Continuing the testing of the USB inrush current protection, I now tried a real-life capacitative load. From previous tests I already know that lowering the reset voltage alone removed inrush current issues for all but one device, namely the iCon i-Creativ.
I now tested it with the M1pre-rc4 that has the current-limiting USB switch. The test setup was intentionally unfriendly, with the lab power supply providing a voltage near the bottom of the permissible range and enforcing strict current limits. I could still produce a significant drop, but it wasn't bad enough to cause a reset: http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/m1rc3/usbpwr/usb-icreativ-50us.png The oscilloscope probe used here has a linear error of a few percent, so the real voltages are about 100 mV higher. So this means that we're about 200 mV above the threshold voltage of the reset chip. With a power supply similar to the one shipped with M1, the results were even better, as expected: http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/m1rc3/usbpwr/usb-icreativ-50us-psu.png More details can be found here: http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/m1rc3/usbpwr/README2 Conclusion: I think lowering the reset chip's threshold voltage combined with the current-limiting USB switch does provide adequate protection of the 5 V rail under reasonable operating conditions. With the USB power switch, the M1 will withstand a certain amount of abuse. Excessive loads on a USB port (i.e., a short) may make it reset but cause no lasting damage. What's next: we still need to verify that the USB power switch doesn't cause trouble for properly behaving USB devices. I don't expect to find any trouble there besides electromechanical instability (due to the way this prototype was made), but I'll try to use the M1pre-rc4 for USB protocol work, so it'll get exposed to a number of devices. - Werner _______________________________________________ http://lists.milkymist.org/listinfo.cgi/devel-milkymist.org IRC: #milkymist@Freenode
