USB:

Under normal circumstances: the host terminal D+ D- is connected to a pull-down 15K resistor, and the Device terminal, if it is a full-speed device, pulls up a 1.5K pull-up resistor on D+, and pulls up a 1.5K pull-up resistor on D- for low-speed devices. Resistance accuracy: +-5%.

Actual waveform:

1. Insert the device, D+ is pulled high

2. The host drives the data line to the reset state SE0 for at least 10ms (Reset signal)

What is SE0, refer to the table below:

After the reset is complete, D+ returns to high:

Reset signal: SE0 state remains for 10ms. The host sends a Reset signal to reset the device before communicating with the device.
The device is configured to the default unconfigured state. The reset signal lasts at least 10ms. If a device sees its upstream end
If the SE0 state of the port exceeds 2.5us, it will be treated as a reset signal. Reset within 10ms of clearing the reset signal
After the recovery time, the Hub must be able to receive all Hub requests, and the device must also receive SetAddress requests,
Failure to receive the request will result in the device not being recognized by the USB system.
3. USB communication:

Numbers form domains, domains form packets, packets are divided into different things (PID domain distinction), and transactions finally form different transmission types (interrupt transmission, parallel transmission, batch transmission and
control transfer)

SOF 0 waveform:

• number:

Transmission order: from bit to high bit (LSB.....MSB)

NRZI encoding (non-return-to-zero encoding) that is: if the input data is 0, it will be encoded as a level inversion, and if the input data is 1, it will be encoded as a level without inversion.

• Field: Tells the serial interface engine that data is about to be transmitted, and is also used to synchronize the local clock with the input

1. Initial SOP: Switch from IDLE state (J state) to K state
2. SYNC: 7 0+1 1
3. Identification field (PID): 8 bits in total, only 4 bits are used, and the other 4 bits are the inversion of PID0~PID3 to verify the PID.

      

   If it is a frame, the PID is 0101B

4. Frame number, the waveform of frame 1 is more obvious, and frame 0 is all 0, it cannot be seen that the lower bits are transmitted first
5. CRC
6. EOP

Interval 1ms, SOF 1 waveform: