USB Isolator Most Frequently Asked Question

ADuM4160 PIN pin grounding

Q. I am getting “USB device not recognized” error message – what do I do? Also, since the isolator is soldered into my circuit, “reconnect peripheral” suggestion seems too difficult to follow.

I decided to write this article after receiving several e-mails from people who bought my isolator. While setups described in those e-mails were different, the problem was the same – a PC refusing to recognize the device connected through the isolator. Here I will try to explain what is happening and also share my ideas how to troubleshoot and possibly fix the problem.

When nothing is connected to USB port, the bus is held at ground level with pull down resistors on the host side. USB device, when connected, pulls one of bus lines up, often times also with a resistor connected to Vbus and data line. Host sees it, sends bus reset and tries to query the device. If device is answering, host keeps querying the device and eventually enumerates it. When device is enumerated, application takes over.

If device is not answering (like for example, when self-powered device is turned off), host will give up and post “Device not recognized” message. To get attention from the host, we need to generate bus event, i.e., disconnect the peripheral and connect it back again.

If we are dealing with typical copper-wire-connected USB host and peripheral, which end of USB cable gets disconnected and reconnected would not matter. However, when host and peripheral are talking through ADuM4160, this is not always the case. I tested about 50 different USB devices and found out that while breaking the connection on the peripheral side works every time, host side disconnect does or does not help depending on device.

It is possible to simulate device disconnect without breaking actual connection by pulling PIN pin of ADuM4160 to ground. Normally it is held at 5V by R8 pull up resistor. This resistor is located next to peripheral side USB connector and marked 1M above it. Left side of it, the one right below the “1M” mark is connected to PIN pin (12). The closest ground point is right pad of bypass capacitor, which sits above R8. An experimenter with good eyesight and steady hand can easily connect those two points with tweezers. However, if said experimenter is not feeling comfortable operating tweezers inside powered circuit, a safer arrangement is shown on the title picture.

Disconnecting peripheral, either directly or by driving PIN pin, with power applied to the isolator and host side connected, will most likely solve device detection problem. If you still having issues with the circuit, please let me know, I will try to help.

Oleg.

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