USB protocol-detects device connection and speed

When the USB device is connected, the USB system automatically detects the connection and identifies the data transfer rate it uses. USB uses the method of increasing the pull-up resistor on the d+ or D-line to identify low-speed and full-speed devices .

USB supports 3 types of transfer rates: low-speed transmission of 1.5mb/s, full-speed transmission of 12mb/s, and 480mb/s transmission at high speeds.

When there is no USB device connection on the downstream port of the host controller or hub, the pull-down resistor on the d+ and D-line causes the voltage of the two data lines to be close to low (0V), and when the low/full speed device is connected, the current flows through the pull-down resistor of the hub and the device on the d+/d-to form a voltage Since the resistance of the pull-down resistor is 15kω, the resistance of the pull-up resistor is 1.5kω, so a DC high voltage of size (vcc*15/(15+1.5)) will appear on the d+/d-line. , the connection of the low-speed device is connected by a solid line drawn resistor, and the line-drawn resistor is connected to the full-speed device. When the USB host detects that the d+/d-line voltage is already close to high, while the other lines remain grounded, it knows that the full speed/low speed device is connected.

At the start of the connection, the high-speed device needs to communicate with the host at full speed to complete its configuration operation. The pull-up resistor on the d+ line is required to connect the 1.5kω. When high-speed equipment is in operation, the d+ line is not pulled up if high speed transmission is used, but if full speed transmission is still used, the pull-up resistor must be used on the d+ line. Therefore, to identify a high-speed device, a software-controlled switch is required between the pull-up resistor and the d+ line, which is usually integrated into the USB device interface chip.

(Full speed pull-up d+, low-speed up-pull-D, high-speed pull-up required control)

USB protocol-detects device connection and speed