We are using a TS-7260 SBC to control an Ocean Optics spectrometer (a MayaPro)
and are having problems with the USB system. The problem has to do with the
fact that the MayaPro consumes very near the USB limit of 500mA. It appears to
exceed significantly (>600mA) during power-on of the USB device.
The first problem I had was that the internal 5V charge pump cannot deliver
this much current. I solved this by plugging the TS-7260 into a regulated 5V
supply and inserting JP7, which then directly puts the regulated 5V to the USB
power. This helped a lot, but didn't solve the full problem.
Now, I see the USB device in two different power states. When plugging in the
device or turning on the computer, I sometimes see one or the other state. One
is the "normal" working state, where it draws 470mA and has a voltage across
the USB nominal 5V of 4.6V. I think the voltage drop is partially on cables,
contacts, and possibly the MIC2026 chip on the TS-7260. The other power state
is "high current", where the USB device draws 690mA, but only has 2.7V voltage
across the nominal 5V. I believe the other 2.2V is dropped in the MIC2026 chip
because that chip becomes very hot in this state. From looking at the MIC2026
data sheet, I believe that it has gone into overcurrent or thermal shutdown,
which should only happen at 1000mA. It might be that the USB device spiked
that high, but I don't know.
Another funny observation is that when I start with the TS-7260 off and the USB
device plugged in, here's what see upon power up. The TS-7260 starts to boot,
and the current of the USB device goes in the high-current state. The MIC2026
gets very hot. The TS-7260 boots fully into runlevel 3, and then when I load
the USB modules, the USB device suddenly switches into its operational state
(normal current). The thing that fixes the system is the loading of the module
"insmod usb-ohci-ep93xx". The current goes normal right after loading this
module. I have no clue what is happening here. Possibly that module
momentarily interrupts the USB power and resets the USB device? Possibly the
USB device is drawing a high current before it enumerates onto the USB system?
My question is if anybody has seen this problem before, and figured out a
workaround.
Thanks!
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