What are Q2 and Q3 for? « LeafLabs Garden

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What are Q2 and Q3 for?

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Started 5 years ago by importer
Latest reply from importer

importer
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on Fri 25 of Dec., 2009 22:28 EST, by Nic

I've been looking at the schematic, and I'm wondering what the circuitry on the USB port is for. It's based around Q2 and Q3, and it looks like it pulls D+ up to 3.3V while DISC is driven low if there's 5V coming in the USB port. I've seen the same circuitry on other STM32 boards, but haven't yet found an explanation for it in the mountains of documentation for the chip or other boards.

I take it DISC is short for "discover", but isn't the same thing accomplished by sensing USB_P?

Posted 5 years ago #
importer
Inactive

on Mon 28 of Dec., 2009 13:39 EST, by LarryP

Nic,

I'm not sure, either (I haven't done any USB hardware.)
However, it seems to me that the same functionality could be done with 2 of the 6 gates of a hex inverter (e.g. '14 schmitt trigger), leaving 4 unused gates to buffer other output lines.

-- Larry

Posted 5 years ago #
importer
Inactive

on Mon 28 of Dec., 2009 17:02 EST, by poslathian

Nic & Larry,
The USB protocol specifies that new bus devices announce themselves to the host operating system via pulling either D+ or D- up (via a pullup resistor). Which line gets pulled depends on whether the device is a full speed or high speed device (I cant remember which is which..i think FS pulls up d+?).
USB_DISC handles this operation. If you reset USB_DISC, then the host computer will see the board as a USB device and begin the enumeration. Similarly, setting the USB_DISC pin will disconnect the device, which might be desirable to initiate a usb reset or if you just want to steal power from USB and not actually connect.
The USB_P pin is for the device to recognize that it has been plugged in at all. For example, if you are powered off of a battery, you might disable the USB peripheral altogether, then re-enable it once USB_P is high.

Posted 5 years ago #

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