Electrical – What’s the hardware and software detect mechanism of the cellphone Micro USB devices attached and detached

cellphoneusb

What's the hardware and software detect mechanism of the Android cellphone Micro USB devices attached and detached?

schematic

simulate this circuit – Schematic created using CircuitLab

  • Hardware Schematic
    hardware sch

Fig1 is my application draft, and more details are as follows:

  1. My cellphone support OTG function and is enabled

  2. As the picture says, my device has connected the ID pin to the ground in order to get the power of 5V.

  3. I connect the D- pin to 5V so that it tells the phone this is a low-speed device.

  4. I have an APP installed on my phone to supervise if there is a device plugged in.

    My device is a non-standard USB device, I don't use the D+/D- pin to communicate with the phone

In my point of view, as soon as the device plugged in, firstly the ID pin of the micro USB receptacle get a falling edge and thus tell the phone there is a device plugin and the device is working as a HOST.

so, anybody familiar with the process of the detect?

I upload the PCB board photo so you can see clearly about my situation.
the PCB board

update : I have measured the output of VBUS when I insert the device, and the VBUS=5V, also as a power indicator a LED is mounted on the PCB, it works well.

Ultimate question: If my hardware connect is right? and then can somebody give me some instructions about the software code?

Best Answer

The simplified legacy OTG works in the following way.

Initial state: The OTG device/host (phone in this case) has ID pin pulled up, this is correct. VBUS is inactive, no voltage supply. ID = HIGH, VBUS = low.

CASE 1: a cable with u-B end is plugged in; the other end is Type-A plug, which goes into USB host (PC for example). In the u-B connector, the ID pin is floating. The phone senses two conditions: ID=HIGH, VBUS=HIGH (normal PC host supplies VBUS). The phone turns its software stack into device mode, will pull D+ up to 3.3V with a 1.5k resistor, and PC host sees the phone either as a mass storage device (old fashion), or MTP - media transfer protocol device.

CASE 2: a cable having ID-pin grounded is plugged in. The phone senses ID=LOW, and VBUS=don't care, normally low as well. This makes the phone to load the USB host software stack, which will turn the VBUS on, and act as a normal USB host. If the other end of a cable is connected to a device, it will pull-up D wire that corresponds to device basic speed.

In theory, the phone receptacle is supposed to be of micro-AB style, and the cable in case2 should have micro-A type shell (rectangular). But this is confusing for customers, and most manufacturers resort to using of u-B receptacle. The OTG swap is done by illegal micro-B plug, with illegally grounded ID pin.

In reality, in CASE1 there is a step of identifying charger signature before the data connection occurs, but this is a different topic.

So, basically, you got it right.

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