Tie ID pin on USB Micro Receptacle to GND?

[Rudi]

Currently I am designing a small module based on the FT201X I2C slave IC from FTDI. This module will act as a classic USB Device: no OTG, no charger application etc. The FTDI chip will be connected to USB using a Micro USB A/B type receptacle. I am not sure, if the ID pin (pin 4) of this receptacle should be left unconnected or should tied to GND.
 
In chapter 8.2 of the FT201X datasheet, the schematics show the ID pin as unconnected. From the USB-IF specifications and other related documents I assume, the ID pin is always an input in non OTG capable USB devices. But, on the other hand, I see many schematics of similar devices where the ID pin is internally connected to GND.

The very fast FTDI support recommends an unmounted 0R resistor in case of concerns, but cannot point me to an USB-IF specification, which answers this question.
 
Can you please give me some advice for the correct usage of the ID pin?

Thanks, Rudi

[Jan Axelson]

The ID pin tells an OTG host whether an attached plug is a Micro-A or Micro-B and thus whether to treat the device as a device only or another OTG device.

The Micro-USB specification included in the USB 2.0 specification's zip file says this about the ID pin:

The ID pin on a Micro-A plug shall be connected to the GND pin. The ID pin on a Micro-B plug is not connected or is connected to ground by a resistance of greater than Rb_PLUG_ID (100kΩ MIN).

The MicroUSB ECN specification included in the USB 2.0 specification's zip file increases the recommended resistance to 1M ohm.   

Note that these requirements are for plugs (on cables), not receptacles.

I'm not clear from your description whether the Micro-AB receptacle is on your device or on the device it attaches to.

If your device is attaching to a device with a Micro-AB receptacle, it will attach using a Micro-A plug, whose ID pin will connect to the ground pin in the plug.

If your device functions only as a device, it should use a series-B, micro-B, or type C receptacle. Micro-AB receptacles are reserved for OTG devices.

Regardless, if you are informally using a Micro-AB receptacle on your device, the cable will attach to the device using a Micro-B plug, whose ground pin will be unconnected or connected to ground via > 1 M ohm.

If the device then attaches to a host using a series-A plug, the host receptacle has no ID pin so it doesn't matter what you do with it. If the device attaches to a host using a micro-A plug, the host will expect to see a high resistance to ground on the ID pin.

[Rudi]

Hello Jan,

the Micro-AB receptacle is connected to the FTDI chip via the PCB of our module. The host will be almost always a PC, but in the future possibly a tablet or even a smart phone.

I will check to use on our module a Micro-B receptacle (instead of the Micro-AB) and leave its ID pin unconnected.

Thanks again for your very fast clarification, Rudi