Confusion about ID-pin used in USB-OTG

allankliu — Sat, 16 Mar 2013 02:51:27 GMT

KL25Z is one of cheapest microcontrollers with USB-OTG. I am going to design a USB-OTG board as Android peripherals, something like ADK, USB host shield for Arduino, IOIO based upon PIC24F256.

I have following components:

Samsung Galaxy Nexus i9250 as Android equipment, with USB host controller APK;
FRDM-KL25Z, which is configured as USB device only. Therefore, I can only use its as USB CDC device and its OpenSDA debugger for external target;
KL25Z128 sample, to be applied from Freescale, for my new OTG board.
TPS2055A USB power switch, to turn on/off VBUS from my new OTG board.
LDO for on-board circuit 3.3V, KL25Z128 on-chip LDO is used as backup via a jumper.

Now, I am confused about ID-pin in mini/macro A/B sockets and cables.

I have a micro B OTG cable with i9250, which is macro B plug (to mobile) and standard A acceptance. By chaining it to other cables, including standard A-standard B, standard A-mini B, standard A-macro B, it works flawlessly with many USB devices, such as ST-Link debugger (B), FRDM-KL25Z OpenSDA debugger (mini B), CDMA feature phone (macro B).

Since macro B-Standard A acceptance plus standard A-macro B cable becomes a macro B to macro B cable. I swap the plugs between Nexus and other macro B devices. I found the VBUS output is disabled after swap. After checking specification, this chaining cable is not encouraged, and its blocks ID-pin, because ID-pin is not available in standard A sockets. Therefore, ID-pin of OTG cable must be always be GND. And ID-pin of standard A-macro B cable must be always open or 1MOhm to GND.

What kind of OTG cable is best choice if I want a single cable for connecting OTG devices? I can not find many macro A cable from the market, including macro A-macro B cables. Although macro A/B acceptance is available. Actually mainstream OTG devices usually use macro B acceptance, or mini A/B acceptance like M52259DEMOKIT. Can I use macro B acceptance in device A as well as device B?

How ID-pin be detected by A/B devices? According to USB spec, if ID-pin is GND, it is device A, if ID-pin is open, it is device B. Device A source VBUS. After HNP, VBUS will be switched to device B. However, how this happens if ID-pin is connected both devices? It only makes sense if ID-pin is isolated between A/B, otherwise grounded ID-pin will be read by both devices and start VBUS contention.

What is suggestion on A/B devices' acceptances, GND or open? If ID-pin ground and open is done in cable, ID-pin of socket should be open. PC USB cable is ignore ID-pin and always offer VBUS. If ID-pin is grounded, then device will always offer VBUS as well.

Is it necessary to detect VBUS with ADC? I am always afraid of VBUS contention, is that necessary?

I found anther thread for ID-pin as Re: Kinetis K40 USB OTG

An external PHY is used, it seems ID-pin is programmable. Why a OTG device wants to define different type by programming ID-pin?

Re: Confusion about ID-pin used in USB-OTG

allankliu — Fri, 22 Mar 2013 04:44:22 GMT

After internet research, I found some pix describes USB macro A to macro B cable. Obviously, ID-pin in device A is always grounded, and ID-pin in device B is always open. The ID pins of each side are disconnected in cable.

As a result, device A detects grounded ID-pin should offer VBUS, and device B detects open-ended ID-pin should not offer VBUS. Device B can only source VBUS after HNP negotiation.

Sorry for Chinese in attached pix, but it is quite clear. Meanwhile USB user guide KLQRUG.pdf (pg98) presents OTG schematics as well.

topic Re: Confusion about ID-pin used in USB-OTG in Kinetis Microcontrollers

Confusion about ID-pin used in USB-OTG

Re: Confusion about ID-pin used in USB-OTG