[Milkymist-devel] reset circuit vs. USB inrush current
werner at almesberger.net
Sun Nov 13 11:22:40 PST 2011
In order to determine at what reset voltage we'd be safe with a
design monitoring only the 5 V rail, I've tested what effect
connecting USB devices has on the 5 V rail.
In all these experiments, the M1 was either sitting at the desktop
or rendering. I didn't see the 5 V rail change noticeably between
desktop and rendering. I measured 5 V with a clip directly at the
reset chip and with the ground reference on a capacitor nearby. The
clip has a ~30 cm unshielded cable, so we pick up a bit of noise.
My M1 is powered by a Minato 5 V 2500 mA switching power supply.
This is not the official M1 power supply and I don't know how it
compares to it.
In standby my scope sees the 5 V rail around 4.97 V. Under load
(desktop or rendering), it drops by about 200 mV.
Let's begin with an old generic USB mouse. It has "Kensington"
written on it. lsusb calls it "Novatek Microelectronics Corp".
Rubberized ball, three buttons, no scroll wheel:
The bottom cursor is at the nominal threshold voltage of the reset
chip. We can see that the 5 V rail drops a little but all is well.
Next, my Rii RF mini keyboard with integrated touchpad. The RF
dongle is a little greedier but still doesn't cause trouble:
Then I tried a USB-MIDI controller, the Korg nanoKONTROL2. This
is a full-speed device. This time we got close to the limit:
Finally, I connected the flagship of my collection of input
devices, a HHKB Professional. This is also a full-speed device.
This time things went too far and the reset chip had to do its
duty, kicking the M1 back into standby. (Or at least that's what
I suppose happened - I didn't check the reset signal.) This
happened each time I connected the HHKB while none of the other
USB input devices I tried had any issues.
Outside the competition, I've also tried my atusb:
As we can see, it has a hidden talent as master assassin. Its
direct capacitative load on VBUS is 10 uF, with 4 uF waiting on
the secondary side of the main regulator and another 2 uF behind
Conclusion: since I used a 3rd party power supply, the results
are only indicative. They show that the 5 V rail of M1 can drop by
about 400 mV when connecting USB devices that can be considered
light power users.
This means that a reset circuit operating from the only externally
regulated 5 V input with a 4.4 V threshold voltage is likely to
experience resets when connecting USB devices.
If we stick with the idea of monitoring only a single input, which
would then have to be the 5 V rail, I would recommend using a reset
chip with a threshold voltage of 4.0 V. This will still let the
regulator of the 3.3 V rail produce a 3.0 V output under worst-case
conditions, while providing a safety margin for USB devices with
high inrush current and for power supplies providing a low "5 V"
These experiments should be confirmed with other power supplies,
including an official M1 power supply.
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