USB Made Simple

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Part 1

Part 2

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Part 2 - Electrical

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Cables

USB cables have been designed to ensure correct connections are always made. By having different connectors on host and device, it is impossible to connect two hosts or two devices together.

Unfortunately it is possible to buy non-approved cables and adapters with illegal combinations of connector. These may be useful in certain development situations, but can lead the unsuspecting user to make connections which can easily damage their equipment.

Cables - Electrical

USB requires a shielded cable containing 4 wires.

Two of these, D+ and D-, form a twisted pair responsible for carrying a differential data signal, as well as some single-ended signal states. (For low speed the data lines may not be twisted.)

The signals on these two wires are referenced to the (third) GND wire.

The fourth wire is called VBUS, and carries a nominal 5V supply, which may be used by a device for power.

Makeup of USB Cable

'A' Plug, 'B' Plug and 'Mini-B' Plug

Connectors

As stated above, USB uses different connectors on host and device to enforce correct connections.

"A" receptacles point downstream from a Host or Hub, while "B" receptacles point upstream from a USB device or hub.

Series A plugs mate with A receptacles, and B plugs mate with B receptacles.

Standard "A" and Standard "B" Plug and Receptacle Pin Assignments

Contact Number	Signal Name	Typical Cable Colour
1	VBUS	Red
2	D-	White
3	D+	Green
4	GND	Black
Shell	Shield	Drain Wire

A mini-B plug and receptacle has also been defined as an alternative to the standard B connector on handheld and portable devices. The mini-B connecter has a fifth pin, named ID, but it is not connected.

Mini-B Plug and Receptacle Pin Assignments

Contact Number	Signal Name	Typical Cable Colour
1	VBUS	Red
2	D-	White
3	D+	Green
4	ID	no connection
5	GND	Black
Shell	Shield	Drain Wire

Cable Types

The USB specification defines three forms of cable:

A high/full speed detachable cable with one end terminated with an A plug and the other end with a B or mini-B plug.
A captive high/full speed cable where one end is either hardwired to the vendors equipment or connected via a vendor specific connector and the other end is terminated with an A plug.
A low speed version of 2.

The maximum length of a high/full speed cable is determined by the attenuation and propagation delay. But, for a low speed cable, it is the signal rise and fall times that determine the maximum length. This forces the maximum length for low speed cable to be shorter than that for high/full speed.

Power Distribution

A device (or hub) can only sink (consume) current from its upstream port.

A 'self-powered' device is one which does not draw power from the bus.

A device which draws its power from the bus is called a 'bus-powered' device. In normal operation, it may draw up to 100mA, or 500mA if permitted to do so by the host.

A device which has been 'Suspended', as a result of no bus activity, must reduce its current consumption to 0.5 mA or less.

If a device is configured for high power (up to 500 mA), and has its remote wakeup feature enabled, it is allowed to draw up to 2.5mA during suspend.

Device Powering

The availability of a 5V supply is a very attractive feature of USB, and can simplify the design of a device considerably. And a device with a single connection is also attractive to the user.

However before designing a bus-powered device it is well to consider the limitations of this approach.

The voltage supplied can fall to 4.35V at the device. There can also be transients on this taking it 0.4V lower, due to other devices being plugged in. Your device needs to cope with these voltage levels.

The standard unit load available is 100mA. No device is permitted to take more than this before it has been configured by the host. It must also reduce its current consumption to 2.5mA* whenever it is 'suspended' by a lack of activity on the bus. However it is not required to obey this rule for a period of 1s from when it connects.**

It should be remember that of this 2.5mA, the required 1.5k pullup resistor is already drawing 0.3mA. This leaves you a budget of 2.2mA to power the rest of your device circuitry. If the device contains a micro-controller it will need a sleep mode which meets this requirement, but 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.

A device may draw up to 500mA after it has been configured as a high-power device. Being configured is dependent on the Hub being able to supply 500mA, which implies a self-powered hub. So there is always a degree of uncertainty whether more than 100mA will be available. It would be well to offer the option of external power via a socket on such a device.

Devices requiring more than 500mA are obliged to be self-powered. The practice of attempting to draw power from two adjacent USB ports, using a modified cable, is not permitted and can easily damage the ports.

* The USB 2.0 core specification specifies the value 0.5mA for suspended standard load devices, but this value was superceded as a result of a later ECN (Engineering Change Note).

** The same ECN removed the need to suspend during the first second after connect.

"Hot-Pluggable"

To achieve the goal of being able to plug a device into and out of a running system, some design rules must be followed. Firstly it is important to realise that if you pull a plug out at the far end of the cable from the device while current is being drawn, then the cable will develop a potentially large flyback voltage across your device. The specification suggests that a minimum solution to this is to place a capacitance of at least 1uF across Vbus and GND.

The second thing to consider is that when you plug your device in, any capacitance between Vbus and GND will cause a dip in voltage across the other ports of the hub to which you are connecting. To limit the consequences of this (such as crashing other devices), the specification places a maximum on the value of capacitance across Vbus and GND of 10uF.

For the same reason, the hub port supply must be bypassed with at least 120uF.

Self-Powered Devices

When designing a self-powered device, remember that you must not pull a D+ or D- line above the Vbus voltage supplied. This means that you must, at the very least, sense when Vbus is connected.

The D+ or D- resistor should, strictly speaking, be pulled up to a 3.3V supply 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.

If you pull, say D+, high in the absence of Vbus then you will risk faulty operation with On-The-Go hosts. (See later).

Summary

We have looked at the electrical requirements of USB.

Coming up...

Next we will examine the elements of signalling which are used by the USB protocol