ik1wvq
Member
HI to all,
I do not understand the use of "DISC" (PB9) line and the two transistors (Q1-Q2) near the USB connector.
my board is "MAPLE MINI".
I ask this because my need is to use the whole PA port as input, and I do not understand how "disconnect" the USB port from PA11-PA12 .
Is sufficient to issue "USBSerial.end()" to free the PA11-PA12 ???
thanks in advance and best regards
Mauro IK1WVQ
gbulmer
Moderator
@ik1wvq - You have correctly identified that PA11 and PA12 are USBDM and USBDP.
However, the USB specification allows for different communication speeds. A USB 1.2 device signals the host which speed it is able to work at by pulling up either USBDM or USBDP with a resistor to 3.3V or (a little) higher.
The pull-up could be a fixed resistor, connected to 3.3V. Once pulled-up, the host asks the device for some details about itself ('enumeration'), and then acts accordingly.
However, Maple is more flexible than a single fixed USB device. Maple can change the type of USB device it implements dynamically. That requires the pull-up resistor to be activated under program control. That is what the DISC pin is doing. It can pull up the USBDP line, to signal its speed (12Mbits), and that it is ready to 'enumerate'.
So if the DISC pin is active, the USBDP signal will be pulled high. Hence you need to disable the DISC pin so that it doesn't pull USBDP high.
IIRC issuing a "USBSerial.end()" will disable the DISC signal, and the two USB pins can be used as GPIO (don;t connect them to a USB host, of course).
(Full disclosure: I am not a member of LeafLabs staff.)
Mauro
In addition to the excellent reply from @gbulmer.
I don't think pb9 is broken out to a pin on the Maple mini, so I don't think you can physically access it unless you solder to the board e.g the processor pin.
As a coincidence I have been working on adding PB9 back into a generic implementation of libmaple, and I now have it working on a generic stm32f103c8 board
You can download and try my repo and look at the changes to the PIN MAP array etc
Or just modify the enum in board.h and then PIN MAP array in board.cpp
gbulmer
Moderator
@rogerclark - there is no need to get at PB9, if all ik1wvq wants to do is disable the pull-up. By disabling the pull-up, in software, the USBDP/USBDM signals will not be pulled up by the pull-up connected to the PB9 DISC signal.
FYI - the Open Source Community design for Maple-compatibles at https://github.com/gbulmer/openstm32hw, for example my later Orone-mini designs (https://github.com/gbulmer/openstm32hw/tree/master/Orone-mini-v0) e.g. Orone-mini-S8J-v0r001 has DISC broken out onto a 0.1" header. It was one of the improvements Siy and I added.
You must log in to post.
