T114 GPS Power Management: Power Cycling Behavior

#1

Hi everyone,

I’m using the T114 V2 board with the 8‑pin GPS header, and I’m trying to understand how power is managed for the optional GNSS module — both in Heltec’s default setup and for custom modules.

Specifically:

  1. Does the T114 supply a VBAT (backup power) line on the GPS connector, e.g., tied to the LiPo battery, to support RTC/ephemeris retention on GNSS modules?
  2. When using Heltec’s optional GPS module (L76K-based, I believe), is the GPS powered off between measurements , or left on continuously?
  3. Is there any software control (e.g., GPIO for enable/power) to manage GPS power state from the Meshtastic firmware?

I’m evaluating whether to use a different GNSS module with better cold-start behavior, so understanding the power strategy is critical — especially for low-power use cases.

Thanks in advance for any insight or schematics clarification!

#2
  1. Without the L76 module schematic, it’s hard to be definitive but probably, implied by the VDD_nRF on pin 7 of the GNSS connector that is always powered - a multimeter would confirm.
  2. That would be up to you via the Vext_Ctrl - again implied by the connector having this on pin 6.
  3. Yes, it is a compile option but at present it’s only linked to the triple-click on the user button. But can be extended to do more, probably best as a plugin.
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#3

Thanks Nick,

Thanks for your comment. I’m not very familiar with Meshtastic codebase just yet but on both Ve_3V3 and VDD_nRF on header pins 7 and 6 are constantly on 3v3 (I have done multiple measurements)
Also VDD_nRF seems to be the main power supply where Vext_Ctrl (from nRF52) controls a P-MOSFET that powers VE_3V3

Ve_3V3 powers:
The OLED Display
The GNSS module via GPS header pin 7
as well as other peripherals as the leds

Powering off Ve_3V3 via Vext_Ctrl disables both the GPS and display so not an ideal solution.

Another observation is that the Pin 3 of the GPS Header is labeled “WakeUP”
It is directly connected to nRF52 GPIO P1.02 this can imply that probably intended as output from nRF52 to wake the GPS out of standby.

It would be nice if any Heltec folk can chip in here.

#4

Unless you have code that turns off the Vext_Ctrl, this is not surprising.

You could check the data sheet for the L76

Look around, see when / if that happens.

Short answer, we have the resources in the form of the schematic and the body of experience of habitual / long-term Heltec module users, your observations mirror mine but you’ve not said what you want to achieve in terms of turning things on & off or what your end goal is. Context is everything and allows for more nuanced responses.

#5

Amazing, so my use case is that I want to take the position and transmit it regularly every 1min. Expecting an initial cold start lock of 1-5mins from the GPS module on start-up and then a 3-9s hot start lock. I want to replicate the Lora 32 V3.2 behaviour where using an external mosfet and the GPS_EN pin the firmware was cutting off power to the module after the lock before enabling it again for the next measurement saving power.

Good news is I managed to validate this functionality is indeed supported from the T114 & L76K GPS module apparently using the wakeup pin 3.

Power consumption is 48-49mA with TFT, BLE, GPS on and falls to 18-19mA after lock is acquired. This is achieved by setting low the wakeup pin 3 in the L76K module as described here

When the wakeup pin 3 of the L76K module set to low state, the module consumes almost no current. I tested that by removing the module altogether and setting the GPS state to “OFF”. Then I got the same 19mA consumption as before with TFT and BLE on.

I’m amazed the power consumption figures are even less than advertised. Good job over there folks!

For future reference, the Meshtastic firmware handles the wakeup state here
here

Thank you for your help

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