Introduction to USB—Part I

Introduction to USB--Part I--Page 2.

USB MINI CONNECTORS

Mini connectors were added in USB 2.0. They are an alternative to the standard B connector, and are used on handheld and portable devices; the Type B connector is too large to be easily integrated into these devices. The mini-B connecter has a fifth pin, named ID. but it is not connected. Mini connectors have fallen out of use with the development of micro connectors for smaller handheld devices.

USB ON-THE-GO (MICRO) CONNECTORS
USB OTG devices use Micro-USB connectors. These connectors are smaller, which makes them more useful for handheld devices, and support the use of the ID pin to determine the device type. An OTG device is required to be able to detect whether a Micro-A or Micro-B plug is inserted by determining if the ID pin resistance to ground is less than 10Ω MAX or if the resistance to ground is greater than 100KΩ. If the electrical resistance on the ID pin is less than 10Ω MAX, the pin shall be treated as ID = FALSE, and a resistance greater than 100kΩ MIN shall be treated as ID = TRUE. “USB Stacks” on page 61 for more details about OTG usage in embedded systems.

1-2-2 ELECTRICAL SPECIFICATIONS

In most embedded system designs, adherence to the USB electrical specifications is guaranteed by the USB controller used, whether that is an external controller, or one that is integrated into an MCU. Most USB controller vendors provide excellent reference materials that you can use as the basis of your product design.

So, the main issue you should consider is compliance testing, if you want to get the USB logo on your product. See “Compliance” on page 64 for more information. Once a device has indicated its presence by setting the signal lines with the correct set of pull-ups resistors, the host controller is responsible for all further traffic on the bus.

A high-speed device begins by connecting to the host/hub as a full-speed device. Once attached, it will perform a high speed chirp during reset, and if the hub supports it, the device establishes a high speed connection. If the device operates in high speed mode, then the D+ line is pulled high to balance the line.

Note: A high-speed device must not support low speed mode. A USB 2.0 compliant downstream-facing hub or host must support all three modes: high speed, full speed and low speed.

USB supports hot plugging with dynamically loadable and unloadable drivers. The user simply plugs the device into the bus, and the host detects this addition, interrogates the newly inserted device, and loads the appropriate driver (provided a driver is installed for your device).

 

Quick facts about the electrical characteristics of USB cabling:

 

• In USB 2.0, a unit load is defined as 100 mA. A device may draw a maximum of five unit loads, for a total of 500 mA from a single port.
• Either the D+ or the D- line will be pulled high. If D+ is high, the device is Full or High Speed. If D- is high, the device is Low Speed.
• High Speed negotiation protocol occurs during the Bus Reset phase
• After detecting the reset signal, a high speed device will signal the host with a 480 Mbps chirp.

 

1-2-3 HIGH SPEED

High-speed additions to the USB specification were introduced in USB 2.0 as a response to the higher speed of FireWire. It was added as an afterthought, and was designed to maintain compatibility without compromising performance. High Speed provides a maximum theoretical throughput of 480 Mb/s using the same cables as Low/Full Speed USB.

This is achieved by having each end of D+ and each end of D- terminated with a 45 Ohm resistance to ground when the link is conveying high speed data

A high-speed transceiver current source injects a current of 17.78 mA (derived from the positive supply) into either the D+ or the D- line. A voltage of 400mV on the line is generated. A differential receiver detects the differential state of the line.

In fact, the 45 ohm resistors are provided by the Full Speed / Low Speed circuitry, at each end of the link, applying a Single Ended Zero. The FS/LS controller is designed to provide as accurate a termination resistance as possible. Removing the high speed transceiver current source from the circuit re-establish the line conditions as defined for Low/Full Speed USB.

 

Next:  Powering USB Devices

 

µC/USB Device: Universal Serial Bus Device Stack for the Renesas RX63N, Chapter 2, by Christian Legare, is excerpted with permission. The text is available at Micrium.