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Another QuickStart external power question (or two). — Parallax Forums

Another QuickStart external power question (or two).

abecedarianabecedarian Posts: 312
edited 2014-07-01 22:47 in Propeller 1
Background:
With my EFI project, the ECU gets 12v-15v from the motorcycle. The MC33814 chip being used on the board has a built in regulator that provides two 5v supplies- one only regulated, intended to power the MCU, and the other regulated and protected against shorts, et cetera, used for external sensors. For prototyping, I'm using a QuickStart board.

The ECU will have a 12v connection and the electronics are switched on or off by the key switch. So, always-on 12v, part time 5v.

What I'd like to accomplish is have the QS board be able to use USB to PC in 3 different scenarios: ECU on the bike but key off, ECU on the bike with key on and ECU disconnected from the bike. The first and last situations would require the QS to receive power from USB while the second precludes power from USB.

I'm thinking that if the /USB_POWER_ENABLE pin can be tied to the 12v supply somehow, this would be the simple solution as that would inhibit USB power any time the ECU is connected to the bike, running or not, and allow USB power if the ECU is off the bike / 12v source.

If this is feasible, would a pull-up resistor of 10K (or ???) ohm between /USB_POWER_ENABLE and 12v suffice?

I'm also open to suggestions.

Thanks.

Comments

  • David CarrierDavid Carrier Posts: 294
    edited 2014-07-01 14:46
    On Rev B QuickStart boards, pulling /USB_POWER_ENABLE to 12 VDC through a 10 kOhm resistor should work. On Rev A boards, it could damage the board, so it isn't recommended. The "Rev" label is on the right side of the board, below the QuickStart label.

    — David Carrier
    Parallax Inc.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2014-07-01 15:07
    Is there a safe way to use an external supply when the QuickStart (Rev A) is plugged into the USB?

    I generally (actually always) disconnect power whenever I plug a USB cable into the QuickStart. It would be nice if there were an easy way to leave the power connected.
  • David CarrierDavid Carrier Posts: 294
    edited 2014-07-01 15:49
    The Rev A QuickStart doesn't have a protection diode on its /USB_POWER_ENABLE line, so to drive it high, you have to pull it to the USB supply. Driving the pin 0.6 volts, or more, higher than the USB supply voltage can damage the FT232R, and driving it more than a few hundred millivolts lower than the USB supply voltage may not completely disconnect the USB supply voltage from the rest of the board. If you can keep power in range, it will work, but you have to be careful, because USB ports power supplies can range from 4.25 to 5.5 volts.

    — David Carrier
    Parallax Inc.
  • abecedarianabecedarian Posts: 312
    edited 2014-07-01 22:47
    Thanks for the reply, David.

    Well that's a bummer though; I have a Rev A. Any suggestions that may help accomplish what I'm looking at?
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