Embedding USB - The Implementation Challenges and Limitations

USB 2.0 Star network configuration  Only one host  Maximum of seven levels of tiers, hub at center  Host and hubs provide power to slaves  Maximum of 5m per cable  Maximum of 127

Renesas Electronics America Inc.

Embedding USB - The Implementation Challenges and Limitations

Micriµm

Renesas Technology & Solution Portfolio

 Universal Serial Bus: Serial bus standard to interface

devices

 Peripheral communication protocol of choice for keyboards,

printers, and memories

protocol stack from USB Device protocol stack and

On-The-Go

 It goes over the design implications and some solutions.

USB for Embedded Systems

USB Products

1969 Serial port (RS-232C)

1995 The USB consortium is formed by Compaq, DEC, IBM,

Intel, Microsoft, NEC and Nortel

1996 USB 1.0 standard was released Plug Fest

1999 USB 1.1 was released Corrected bugs

2000 USB 2.0 was released specifying the implementation of a

480 Mbps bus

2001 USB On-The-Go (OTG) supplement was added which

allowed devices to talk to each other

2005 Wireless USB standard was released which is a point to

point wireless communications link

Brief History

USB 2.0

 Star network configuration

 Only one host

 Maximum of seven levels

of tiers, hub at center

 Host and hubs provide power to slaves

 Maximum of 5m per cable

 Maximum of 127 devices (hubs included)

 Host schedules and initiates data transfers

 Devices implements one

or more functions

Architecture Topology

USB Speed Low Speed Full Speed High Speed

Speed 1.5 Mbits/sec 12 Mbits/sec 480 Mbits/sec

USB 1.1

 USB 1.1

 OHCI – Compaq, Microsoft, and National; hardware burden

 UHCI – Intel, software burden

 EHCI – USB with Intel, simplified software and reduced cost

 SuperSpeed - Hewlett-Packard, Intel, Microsoft, NEC, NXP

Semiconductors and Texas Instruments

 November 17, 2008

Host Controllers

 Device

 A device is a logical or physical entity that performs one of more functions.

 A composite device has one address but multiple interfaces or

association of interfaces each providing a function (i.e a function printer, scanner, fax, copier)

 A compound device contains a hub with one or more

permanently attached devices A host treats the hub and its functions as if they were separate physical devices.

Device

Root Hub (RH) = embedded hub part of the Host

 Provides 1 or more attachment points (i.e ports)

 Equivalent to external Hub – Presents standard hub functions – Presents standard states as other USB devices

Host and Root Hub

Responsible for:

 Device connection/disconnection

 Device reset

 When the Host stack starts, the 1st device to be enumerated

is always the Root Hub (RH)

which contains features supported by the RH

 bNbrPorts: Number of downstream facing ports that this hub

supports

 Device attachment/detachment event

 Port power/enable/reset/suspend/resume events

Host and Root Hub

Enumerating Function 1

Enumerating Hub Hub descriptor

Enumerating Function 2 & 3

bNbrPorts = 2

bNbrPorts = 2

Mechanical Specifications

Mechanical Specifications

Cable:

• Cable is made of 4 28-AWG conductors:

• Data pair is twisted

• Maximum cable length is 5 meters

Connector:

• Connector Type A connects to host

• Connector Type B connects to device

• Connector power pins are longer for hot-swap

1 2 3 4

Type A USB Connector

Type B USB Connector

1 2

3 4

D+ D- + -

D+

+

D

-Mini-B Plug

Mini connectors were added in USB 2.0

and OTG supplement

An alternative to the standard B

connector on handheld and portable

devices The mini-B connecter has a

fifth pin, named ID It is not connected

Number

Signal Name

Typical Wiring Assignment

Shell Shield Drain Wire

Micro connector Pin Assignments

OTG Connectors

Micro-A USB Connector

Micro-B USB Connector

1 5

White insulator

1 5

Black insulator

Electrical Specifications

 The complexities and speed of the USB protocol are such

that it is not practical to expect a general purpose

microcontroller to be able to implement the protocol using

an instruction-driven basis

portions of the specification, and the circuitry grouping which performs this function is referred to as the Serial Interface

Engine (SIE)

Serial Interface Engine

Device Detection

 Current is limited to 100mA

 Either the D+ or the D- line will be pulled high

 D+ is high, the device is a full or high speed

 The USB bus 5V supply is a very attractive feature A USB

device with a single connection is also attractive to the user

 Supply voltage can fall to 4.35V at the device

 When other devices are being plugged, there can be transients

lowering the supply voltage by 0.4V

 Devices can be classified as:

– Bus powered – Self powered – Hybrid powered

Device Powering

 Self-powered devices provide power requirements on descriptor

(see descriptor in the software section) Initially provides 100mA, this can be increased up to 500mA (high power)

 Self-powered hubs have a limitation on self-powered devices of

Device Powering

 There is a suspend mode (no bus activity) In suspend

mode, devices consume up to 500µA (where 200 µA already used by the pull up and pull-down resistors)

which meets this requirement Do not forget that a badly

placed resistor can very easily draw current which you

hadn't expected Measure your suspend current with a meter

feature enabled are allowed to draw up to 2.5mA during

suspend

device is configured

Device Powering

 Place a capacitance of at least 1uF across Vbus and GND to prevent from flyback voltage when unplugging a cable at the far end from the device

GND is limited by the specification to 10uF This is to avoid

other devices to crash because plugging device in, a

capacitance between Vbus and GND will cause a dip in

voltage across the other ports of the hub

with at least 120uF

Hot-Pluggable

 If you pull, say D+, high in the absence of Vbus then you

will risk faulty operation with On-The-Go hosts

D+ or D- line above the Vbus voltage supplied This means that you must sense when Vbus is connected

derived from Vbus, or controlled by Vbus in such a way that the resistor never sources current to the data line when

Vbus is switched off

Self-Powered Devices

 On-The-Go and Battery Charging Specification both add new powering modes to the USB specification.

Power)

 +5 volts DC at up to 6 amps per connector (up to 30 watts)

 +12 volts DC at up to 6 amps per connector (up to 72 watts)

 +24 volts DC at up to 6 amps per connector (up to 144 watts)

of extra power lines for point-of-sale terminals

More Power

SYNC PATTERN PACKET INFORMATION IDLE

SYNC PATTERN PACKET INFORMATION IDLE

SYNC PATTERN PACKET INFORMATION IDLE

(NRZI)

 Given that there are 2 data lines, many different conditions are signaled using them:

Bus States

Single Ended Zero (SE0) D+ and D- low Single Ended One (SE1) D+ and D- high

Data J State:

Low-speed Full-speed Differential '0' Differential '1'

Data K State:

Low-speed Full-speed Differential '1' Differential '0'

Idle State:

Low-speed Full-speed D- high, D+ low D+ high, D- low

High Speed

Packet Sync

Just prior to the packet sync, both data lines are low The sync is sent using the NRZI sequence

KJKJKJKJKJKJKJKJ KJKJKJKJKJKJKJKK

A hub may drop up to 4 bits from the sync pattern After 5 hubs the

sync field of a packet may be only 12 bits long.

 On a low or full speed link, a brief Single Ended Zero (SE0) state is used to indicate End Of Packet (EOP), and idle is indicated by a J

condition.

 On a high speed link, the idle state is effectively a SE0, so that

state is not available to indicate EOP, and a different method for

indicating the end of packet is used

 During normal data transmission there can not be a run of more

than 6 1's in a row, because a 0 is automatically inserted (and will

be removed on reception), for clock recovery reasons.

 At high speed, the EOP is indicated, by deliberately sending a byte which contains a bit-stuffing error; '01111111' This applies at the end of all packets except SOF.

 Each high speed SOF packet is terminated with 5 NRZI bytes

containing bitstuffing errors: 01111111 11111111 11111111

-11111111 11111111 This pattern allows the 'disconnection

envelope detector' to detect a rise in data amplitude above 625

mV, in the event that the device, along with its termination

resistors, has been unplugged.

End of Packet

Negotiating High Speed

A high speed device will always present itself initially as a Full Speed device (by a 1.5K pullup resistor on D+).

The negotiation for High Speed takes place during the Reset, which is the first thing a host (root or tier hub) must do to a device before

attempting data communication.

The high speed detection handshake is initiated by the device A hub will respond to it, if it is high speed capable.

High Speed Hubs

with Full and Low Speed Devices

Traffic directed at full or low speed devices uses high speed links, as

high speed split transactions.

SYNC IDLE PACKET INFORMATION EOP

PID

Packets

 The fundamental element of communications is the packet

 A packet is made of three parts: START, INFORMATION, and

END-OF-PACKET (EOP)

 At the START of a packet will be a sequence of transitions which is called

SYNC

 The SYNC pattern is a chirp of either 8 bits (full-/low-speed) or 32 bits

(high-speed) where each bit is a J or K state:

KJKJKJKJKJKJKJKJKJKJKJKJKJKJKJKJKK

 The INFORMATION varies from 1 byte up to 1024 bytes

bits long at low and full speed or 32 bits long for high speed and is used to synchronize the clock of the receiver with that

of the transmitter The last two bits indicate where the PID

fields starts

USB Packet Fields

SYNC

PID

type of packet that is being sent

USB Packet Fields

PID

Group PID Value Packet Identifier

1100 ERR (Split Transaction Error Handshake (reuses PRE value))

1000 SPLIT (High-speed Split Transaction Token)

0100 PING (High-speed flow control probe for a bulk/control Endpoint)

 The first byte in every packet is the Packet Identifier (PID)

byte This byte needs to be recognized quickly by the USB

controller This is why it is not included in any CRC checks

It has its own validity check

4 bits by repeating and complementing them, making an 8

bit PID field in total

USB Packet Fields

PID

PID 0 PID 1 PID 2 PID 3 nPID 0 nPID 1 nPID 2 nPID 3

ADDR

designated for Being 7 bits in length allows for 127 devices

to be supported Address 0 is not valid, as any device which

is not yet assigned an address must respond to packets sent

to address zero

USB Packet Fields

ADDR

ENDP

endpoints Low speed devices, however can only have 2

additional endpoints on top of the default pipe (4 endpoints max)

USB Packet Fields

ENDP

 A CRC is a value calculated from a number of data bytes to form a unique value which is transmitted along with the data bytes, and then used to validate the correct reception of the data

 a 5 bits long (CRC5)

and

 a 16 bits long (CRC16)

algorithms used to compute the CRCs

Cyclic Redundancy Code (CRC)

EOP

approximately 2 bit times followed by a J for 1 bit time

USB Packet Fields

EOP

 For a general embedded system, silicon vendors have

resolved most of the hardware and electrical issues for the

embedded engineer You need not be concerned with the

detail of syncs, bit stuffing, and End Of Packet conditions

You should just be aware that the Serial Interface Engine

can recognize the start and end of a packet, and that the

packet contains a whole number of bytes

the software realm

USB Controllers

Software Specifications

 USB has four different packet types:

 Start of Frame Packets

 Token Packets

 Data Packets

 Handshake Packets & Split Transaction Special Token Packets

USB Packet Types

PID FRAME

NUMBER

CRC 5 SOF

8 11 5

0101

indicate the start of a new

frame The SOF packet

consisting of an 11-bit

frame number is sent by

the host every 1ms ± 500ns

on a full speed bus or every

125 µs ± 0.0625 µs on a

high speed bus

(micro)frame An SOF

allows endpoints to identify

the start of the

(micro)frame and

synchronize internal

endpoint clocks to the host

Start of Frame Packets

PID ADDR CRC

5

IN OUT SETUP

8 7 5

1001 0001 1101

EN DP

4

transaction to follow They are always

the first packet in a transaction,

identifying the targeted endpoint, and

the purpose of the transaction

 Data packets contain the payload.

transmitting up to 1024 bytes of data

 Data0

 Data1

Data Packets

Low and Full speed

DATA0 DATA1 DATA2 MDATA

0011 1011 0111 1111

DATA0 and DATA1 PIDs are used in Low and Full speed links as part of an error-checking

system.

When used, all data packets on a particular endpoint use an alternating DATA0 / DATA1 so that the endpoint knows if a received packet is the one it is expecting If it is not the expected

packet, it will still acknowledge (ACK) the packet as it is correctly received, but will then

discard the data, assuming that it has been retransmitted because the host missed seeing the ACK after the previous data packet.

 High Speed mode defines two additional data PIDs, DATA2 and MDATA

 This data packet token has been added as part of a system for controlling

multiple isochronous IN packets during one microframe at high speed For each isochronous IN packet requested, the suffix of the DATAx PID represents the remaining number of packets to be transferred during the current micro- frame.

 This data packet token has been added as part of a system for controlling

multiple isochronous OUT packets during one microframe at high speed All but the last packet sent during a microframe use the MDATA PID The last packet sent uses DATA0, DATA1 or DATA2 depending on whether one, two or three packets were sent.

Data Packets

High Speed

8

ACK NAK STALL NYET

0010 1010 1110 0110

 Handshake packets are used for acknowledging data

or reporting errors There are four type of

handshake packets which consist simply of the PID.

The device is not ready to answer (High Speed only).

Handshake Packets

C PORT SE

E T

7

1 1 1 2

Split Transaction Special Token Packets

 The split transaction token is used to support split transactions

between the host controller communicating with a hub operating

at high speed with full-/low-speed devices to some of its

downstream facing ports.

 Packet

 Token: OUT, IN, SOF, SETUP

 Data: DATA0, DATA1, DATA2, MDATA

 Handshake: ACK, NAK, STALL, NYET

 Special: PRE, ERR, SPLIT, PING

Hierarchy of Data

Transfer (Control, Interrupt, Isochronous, Bulk)

Transaction 1 Transaction 2 Transaction 3 … Transaction n

Packet 1 Packet 2 Packet 3 … Packet 1 Packet 2 … Packet 1 Packet 2 …

 A successful transaction is

 a sequence of three packets which performs a simple but secure transfer of data.

 there are only 2 packets;

 the handshake packet on the end is omitted This is because

error-checking is not required.

 In each of the following illustrations, the packets from the host

Transactions

 A successful SETUP transaction comprises three sequential

packets This is similar to an OUT transaction, but the data

payload is exactly 8 bytes long, and the SETUP PID in the

token packet informs the device that this is the first

transaction in a Control Transfer (see standard requests)

packet

SETUP Transaction

Token Packet

Data Packet

Handshake Packet

From Host

ADDR SETUP

E N D P

C R C 5

D A T A 0

8 bytes