What type of USB signal is it? Why the pull-down resistor at the d+,d-, how to calculate the specific resistance

USB protocol required, 1.5K pull-up at d+ when indicated to be full speed device, in D-Indicates not full speed device
Some scenarios (such as PNX5230) recommend that the d+/d-pull the 1M resistor is to improve the stability of the data transmission

①usb has master and slave equipment, main equipment: PC, now on the market those plug U-disk can play mp3 "MP3" and the like,
The USB signal is a differential signal, the signal line is d+, D-,。 On the USB host side, the d+,d-is connected by a 15kohm pull-down resistor, while on the USB device side, there is the difference between the high speed and low velocity devices. usb1.0, the 1.1,2.0 protocol has defined high-speed equipment to meet the needs of different situations, these hardware differences are: high-speed equipment: d+ to a 1.5kohm pull-up resistor, D-Do not connect, low-speed equipment is the opposite.
When the USB device is plugged into the host, the d+ is pulled high, D-invariant, and the low-speed device is the opposite. This pull-up process takes about 2.5us of time, the host this time to detect the signal, you can determine the device plug in, and the type of the device, and then start the communication, enumeration ... such as

②usb OTG (on the Go) is the ability to do both host and client
We usually accept the PC as the client to transmit data, as host can be connected to the printer directly to print the photos of the phone
Judge whether the host or the client is by usb_id this pin

When acting as a client, the usb_id is basically dangling (internal pull-up)

If the detected usb_id is pulled down, it is considered as host, outward output
So we need an external client device to pull the usb_id down.

About the USB pull-up resistor, not just a random resistance to any resistor on the OK.

When your USB main device, the d+, D-on a 15K pull-down resistor, so that when no device insertion, d+, D-on always remain low, when the interface from the device can be set by the pull-up resistor at different transmission rates, when the d+ A 1.5K pull-up resistor can be worked in a high-speed mode such as 12MBPs, when the D-connect 1.5K pull-up resistor works in a low-rate mode, such as 1.5MPBs.

The main USB automatic recognition from the device for high speed or low speed on the pull-up resistor in the d+ or D-difference, the resistance of the non-standard will affect the USB automatic identification allocation of resources, there can not be found hardware or use to the general hardware discovery failure, etc., so do not casually pull the resistor, the first standard USB hardware interface circuit.

What type of USB signal is it? Why the pull-down resistor at the d+,d-, how to calculate the specific resistance