USB Current Limiting

[richarthurs]

Generally, plugging the bare OBC into any laptop or USB hub will work.

It was observed that an OBC + radio would cause issues (only tested on Richard's MBP), and would frequently draw too much current and cause the OS to shut down the USB port.

The combined draw of OBC + RF is still likely under 500 mA, so the culprit is likely this clause of the USB spec:

iii) A bus powered high power USB device (one that draws more than 100mA) should use one of the
CBUS pins configured as PWREN# and use it to keep the current below 100mA on plug-in and
2.5mA on USB suspend.

Since the OBC and radio were connected at plug-in, the current drawn would be over 100 mA.

Fix

One fix for this would be to have one of the PWREN pins tied to MCU reset, so the OBC would be drawing very little current at plug-in.

If future stack designs have the ability for the OBC to entirely disable subsystems, the OBC should talk to the FTDI and only enable the subsystems some time after plug in.

It may also be possible to do some other current limiting.

http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT230X.pdf

[richarthurs]

Might something like this be useful? https://www.digikey.ca/product-detail/en/maxim-integrated/MAX22505GTG-/MAX22505GTG--ND/8635404