M1pre-rc4 vs. capacitative USB load
===================================

This is a test of how the current-limiting USB power switch in
M1pre-rc4 handles a "nasty" capacitative USB load. The worst load
identified in previous tests with an M1rc3 with altered reset
logic (but no current-limiter) [1] was an iCon i-Creativ.

[1] http://lists.milkymist.org/pipermail/devel-milkymist.org/2011-December/002488.html

The M1 was set up in the nominal 5.0 V configuration described in
README, which results in a supply voltage that's about 200 mV
below what M1 should normally receive:

Source	Input		J21/J3
-------	---------------	---------------
5.00 	4.81/4.86/4.90	4.47/4.62/4.72

The worst drop of the 5 V rail that could be observed in about a
dozen hotplug tries was about 450 mV [2]. Note that the probe used
in these measurements has a linear error that cause readings to be
about 2-3% low (~100 mV in this case), i.e., the lowest voltage
was still about 200 mV above the threshold of the reset chip.

[2] usb-icreativ-50us.png

The tests above were done with a lab power supply that has very
fast current limiting. A test with a 5 V switching power supply
similar to the one shipped with M1 showed a reduced drop [3] of
only about 350 mV.