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can quickstart board power 3.3v gps? — Parallax Forums

can quickstart board power 3.3v gps?

Dwayne DibbleyDwayne Dibbley Posts: 63
edited 2011-07-20 10:22 in Propeller 1
Hi, should the Quickstart Board be able to supply a 3.3v GPS pulling about 93mA ?

measuring pin 39 and 38 on J1 gives me 3.3v no problems

I have connected the GPS GND to pin 39 of J1 and the 3.3v of the GPS to pin 38 of J1, but as soon as the unit is connected the quickstart shuts down ( EEPROM has sample code that flashes LED's and the LED's stop flashing when gps is connected ) and when i unplug the GPS the sample code starts up again.

My quickstart is powered by the PC USB.

I have a USB TTL convertor that powers the gps fine so the PC USB has enough power.

Thanks

Comments

  • Mark_TMark_T Posts: 1,981
    edited 2011-07-19 18:03
    You can't hotplug it onto the regulated supply, the decoupling caps on the GPS will crowbar the 3.3V line and brownout the Prop...
  • Dwayne DibbleyDwayne Dibbley Posts: 63
    edited 2011-07-20 01:07
    I have also tried with the gps plugged in prior to applying USB power and same result :(. If i power the GPS externally from another power source ie 3.3v + Gnd, and have the RX connected to the propeller, will the RX work on its own or would i need to link the Gnd's?

    I gather the GPS is brownouting the quickstart on startup hence the external power source idea seperate from the quickstart.

    Thanks
  • StefanL38StefanL38 Posts: 2,292
    edited 2011-07-20 02:35
    if you use two or more powersupplies for proper operation the GNDs must be connected.
    Without connecting the GND strange things can happen.

    If the circuit draws high currents (more than 1A) you have to take care where to connect the two GNDs together, because of the voltagedrop
    caused by high currents.

    With 100 mA this doesn't matter.
    keep the questions coming
    best regards

    Stefan
  • Dwayne DibbleyDwayne Dibbley Posts: 63
    edited 2011-07-20 03:12
    Am i correct in thinking the USB power is only drawing 100mA from the PC port. So when the GPS draws 93mA obviously the quickstart only has 7mA left and brownouts. So i could feed in from an external power source to the quickstart ( eg pin 40 and pin 38 with a 9v battery ). This should then give me enough power to run the board and the gps unit? or am i barking up the wrong tree.

    Thanks
  • RaymanRayman Posts: 14,876
    edited 2011-07-20 08:17
    I'm a little suprised you have trouble drawing only 93 mA... I would expect the Quickstart circuit to provide easily 500 mA of 5V power and say 400 mA could be converted to 3.3 V. Maybe you should check what the 3.3 V regulator on the Quickstart is rated for... Maybe it's only 100 mA and that would explain it.

    You could add your own 3.3V regulator to power the GPS off of the 5V USB power...

    Or, maybe that GPS really draws more than 93 mA at startup...
  • Mike GreenMike Green Posts: 23,101
    edited 2011-07-20 08:38
    @Rayman- The issue is the use of USB power. A normal USB device can use up to 100mA. Once a device has initialized itself, it can request more power (500mA). In your case, the power drain of the GPS plus the QuickStart itself immediately exceeds 100mA and the USB host will shut it down. One way around this is to use a powered USB hub. Many of these will just provide 500mA to devices without checking for low power / high power mode.
  • RaymanRayman Posts: 14,876
    edited 2011-07-20 09:11
    I think it's the FTDI 232RL chip that controls the USB power. I've never seen it actually turn the power off due to current draw more that 100 mA.
    You can program the FTDI chip using the MPROG.exe utility. I used to always use it to allow 500mA. But, after a long time, I found that the default setting of 100 mA worked even when drawing 400 mA.

    The PC could also shut down power, but I've only seen that when drawing a lot more that 500mA.

    Actually, now that I think about it.... What is your USB cable connected to Dwayne?
    If it's going to a non-powered USB hub, that would explain your problem.
    You definitely need a powered USB hub, or a direct PC connection.
  • Dwayne DibbleyDwayne Dibbley Posts: 63
    edited 2011-07-20 09:15
    confirmed that the quickstart is only requesting 90mA from the PC ( direct connection ) seen here:

    usb.jpg

    Can i feed an external power source in via J1 as well as having the USB connected ( for easy access to terminal ) ?

    Thanks
  • Dwayne DibbleyDwayne Dibbley Posts: 63
    edited 2011-07-20 09:32
    UPDATE: I was actually plugging into pins 37 and 39 ( slaps face with wet fish ) so i was actually just resetting the quickstart, now plugged into the real pin 38 and 39 and gps powers up - phew

    sorry about that :)

    now onto some programming

    Thanks
  • RaymanRayman Posts: 14,876
    edited 2011-07-20 10:22
    Glad you figured it out.

    BTW: The FTDI chip tells the PC how much current it needs during connection and then the PC tells the FTDI chip it's OK to turn power on. But, I've never seen a PC that actually enforced the requested current limit. That's why I don't bother reprogramming the FTDI chip anymore....
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