Quartz 5

USB

12 min read

Universal Serial Bus

Three speeds:

  • High (480 Mbits/s)
  • Full (12)
  • Low (1.5)
    and then USB 3 came along..

USB is HOST controlled. Only one HOST per bus (bunch of connections). On-The-Go was then tacked on.. but don’t worry too much.

USB addresses allow for 127 ‘slave’ devices. Hubs can be involved, but maximum number of tiers is 6 (root hub included). Limitation of cable is 5 metres.

Star Topology - every device is connected to a “root”. Benefits was that each sub could switch a device if it draws too much current, high speed doesn’t overload low speed… etc. Better than daisy chaining.

The S in USB is serial - data is sent one bit at a time over a wire (or pairs). Two power wires (+5V, GND) and two twisted pair differential data signals (D+, D-). Something about NRZI, but I’m not an electrical engineer.

The B in USB is bus - a way of data transmission.

USB is hot-pluggable. When new device is inserted, the host interrogates it and loads the driver.

Supports Control, Interrupt, Bulk and Isochronous transfers. Each transfer mode provides the designer trade-offs in areas such as error detection and recovery, guaranteed latency and bandwidth.

Isochronous allows a device to reserve a defined amount of bandwidth with guaranteed latency

Electricals

Data

J and K states are weird, they are different depending on speed.

USB uses NRZI (Non-Return-to-Zero Inverted) encoding:

1 → no transition
0 → transition (J ↔ K)

Bit stuffing is used to avoid long runs of 1s (so clock recovery keeps working).
SE0 is both lines low (<0.3V)

USB devices work by pulling the D+ or D- to 3.3v (1.5K). Full speed pulls D+, low speed pulls D-.

The receiver defines a differential ‘1’ as D+ 200mV greater than D- and a differential ‘0’ as D+ 200mV less than D-. The polarity of the signal is inverted depending on the speed of the bus. Therefore the terms ‘J’ and ‘K’ states are used in signifying the logic levels. In low speed a ‘J’ state is a differential 0. In high speed a ‘J’ state is a differential 1.

Remember - J is the “idle state”

Power

Devices obtain power from the V_Bus. It specifies its power consumption in 2mA units. Three classes of USB functions:

  • low-power bus powered functions: up to 100mA
  • high-power: cannot draw more than one unit load (100mA) unless it is configured, then can draw 5
  • self powered: cannot draw more than one unit load, must have provisions in place

Suspend

Now, suspend mode is MANDATORY on all devices. It essentially turns 1 unit load into 500uA. When no activity on bus for > 3.0ms, it has 7.0ms to shut down.

To prevent random suspends:

  • A high speed bus will have micro-frames sent every 125.0 µs ±62.5 ns.
  • A full speed bus will have a frame sent down each 1.000 ms ±500 ns.
  • A low speed bus will have a keep alive which is a EOP (End of Packet) every 1ms only in the absence of any low speed data.

There are also selective suspends. Remote wakeup can also signal host to resume from suspend

Clocks

  • High speed data is clocked at 480.00Mb/s with a data signalling tolerance of ± 500ppm.
  • Full speed data is clocked at 12.000Mb/s with a data signalling tolerance of ±0.25% or 2,500ppm.
  • Low speed data is clocked at 1.50Mb/s with a data signalling tolerance of ±1.5% or 15,000ppm.

Packet Transfer

USB is master-slave. Host initiates all read / write requests.

Little endian - LSBit first.
USB Packet Terms:

  1. Sync - 8 bits or 32 bits for high speed and is meant to synchronize the clock
  2. PID - Packet ID. Complemented (bitwise NOT) and repeated to make 8 bits
  3. ADDR - 7 bits. Address 0 is the special address for “device not yet assigned address”
  4. ENDP - 4 bits. For endpoints
  5. CRC - 5 bits. Data packets have 16. Cyclic Redundancy Checks
  6. EOP - 2 bit times followed by a J for 1 bit time. (K and J states just have to do with the signals, SE0 is both lines LOW)

Packet Types

GroupPID ValuePacket Identifier
Token0001OUT Token
Token1001IN Token
Token0101SOF Token
Token1101SETUP Token
Data0011DATA0
Data1011DATA1
Data0111DATA2
Data1111MDATA
Handshake0010ACK Handshake
Handshake1010NAK Handshake
Handshake1110STALL Handshake
Handshake0110NYET (No Response Yet)
Special1100PREamble
Special1100ERR
Special1000Split
Special0100Ping

Token

For token packets, the three types are:

  • In: USB device that host wants to read info
  • Out: Host wants to send USB device info
  • Setup: Control transfers
    Follows Sync => PID => ADDR => ENDP => CRC5 => EOP

Data

For data packets, there are two types capable of 1024 bytes

  • Data0
  • Data1
    High speed has
  • Data2
  • MDATA
    Follows Sync => PID => Data => CRC16 => EOP
  • Maximum data payload size for low-speed devices is 8 bytes.
  • Maximum data payload size for full-speed devices is 1023 bytes.
  • Maximum data payload size for high-speed devices is 1024 bytes.

Handshake Packets

Only got the PID.

  • ACK: acknowledge package received
  • NAK: device temporarily cannot send or receive data. Used during interrupt transactions
  • STALL: device needs intervention

Follows Sync => PID => EOP

Start of 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.

Endpoints

Endpoints are sources or sinks. Software may write to the EP1 for example. EP1 OUt is “sink” for host => device. EP1 IN is “source”.

All devices must have endpoint zero. Which is for control / status / enumeration.

Pipes

Client software sends data thru pipes. Pipes = connection from host => endpoint(s). Can be one of the 4 Transfer Types. This means that different endpoints can be used for different types of transfers (gaming headset that needs isochronous for audio, interrupt for vol up/down/mute, control for USB stuff)

Two types:

  • Stream pipes: Send any type of data. Predefined in/out. For bulk, isochronous, interrupt. Controlled by host or device (meaning host / device can “control: by filling/emptying buffer)
  • Message pipes. Host controlled. In AND out. Only control transfers

Transfer Types

Control Transfers

Command and status operations. “Paperwork”. Uses best effort delivery.

Up to 3 stages:

  • The Setup Stage: request is sent. Uses the SETUP Token
  • Optional Data Stage. Further divided to IN and OUT Transfers
    • IN: Function ignores if malformed. If correct, respond either with DATA or NAK
    • Out: Device replies ACK on success, NAK if EP is not empty due to processing previous (Hold on, im still processing!). STALL on error
  • Status Stage reports the status of the overall request and this once again varies due to direction of transfer. Status reporting is always performed by the function.
    • IN: If the host sent INs, then host must acknowledge with an OUT + zero length data (ZLP). If error, STALL, if busy, NAK
    • OUT: If the host sent OUT tokens, the function will acknowledge with a ZLP. If error, STALL, if busy, NAK

Interrupt Transfers

USB devices must wait until host polls to interrupt! Guarantees latency, and error detection.

  • IN: Host will periodically poll, configured by endpoint descriptor
  • OUT: Send device interrupt data

Isochronous Transfers

Isochronous guarantees a latency bound, and does NOT guarantee message delivery. For things like audio data, where you need guaranteed latency and dropped packets are fine.

Maximum payload size can be configured, high & full speed ONLY.

Usually provides alternatives, lower bandwidth fallback endpoints

Bulk Transfers

For large, bursty transfers. Guarantees DELIVERY of data. Full & high speed modes only.

  • IN: Ready to receive data
  • OUT: Send data

A Note About Bandwidth

The HOST is responsible for bandwidth management. Usually a percentage is reserved for control transfers.

USB Descriptors

Heart of USB software.

Device Descriptors

Defines Product ID / Vendor ID to load specific drivers, USB version, maximum packet size, etc. Also has # of config descriptors

Configuration Descriptors

Rare. Mutually exclusive, only one configuration allowed at a time. Used for driverlss / drivered scenarios? Host will send a SetConfiguration command to select.

Interface Descriptors

These define logical “functions” - multiple can co-exist. Defines a bInterfaceNumber and bAlternative number. bInterface number defines which interface it is, bAlternative defines what alternative it is.

So for example, you can have (0, 0), (1, 0) and (1,1). (1, 0) and (1, 1) are alternatives.

Endpoint Descriptors

EP0 is special, it is assumed to be configured. All EP descriptors describe non-zero

String Descriptors

Optional, if there’s none, then the string descriptors should point to index 0. Index 0 should be a list of languages?

There’s a format for language codes, and the Get Descriptor request can request that language code.

Weird Naming Convention

b - byte
w - word (2 bytes)
dw - (4 bytes)
i - index
bm - bitmap

On-The-Go

Can act as host or device - used in your phone.

Terminology

  • Function: A specific logical unit of the device (Audio function for camera, video function for camera)

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