USB2.0 Study Note Series (vi): USB2.0 Hardware design needs attention

In designing USB2.0, I found an official design guide for hardware design, some of which are nice. Not just USB design, some of the ideas can be applied to other occasions.

For USB2.0, the full-speed state can reach 480Mbps, bandwidth compared to USB1.0 improved a lot. USB2.0 power supply Power can reach 500mA, in fact, with the USB itself consumption, that is, around 450~480ma.

1.USB power supply

For power supply options for external devices, USB can be used for power supply or self-powered. For an introduction to USB hubs, see another introduction to USB hubs.

The USB power supply architecture is as shown:

External device self-powered USB system architecture as shown:

The above two graphs are a typical application diagram of Vbus power supply, and according to the manual, if the wakeup function is used, the USB control register is required to detect the state of the Vbus.

I design the time, did not use the figure on the 1.5K resistor form, just let USB provide 5v/500ma power supply, power configuration needs to be configured in the USB firmware, hardware circuit as shown:

2.WAKEUP PIN Settings

I designed the circuit, did not provide in the non-data transmission when the sleep, there is data transmission when the wake-up function, but has been in the wake state.

3.USB Reset Circuit

The disadvantages of resetting the simple RC circuit are mentioned in the diagram manuals above. When the USB device is disconnected, the chip is powered off, but the capacitor continues to be charged to the chip's supply voltage, and then it discharges slowly through the resistor. What happens if I reinsert the USB device now? If the capacitance is discharged, but the voltage is not lower than the reset threshold, then the reset PIN will supply the chip, but the voltage on the pin is too high, so the chip cannot be reset at T2. You can temporarily solve this problem by adding a diode in Figure 7, which can speed up the discharge of the capacitance voltage at power-down, but it can only shorten the critical time from disconnection to reconnection. The RC discharge method depends on whether the output voltage of the Ldo input is reduced to 0 V when it is disconnected, depending on the particular LDO and its external components, such as the high capacity capacitance. The most important problem is that the circuit cannot guarantee that the chip
Reliably reset in any case, especially in the case of a USB fast disconnect/Connect.

The following ideal reset solution is given in the manual, which is, of course, the final result.

In practice, I test the situation for the common RC circuit can meet the requirements of this design, as shown in:

4. Crystal Oscillator Requirements

5.USB Differential Impedance Control

On the control of differential impedance, the author's previous blog has been explained, you can go to see the relevant content, here briefly mention.

6.PCB Design Considerations

USB2.0 Study Note Series (vi): USB2.0 Hardware design needs attention