While we are all talking about USB 3.0 in the modern day embedded boards, there is a long way before USB 2.0 to be ignored. USB 2.0 is one of the widely used interface in embedded boards and stands right after the serial port (UART). In fact, many boards have USB 2.0 in place of UART. We all know that USB 2.0 uses differential signalling. Let us now consider some of the important points regarding the USB 2.0.
- The Host port is called an Upstream port and Device port is called a downstream port
- Host controls the communication in USB 2.0 interface
- The HIGH state voltage level for a USB 2.0 signal is 400 mV
- The levels on the USB 2.0 signals are 0 V when no device is connected
- AC coupling capacitors are not required for USB 2.0 interface
- USB 2.0 can support up to a maximum current of 500 mA @5V, which meant power of up to 2.5W. The current can be from 100 mA to 500 mA
- USB 2.0 is a High speed interface which can support up to 480 Mbps speed
- USB 2.0 supports bi-directional data transfer. This meant data can flow both ways between Host and Device.
- USB 2.0 is half duplex communication. Either Host or Device can only transfer data at once.
- In a USB 2.0 communication, the transmission starts from LSB
- Polarity reversal is not allowed in USB 2.0. so, if designers reverse D+ and D- connection, USB 2.0 doesn't work
- If you are probing USB 2.0 signals, you can probe individual signals D+ and D- with a single ended trace
- USB 2.0 allows cable lengths of up to 5m.
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