I have this GPS which requires a back up battery to achieve warm start. I noticed in the schematics that V3.3 and VBAT is internally connected for this GPS board. Therefore, I connect a 3.3V battery to the JST in order to power the board. A microcontroller is used to read from the GPS. However the 3.3V pin on microcontroller is not connected to 3.3V pin on the GPS. They only share a common ground. The reason for this is that I want to use a switch to turn off the power to the microcontroller but leave GPS running in low power mode(off the battery). However, When I turned off the power supply to the microcontroller, the LED (attached to 3.3V of the microcontroller) was flickering weakly. As soon as I unplugged the battery on the GPS, the LED went off.

I am suspecting that the battery is supplying power to the LED but how is that possible? The LED only shares the ground of the battery.

enter image description here

There are couple 0.1uF caps between the 3.3V and GND on the microcontroller side. Also, it uses UART(TTL) to communicate with the GPS.

  • \$\begingroup\$ JST is the same as JP2 on that schematic? \$\endgroup\$
    – Cornelius
    Commented Jun 5, 2014 at 15:11
  • 2
    \$\begingroup\$ How did you turn off power to the microcontroller? Does it have a separate regulator or did you add in a mosfet or a simple physical switch? If so, where did you add it to the circuit? A schematic would be helpful. \$\endgroup\$
    – horta
    Commented Jun 5, 2014 at 15:33
  • \$\begingroup\$ @Cornelius They are the same. \$\endgroup\$ Commented Jun 5, 2014 at 15:49
  • \$\begingroup\$ @horta I used a physical switch to turn off the power of the microcontroller. \$\endgroup\$ Commented Jun 5, 2014 at 15:50
  • \$\begingroup\$ Please draw interconnections between MCU and GPS \$\endgroup\$
    – Vovanium
    Commented Jun 5, 2014 at 16:06

1 Answer 1


It is possible, that GPS did not went to lower power mode when you power off MCU. It seems that there's a pin '24' for this. If MCU is still driving TX pin to Vcc then your MCU have phantom power going through this connection. You should disconnect communication lines when you turn off eighter device to keep off from such effects.


simulate this circuit – Schematic created using CircuitLab

It is always good to avoid phantom power issues. There are some methods for it.

  1. There could be specific interface chips. Unfortunately I do not remember single one.

  2. There are several logic level shifters (like TXS0102) that implement disconnection (Hi-Z) when power on ether side goes low.

  3. Tristate buffers.

  4. Open-L (open-collector/open-drain) signalling with pullup at receiver side.

  5. As a low man's solution a resistor limiting current through signal wire to reasonable value.

  • \$\begingroup\$ The GPS will stream data regardless if anything is reading or not. But if I flip the switch twice, like on-off-on-off, the LED goes completely off. \$\endgroup\$ Commented Jun 5, 2014 at 16:00
  • \$\begingroup\$ It seems something is switched inside GPS. E. g. it detects 'break' signal on RX and shut TX down. \$\endgroup\$
    – Vovanium
    Commented Jun 5, 2014 at 16:07
  • \$\begingroup\$ You are right. The TX pin on GPS is causing this. As soon as I disconnect TX only, the led went off. I am not too familiar with phantom power, is it something that will damage the microcontroller? Or it will just drain the battery faster? \$\endgroup\$ Commented Jun 5, 2014 at 18:43
  • \$\begingroup\$ Phantom may blow protective diodes inside chip if there's much current flow through it. Some other undesired effects may occur, like parasitic thyrisor structure latchups. \$\endgroup\$
    – Vovanium
    Commented Jun 5, 2014 at 19:14

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