I am designing and using a USB 2.0 (High-Speed) specification circuit.
The circuit is shown as below.
Sometimes CPU cannot recognize the USB stick (USB 2.0 High speed).
At first, I changed the resistors from 0 ohms to 27 ohms.
However, CPU still could not recognize the USB stick.
So, when I removed both the 22 pF capacitors (open status), CPU can recognize the USB stick very well.
What is the problem?
I know the role of 22pF capacitors with the series resistors is to remove ringing of the signal causing the EMI.
But when I removed the capacitors, the problem was solved (with still 0 ohms series remaining). Indeed, I can not understand the difference between the existence of capacitors of open circuit (removing capacitor) on the USB data differential signal by using an oscilloscope.
What is the problem here?
Best Answer
With 22 pF to ground, the link won't work. Having 22 pF is a brutal violation of USB 2.0 signal requirements.
When you loaded the data bus with such huge caps, the HS (480 mbps, 240 MHz) signals are severely degraded, below borderline of HS communication. Yet the initial speed negotiation (chirping sequence), which occurs on 10 kHz rate (50 us pulses), passes just fine. So the host receives a valid HS handshake and believes that the device is HS-device, and therefore starts the communication at HS rate. But your 22 pF hammers the HS signal, and all HS protocol fails, or is very-very unreliable. That's where your design fails.
Apparently you have mixed up old USB 1.1 (FS) recommendations with HS fully-terminated transmission line requirements. Urgently remove all your caps (and resistors) from USB data lines.