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k20/k40 USB connections, correct cct design

Question asked by Robert Lewis on Nov 11, 2014
Latest reply on Nov 17, 2014 by Hui_Ma



I have been going through a revision of our k20 board and am concerned that we have an error in the USB design. It works but the flash drive seems too warm.



In review of the Kinetis Freescale.User_Guide, it suggests the following design.


"The DCD module needs to know when the USB connector is plugged in. This can be

made using an I/O signal measuring the status of the VBUS line of the USB connector.

When the VBUS line becomes high, the software must call the start sequence routine of

the DCD module. (see I/O section for more details of the pin configuration)."


We have tied VBUS to +5V, as does the designs I have checked for the Tower Systems so how can the line be anything but high?


Or is this an assumption that the VBUS signal is arrives at a high level at sometime after the processor is stable, and then the processor should check the line before initializing the software?


2). The last question is the VOUT33 line, that seems to be used to charge a coin battery at times, and also power sections of the k40. Is this a normal design? We do not use the VOUT33 regulator at all, because we are using the output of the board 3.3V regulator for all the MPU circuits. Should we create a separate 3.3V VOUT33 bus and then power just the MPU with the VOUT33 output? I also do not understand the comment about enabling the regulator to make sure the pull-up is present?


"Voltage Regulator

The USB transceiver power line comes directly from the VOUT33 (voltage regulator

output). Therefore the regulator must be enabled to make sure that the pull-up is present

when needed."



Voltage Regulator

The USB transceiver power supply comes directly from VOUT33 (voltage regulator

output). Therefore, the regulator must be enabled to supply 3.3 V to the transceiver."


The USB power we are using is 5V to the USB, VOUT33 is not connected, what 3.3v transceiver is being referenced? Do they mean the internal transceiver is enabled via a register bit, if so what has that to do with VOUT33 that doesn't seem to even connect to anything to do with the USB device, which is at 5V?


I have read through a number of the USB posts and do not see these issues addressed. Is this a major mistake on our part or perhaps I am missing the point being made in the documentation.


Any help would be very welcome, thanks.