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USB Help Needed
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rfdes:

--- Quote from: SiliconWizard on July 21, 2019, 04:36:44 pm ---Why would you want to do that in the first place?

--- End quote ---

Yes, it is unusual.  My reasoning is that I have an embedded project where I wish to parallel a USB port with a PS/2 port.  I am using a third party part where the same USB pins also are used for the PS/2 port.  However, the only way to configure the part in production is using USB.  Due to the requirement of EMI protection needed for the PS/2 port, the impedance of the EMI components limits the bandwidth when using the port with USB. The end user will only use the PS/2 capabilities of the device.  My hope was to eliminate the need to use jumpers at production for configuring the device.  Configuration speed is irrelevant, so low speed USB is totally OK in this application.  I could get fancy with the electronics and do some semiconductor switching to accomplish this but the effort isn't really worth it.

thanks to all for their input.
Jim

SiliconWizard:
I see. What is this "third party" part? Do you have any means of modifying its firmware? Do you know which USB class it natively enumerates as?
rfdes:

--- Quote from: SiliconWizard on July 21, 2019, 05:27:43 pm ---I see. What is this "third party" part? Do you have any means of modifying its firmware? Do you know which USB class it natively enumerates as?

--- End quote ---

A company named 'Sprintek' sells a line of USB/PS/2 keyboard encoder parts, so the class of USB would be HID.  We are using this part to support a legacy PS/2 keyboard design for the US military.  There is a way to update the firmware, however, the firmware is proprietary.  I have yet to reach out to the company to see if the firmware exists to support low speed only. Again, I don't really believe there is justification to re-uploading the firmware just to save a 'jumper'.
NorthGuy:
It is all up to the device. The device signals with pullups whether it is FS or LS. FS device has a pull-up on D+. LS device has a pull-up on D-. Host has no means to offer a different speed.
rfdes:

--- Quote from: NorthGuy on July 21, 2019, 07:15:05 pm ---It is all up to the device. The device signals with pullups whether it is FS or LS. FS device has a pull-up on D+. LS device has a pull-up on D-. Host has no means to offer a different speed.

--- End quote ---

Yes, the pull-ups are necessary to signal the host as to the speed capabilities, however, the device still has to be programmed to use low speed as opposed to full speed.  Not being that familiar with the details of USB enumeration, I was hoping that the host could mandate that the device communicate at low speed but as I have now learned, it doesn't work that way.

Take care - Jim
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