1
\$\begingroup\$

I am designing and using a USB 2.0 (High-Speed) specification circuit.

The circuit is shown as below.

USB data line invalid capacitors

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?

\$\endgroup\$
10
  • \$\begingroup\$ Have you tried measuring the signal waveforms in the data lines? Please provide some results. \$\endgroup\$
    – user94729
    Apr 23 '18 at 9:48
  • \$\begingroup\$ Perhaps the one of the capacitors you removed was shorted out? Just a guess. There isn't much any of us can do but guess with the information you have provided. Things like PCB layouts, photographs, scope waveforms would all be useful. \$\endgroup\$ Apr 23 '18 at 9:59
  • 2
    \$\begingroup\$ 22pF seems quite high. Where did you get that figure from? \$\endgroup\$
    – Jon
    Apr 23 '18 at 10:16
  • 1
    \$\begingroup\$ Section 7.1.6.2 of USB 2.0 spec has the details you need. You need to be very careful with termination if you want any sort of reliability in the field. As you can see from the spec, it's a bit more complicated than just adding a cap and resistor. \$\endgroup\$
    – Jon
    Apr 23 '18 at 10:22
  • 2
    \$\begingroup\$ USB 2 operates at 480 Mbit/s maximum. Work out the effect 22pF capacitors have at that sort of frequency and you'll see why nobody puts them in their designs. \$\endgroup\$
    – Finbarr
    Apr 23 '18 at 10:23
7
\$\begingroup\$

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.

\$\endgroup\$
6
  • \$\begingroup\$ Is the role of cap as EMI suppression valid only for USB1.1?? Not 2.0? \$\endgroup\$
    – 유영근
    Apr 24 '18 at 23:46
  • 1
    \$\begingroup\$ The "EMI suppression cap" is a blast of deep past, when full-speed drivers didn't have any means to control the signal edge rising time, so the lines would ring like hell without R and C. Not anymore, all USB 2.0 HS-compliant drivers must have edge control, otherwise, with the absence of caps and resistors for HS mode, they would not pass USB silicon certification. So caps can't be used, and there is no needs for any caps, since the edges are already bandwidth limited. \$\endgroup\$ Apr 25 '18 at 0:05
  • \$\begingroup\$ Then, are 15kohm resistor(pull-down) also not needed?! Why could this pull down resistor not work as pull down in this design??? \$\endgroup\$
    – 유영근
    Apr 25 '18 at 0:06
  • \$\begingroup\$ Usually the 15k pull downs are also incorporated in PHY circuitry for downstream-facing ports. What kind of "CPU" are you using? \$\endgroup\$ Apr 25 '18 at 0:08
  • \$\begingroup\$ Samsung CPU based on ARM 8 or 9!! \$\endgroup\$
    – 유영근
    Apr 25 '18 at 0:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.