Weird hardware mod ???

Werner Almesberger werner at
Mon Feb 2 03:12:46 CET 2009

Rask Ingemann Lambertsen wrote:
>    A separate connector for power supply + FET connected as per the PCF50633
> manual, so the USB port is available even when charging.

I thought about this for IDBG. If you have power come in from only
one USB source, you could share the USB current limiter in the PMU.

If you may have power from both sides, you'd need a current limiter
that can be set to 100/500 mA on the "extra" port. If you want to
support also the wall charger, you need an ADC and thus a different
controller chip. (Suitable choices are available, they're just a tad
more costly.)

The current limiter is needed - if you accept dual input - to avoid
situations like both USB ports providing 500 mA, so you'd set the
limit to 1000 mA. But then one gets disconnected and until
software has responded, you'll be drawing 1000 mA from the poor
remaining source.

This would be particularly bad if the software that has to respond
is the main CPU and it has just crashed. The conditional probability
of someone pulling power when the CPU has crashed might be rather
high, considering that one may attempt a power-on reset ...

If you only admit a single USB source at a time, all you need is a
switch and access to EN_USBHOST. Something like the Fairchild
FPF1003A (1x1.5 mm BGA) or FPF1007 (2x2 mm MLF) should do nicely.

Things get much easier if you design with just a single type of
source in mind without having to handle all the possibilities of

- Werner

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