M1pre-rc4 vs. shorted USB
=========================

These are the results of supplying an M1 equipped with a
current-limiting power switch from a lab supply and shorting or
(over)loading one USB port. Note that this test involved only
resistive loads.

Source	Input		J21/J3		Boot	Short USB	Load USB (2 Ohm)
-------	---------------	---------------	-------	---------------	---------------
4.50	-		-		No	-		-
4.60	4.41/4.46/4.50	4.03/4.20/4.33	Unstbl.	Reset		4.04/4.09/4.33
4.70	4.51/4.57/4.60	4.15/4.31/4.43	Unstbl.	Reset		4.16/4.22/4.43
4.80	4.61/4.66/4.70	4.22/4.40/4.52	Yes	Reset		4.25/4.32/4.53
4.90	4.71/4.76/4.80	4.29/4.47/4.60	Yes	Reset		4.39/4.46/4.62
5.00 *	4.81/4.86/4.90	4.47/4.62/4.72	Yes	Reset [1,2]	4.48/4.55/4.72
5.10	-		-		Yes	Reset		-	   [3]
5.20	-		-		Yes	Reset		-
5.30	5.10/5.16/5.19	4.61/4.86/4.99	Yes	Unstbl.		-
5.40	-		4.85/5.01/5.10	Yes	4.89/5.01/5.11 [4]  -

* Voltage at source: 4.99/5.01/5.01 V

- not measure either because measurement was not applicable or
  because the result would have been of limited interest.

All voltage measurements min/avg/max, over 3000 samples. Flickernose
was rendering "The Tunnel" at the time. The M1pre-rc4 was running
the vanilla 2011-07-13 installation.

"Source" is the nominal supply voltage, from a Leaptronix mPP-3040D,
with current limit set to 3.0 A.

"Input" is the voltage measured on the supply cables near the M1's
DC input.

"J21/J3" is the voltage on the internal 5 V rail measured across
J21.1 and J3.14, with a Fluke 8845A with averaging set to 1 PLC.
[5] shows the measurement configuration.

"Boot" indicates whether the M1 can boot out of standby with the
respective voltage and current limit. If only unstable operation
was achieved, M1 only sometimes booted or reset after running for
some minutes.

"Short USB" indicates what happens if one USB port is shorted
through labsw. In all most, the voltage drop on the 5 V rail was
sufficiently large to cause a reset. [1] and [2] show the internal
5 V rail in channel 1, and the external VBUS on channel 2. Note
that the resistive probes used have a few percent of linear error.
[6] shows the external setup on the USB port.

If setting a supply voltage well above the nominal 5.0 V, even
shorts on USB can be survived. At 5.30 V and 5.40 V, resets still
occurred frequently. [4] shows the 5 V rail (barely) escaping a
reset.

"Load USB (2 Ohm)" repeats the same experiment with a 2 Ohm series
resistor added. [3] shows the response. Occasional resets occurred
at nominal voltages of 4.80 V or below.

[1] usb-short-100us.png
[2] usb-short-10us.png
[3] usb-2R-200us.png  
[4] usb-short-5V4.png
[5] tap-j21j3.jpg
[6] tap-usb.jpg    
