m1rc3/usbpwr/: M1pre-rc4 USB short-circuit tests

m1rc3/usbpwr/README

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+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 *	4.71/4.86/4.90	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 10 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