sent at least once every millisecond on a low speed link, in order to keep the device from suspending.


What is the SIE? A typical function USB hardware interface is shown below

usb protocol

The SIE is the frontend of this hardware and handles most of the protocol signaling. The SIE typically comprehends signaling up to the transaction level. The functions that it handles could include:

  • Packet recognition, transaction sequencing
  • SOP, EOP, RESET, RESUME signal detection/generation
  • Clock/Data separation
  • NRZI Data encoding/decoding and bit-stuffing
  • CRC generation and checking (Token and Data)
  • Packet ID (PID) generation and checking/decoding
  • Serial-Parallel/ Parallel-Serial Conversion

A typical implementation of an SIE with these functions takes about 2500 gates. So the module itself is fairly small; and the functionality is straightforward. In spite of this apparent simplicity, it is possible to end up with a design that doesn’t work reliably i.e. a design which is not robust. I will point out some reasons for problems from a design.

  • Sources of robustness problems

The primary source of robustness problems is the existence of multiple clock domains in the SIE, some of which are asynchronous to each other. If signaling between these domains doesn’t adhere to synchronization rules, intermittent problems can result. These problems are invariably difficult to track down and fix.

Other areas which have the potential for robustness problems include:

  • out-of-band signal handling on per-packet basis
  • bit stuffing/unstuffing
  • special casing for setup, iso etc
  • special casing for low speed
  • suspend /resume support

The following text will review some of these areas in turn and discuss techniques to address the problems.

  • Multiple clock domains

The typical SIE has to deal with four clock zones in three domains:

  1. · USB host 12Mhz clock or receive clock
  2. · internal 4x clock (48Mhz) and transmit clock (divided by 4 version)
  3.  · SIE backside clock or interface clock
  • Race Conditions in the transmit domain

The clock zones in the second domain are synchronous; however race conditions could occur in signaling between the 1x and 4x sub domains because the 1x clock is derived from the 4x clock. This may be a bigger problem in some target technologies than in others. The problem is exacerbated by the need to switch the hardware between transmitter and receiver clocks.

Since the USB is half duplex several of the modules in the SIE can be shared between transmit and receive e.g. the crc logic . Since every USB transaction includes receive and transmit phases, the state machines carry state between the phases. So there is a need for a means to reliably multiplex between receive clock and transmit clock.

  • Packet delimiters and out of band signaling

Precise detection of packet delimiters is crucial for robust SIE operation. Each packet has a start delimiter (or sync) and end delimiter (or EOP). The nominal sync field consists of an NRZI KJKJKJKK pattern. Even though this is an in band (made up of differential signals) pattern, the initial bit may be distorted due to hub turn on behavior .

  • Bit stuffing and unstuffing

Bit stuffing and unstuffing can be implemented by putting the state machines and datapath on hold while stuffing or stripping the extra bit. Bit unstuffing near the EOP needs to be handled carefully as explained above.

Although most transactions are three phase, ISO transactions are only two phase and the state machines need to comprehend this. Similarly SETUP transactions are identical to OUT transactions except that they cannot be NAKed or STALLed. The data buffering and the state machines need to take this into account. Data toggle sequencing logic at a bidirectional endpoint should take into account the specific requirements for the starting toggle sequence of each stage of a control transfer.

Low speed signaling is identical to full speed signaling except for the inversion of polarity. But low speed devices need to comprehend that while most data entities are defined in terms of number of bits, the se0 width for reset is not. Low speed devices should also be able to handle keep-alive signals (bare EOPs) correctly.

USB Protocol in Depth – Protocol Layer

In this article I’ll try my best to explain the most part of the protocol layer of USB specification.

I’ll talk a little about the transfers, but will not cover a lot, maybe I’ll make an article just about the transfers, but what you need to know is that for each use you have a different type of packages.

In the last section we could check the physical layer that will interface the connector and will work together with the SIE, but as the SIE isn’t very well define in the specifications this could lead to great confusion, as many manufacturers could implement the SIE together with the Protocol layer, or some of the functions of the protocol.

As what I define about SIE in the last post, SIE will handle the signaling and the interface with protocol layer, to let things easier let’s consider that the SIE will interface with protocol layer by UTMI (USB 2.0 Transceiver Macrocell Interface) or ULPI (UTMI+ low pin interface), that’s the usually you can find in most standard USB 2.0/3.0 transceiver integrated circuits.

Other consideration is that I’ll need to define the 4 types of transactions that exists on USB protocol:

  • Control Transfers Used for sending commands to the device, make inquiries, and configure the device.
  • Interrupt Transfers Used for sending small amounts of bursty data that requires a guaranteed minimum latency.
  • Bulk Transfers Used for large data transfers that use all available USB bandwidth with no guarantee on transfer speed or latency.
  • Isochronous Transfers Used for data that requires a guaranteed data delivery rate. Isochronous transfers are capable of this guaranteed delivery time due to their guaranteed latency, guaranteed bus bandwidth, and lack of error correction. Without the error correction, there is no halt in transmission while packets containing errors are resent.

The protocol layer manages the end-to-end flow of data between a device and its host. This layer is built on the assumption that the link layer guarantees delivery of certain types of packets and this layer adds on end to end reliability for the rest of the packets depending on the transfer type.

Here we will discuss the following concepts in detail:

  • Types of packets
  • Format of the packets
  • Expected responses to packets sent by the host and a device
  • Support for Streams for the bulk transfer type
  • Timing parameters for the various responses and packets the host or a device may receive or transmit

One first thing if you look at the USB communication from a time perspective, it contains a series of frames, in your time slot. Each frame consists of a Start of Frame (SOF) followed by one or more transactions. Each transaction is made up of a series of packets. A packet is preceded with a sync pattern and ends with an End of Packet (EOP) pattern. At a minimum, a transaction has a token packet. Depending on the transaction, there may be one or more data packets and some transactions may or may not have a handshake packet.

Packet Types can potentially represent four packet types:

1.      Token packets

  • Initiate transaction
  • Identify device involved in transaction
  • Always sourced by the host

2.      Data packets

  • Delivers payload data
  • Sourced by host or device

3.      Handshake packets

  • Acknowledge error-free data receipt
  • Sourced by receiver of data

4.      Special packets

  • Facilitates speed differentials
  • Sourced by host-to-hub devices

I’ll cover Token, Data and Handshake packets, special packets will be cover maybe in a future article about Hubs, leave a comment if you want an article about USB HUB.

Token packets always come from the host and are used to direct traffic on the bus. The function of the token packet depends on the activity performed, the format for a token packet is shown in the image below.

usb protocol

Another token packet is a SOF (start of frame) packet, show in the figure below

Data packets follow IN, OUT, and SETUP token packets. The size of the payload data ranges from 0 to 1024 bytes depending on the transfer type. The packet ID toggles between DATA0 and DATA1 for each successful data packet transfer, and the packet closes with a 16-bit CRC. The format is shown in the figure below

Handshake packets conclude each transaction. Each handshake includes an 8-bit packet ID and is sent by the receiver of the transaction, the format is shown in figure below

I know that I’m missing more information about each bit in PID, ADDR, ENDP, DATA, Frame Number and CRC5/16, but this article is already to extensive, if you want know more details, leave a comment.

How the USB is a protocol with handshake, usually with some packets will be expect some responses, to simplify let’s consider that always an token will be sent, followed by a data packet or receiving a data packet always will be a handshake packet involved, how the article is going a little extend, leave a comment if you want an article about the transfers.

All this is valid for each type of transfers, which can be a control, interrupt, bulk or isochronous transfers.

Other concept is that each 1ms is a frame that contain a SOF and can contain many packets. In high speed an SOF is sent out every 125 us and frame count is only incremented every 1ms.

USB in Depth – xHCI

USB in Depth xHCI

In terms of USB Standards, xHCI will be implemented in the Host side of the USB communication, from the Endpoint to the software driver.

Have in mind that, we are in the Host side of an USB 3.x version (With “SuperSpeed”) but still compatible with USB 2.x and 1.x. The xHCI was created to replace OHCI, UHCI and EHCI. Other important thing is that, all host controllers need to implement hub functions, all theses things I don’t cover in my last articles, and I’ll not cover in this, if you want, leave a comment and I’ll cover theses topics, this topic is created due a comment!

Another point is that in USB3.x you have a PIPE interface too instead of only UTMI+.

The first point you need to consider is the Endpoint, i talked a little about Endpoint in the others articles, here and here. Everything from the pins until the Endpoint it’s valid and the same here. But there is a new “player” the “Rings”, and there is three of them: Transfer Ring, Event Ring and Command Ring and each Endpoint has his Transfer Ring.

Another new concept is Transfer Request Block, that is a data structure construct in memory by the software to transfer a single physically block of data between host memory and the Host Controller. Contain a single data buffer pointer, size of the buffer and some additional control information.

Same new concepts are, Device Contexts and the Device Context Base Address Array. The Device Context is used to report the device configuration and state information to the system software and consists of 32 data structures ( index = 0 for Slot Context and the remaining (1 to 31 are Endpoints Context). The Device Context Base Address Array is the based lookup table for accessing the Device Context in each slot.

I’ll focus in the data transfer and not cover Event Ring Segmented Table, Event Ring, Command Ring. Other thing considering in the standard is an PCI config space, but I will not cover, is away out of this scope.

Other thing to consider is the registers of the xHCI, they are:

  • Doorbell Array – The Doorbell Array (up to 256 Registers of 32 Bits) is defined in the array for each Device Slot. System Software utilizes theses registers to notify the Host Controller that it has Device Slot related work for the Host Controller to perform.
  • Runtime Registers – Is referred as Runtime Base too, and each register multiples of 32 bits in length, is used to control microframe and interruptions.
  • xHCI Extended Capabilities – If the Host Controller implements any extended capabilities, it specifies a non-zero value in the xHCI Extended Capabilities Pointer field
  • Operational Registers – The Operational Registers, referred as Operational Base, are registers to support the operation of the USB xHC.
  • Capability Registers – This registers specify the limits and capabilities of the host controller implementation

Host Controller Initialization

When the system boots, host controller is enumerated, assigned a base register for the xHC register space and the system software sets the Frame Length Adjustment (FLADJ) register to a system-specific value.

Some tasks of the system software need to perform are:

  • Initialize the system I/O memory maps, if supported
  • After hardware reset, wait until the Controller Not Ready flag in the USBSTS is ‘0’ before writing any xHC Operational or Runtime Register
  • Program the Max Device Slots Enabled field in CONFIG register
  • Program the Device Context Base Address Array Pointer (DCBAAP) register
  • Define the Command Ring Dequeue Pointer
  • Initialize Interrupts
  • Write the USBCMD to turn the Host Controller ON.

At his point the host controller is up and running and the Root Hub ports will begin reporting device connects, etc, And the system software may begin enumerating devices.

Just remember that USB2.x devices require the port reset process to advance the port to the Enabled state.

USB Device Initialization

The USB Device initialization process is the same, whether the device attached is an HUB or any another Function.

After a Hardware Reset, HCRST, or command to the PLS = RxDetectState, all Root Hub ports shall be in Disconnected state and when a USB device is attached to a port that is in Disconnected state all the protocol process will start.

I will not go into details in this step, it’s a long process that you need to be sure to follow all steps for correct operation, you can check the standard to get all steps.

Transfer Request Block (TRB)

I’ll say that to start understand how everything get together you need to understand the TRB’s, they will make the interface from what you know about USB transfers (Isoch,Interrupt, Control and Bulk) to the xHCI software and hardware controllers. Each of the USB transfers have one TRB related, the template is shown in the image below.

usb protocol

Each transfer will have your own parameters, status, control, etc fields, with this basic structure.

The TRB Ring make the management of the TRB’s. The TRB Ring is a circular queque of TRB data structures and there is 3 basic types: Transfer, Event and Command.

Command Interface

The Software places commands on the Command Ring, through the Command Ring Control Register (CRCR), then rings the Host Controller Doorbell Register to notify the hardware. Some commands are:

  • Enable/Disable Slot
  • Configure/Reset/Stop Endpoint
  • Reset Device
  • Force Event/Header

Doorbells Registers

The doorbells are an array of 256 32-bit registers that reside in MMIO space and are indexed by device slot Id. Each Doorbell has an Endpoint associated to it.


The xHCI will manage all the USB transfers in the host, making the bridge between hardware and software.

With theses definitions in mind, i think you can go over the standard and understand better, as always that’s no possible to cover all in theses small articles.

You can always leave a comment, can ask to cover any topic as few people already did it, this topic included. The next Topic will be PCIe.


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