can a female A USB port be used on an OTG host?

Question

I'm designing a USB device that will act as a host all the time. Instead of using a mini-AB OTG receptacle i'd like to use a classic female A receptacle on the PCB. Is this allowed and can this potentially cause problems?

Clarification: I'm asking this question because I have a device where I would like to use the OTG functionality, i.e. give the user the possibility to hook it up to the computer OR to a peripheral, but I can't use the mini/micro OTG connectors and want to use classic USB A and B connectors. Ideally I would want the OTG port to be able to communicate both as a device and a host but that is probably impossible (?) so if I can find a way to allow the user to EITHER connect the device as a host or as a device using ONE of the two ports, that would be enough.

Clarification #2: I primarily want to use the OTG port as a host port, but if I can use it as a device port as well that is an extra feature on the device I'm working on which will make it more attractive. However, I do not want to use mini or micro connectors. I get that they are the new trend and all that and that they are great for smartphones and tablets but for the application I'm working on we can't and don't want to use them. I'd like to thank all who answered already whether I can use a type A host port. Any hints on how I could add a B port as well connected to the same OTG port, and let the user connect the product as a host or as a device, but not at the same time, but using a type A and B connector rather than a single micro or mini USB connector are very welcome. Confusion for the user is not an issue here since we can make that perfectly clear in the manual (e.g. you can use the host or device port but not at the same time - if you use the host port then the device port won't work or the other way around). Hope this makes it clear.

Answer 1

You can certainly use OTG hardware in the microcontroller to do a USB host only. You don't need to use the OTG extensions for anything. In fact, you shouldn't even mention anything about OTG. What you are doing is a host, and thus you should use a female A connector. And if you intend to connect to a USB flash memory, what other connector could you use? Traditional USB flash memories generally are always plugged to A connectors.

So go right ahead and use the female A connector. You're using it exactly as it's supposed to be used. Remember that you need to provide 5V power to the female A connector.

Answer 2

The physical connector is not mandatory for it to work. It's mandatory to be in compliance, and to frankly not confuse the end user. If your device is going to be a full time host, regardless of its OTG capabilities, then go right ahead and use the full size USB A female connector. That's a perfectly legit use. In this case, the ID pin of your microcontroller should be permanently wired on your PCB for host mode, if you can't do it in your code.

Answer 3

Yes, the standard Type-A receptacle is still a standard way to have the USB host design.

However, the industry is moving towards Type-C connectors. The Type-C may look over-complicated at first, but you can elect to use a minimal Type-C configuration (USB2.0 only, CC pins 56k pullup) to provide the basic host functionality, yet to be fully USB compliant.

But if you do want to use OTG functionality (as per question clarification), it is not possible to accomplish this with Type-A receptacle, because the A-connector does not have the necessary ID pin to switch the function.

The OTG port has only two physical data wires, D+, and D-. The bus CANNOT BE DEVICE AND HOST SIMULTANEOUSLY! It either one, or another. Inside your MCU there are two different logical functions/controllers, a host controller, and a device controller. They SHARE a single USB PHY, by means of internal multiplexer of two different data paths.

However, you can put two USB connectors, Type-B receptacle, and type-A receptacle, connect D+/D- electrically in parallel, but use the VBUS from B-connector as OTG switch signal. This will constitute an ugly kluge, it contradicts every USB specification, and it will cause massive user confusion.

Answer 4

Any hints on how I could add a B port as well connected to the same OTG port, and let the user connect the product as a host or as a device, but not at the same time, but using a type A and B connector rather than a single micro or mini USB connector are very welcome.

Afaict what you want to do in this case is

1. Use a USB mux chip.
2. use the VBUS line from the B connector as a control line.

When Vbus is present on the B connector you switch the MUX to the B connector and signal the processor to go into device mode (e.g. by driving it's ID pin high*).

When Vbus is not present on the B connector you switch the MUX to the A connector and signal the processor to go into host mode (e.g. by driving it's ID pin low*).

If the two sockets are close together, you don't care about compliance with standards and you trust your users not to plug in both cables at the same time you may be able to skip the mux chip.

* Watch the voltage level specifications for the ID input pin though, you probablly don't want to connect it directly to Vbus.