A USB device must indicate its speed by pulling either the D+ or D- line high to 3.3 volts. A full speed device, pictured below will use a pull up resistor attached to D+ to specify itself as a full speed device. These pull up resistors at the device end will also be used by the host or hub to detect the presence of a device connected to its port. Without a pull up resistor, USB assumes there is nothing connected to the bus. Some devices have this resistor built into its silicon, which can be turned on and off under firmware control, others require an external resistor.
For example Philips Semiconductor has a SoftConnectTM technology. When first connected to the bus, this allows the microcontroller to initialise the USB function device before it enables the pull up speed identification resistor, indicating a device is attached to the bus. If the pull up resistor was connected to Vbus, then this would indicate a device has been connected to the bus as soon as the plug is inserted. The host may then attempt to reset the device and ask for a descriptor when the microprocessor hasn’t even started to initialise the usb function device.
Other vendors such as Cypress Semiconductor also use a programmable resistor for Re-NumerationTM purposes in their EzUSB devices where the one device can be enumerated for one function such as In field programming then be disconnected from the bus under firmware control, and enumerate as another different device, all without the user lifting an eyelid. Many of the EzUSB devices do not have any Flash or OTP ROM to store code. They are bootstraped at connection.
You will notice we have not included speed identification for High Speed mode. High speed devices will start by connecting as a full speed device (1.5k to 3.3V). Once it has been attached, it will do a high speed chirp during reset and establish a high speed connection if the hub supports it. If the device operates in high speed mode, then the pull up resistor is removed to balance the line.
A USB 2.0 compliant device is not required to support high-speed mode. This allows cheaper devices to be produced if the speed isn’t critical. This is also the case for a low speed USB 1.1 devices which is not required to support full speed.
However a high speed device must not support low speed mode. It should only support full speed mode needed to connect first, then high speed mode if successfully negotiated later. A USB 2.0 compliant downstream facing device (Hub or Host) must support all three modes, high speed, full speed and low speed.
Download & Links
- http://www.usb.org - The USB Implementers Forum, Inc. (USB-IF)
- Beyond Logic