LoPy Specs

[ssmith]

I'm wondering if there are better specs in the works for the LoPy module. What is currently published is not really sufficient to do a product design using the module. Some of my questions:

• Power input

  • If powering from VIN
    • Can the 3.3V rail be used for other power on the board?
    • If so, what is the max power that can be supplied by the 3.3V?
    • What is the efficiency of the 3.3V supply? What is the quiescent current of the supply?
      • In a battery powered sensor design 1uA is a lot of current.
  • If powering from 3.3V
    • The spec states "The input voltage must be exactly 3V3, ripple free and from a supply capable of sourcing at least 500mA of current"
    • This is not a spec or reality. What is the range for each. I would assume it is the same as the ESP32 and the SX1276?

• Knowing the pin mapping between the module pins and the ESP32 pins would be very helpful.

• Knowing if any pins need pull-ups or pull-downs for minimum power during sleep modes.

• What are the specs for each of the GPIO pins. When designing for minimum power consumption the difference between a 20K pull-up and a 100K pull-up is huge.

  • What are the values of the internal pull resistors?

Thanks,
Steve

[jmarcelino]

@ssmith
Yes it's perfectly fine.

If you measure at the 5V pin while powering from the 3.3v you'll see the voltage minus the minus the internal diode drop of the high side FET (should read about 3v or slightly less).

I wouldn't recommend leaving other devices attached to 5V pin as they may be powered by that backfeed through the diode and potentially overheat it, but I don't see a problem to leave it floating.

There may be some very small extra consumption (a few μA? I'd need to order the TI eval kit to be sure) caused by this so, for optimal power consumption, you really should go plan for the L01 OEM module soldered on the PCB.

[ssmith]

@jmarcelino, Is it a good idea to power a LoPy from the 3.3V pin? There is no diode to prevent backfeed into the LM3281.

[jmarcelino]

@ssmith said in LoPy Specs:

• If powering from VIN
  • Can the 3.3V rail be used for other power on the board?
  • If so, what is the max power that can be supplied by the 3.3V?

That's in the pinout diagram
https://docs.pycom.io/pycom_esp32/_downloads/lopy_pinout.pdf

"up to 1.2A maximum load" on the 3.3V pin

• What is the efficiency of the 3.3V supply? What is the quiescent current of the supply?

From the reference schematic they use the LM3281
http://www.ti.com/lit/ds/symlink/lm3281.pdf

• In a battery powered sensor design 1uA is a lot of current.

For such a sensor you'd really want to use your own regulator optimised for your application and ideally use the L01 OEM module instead of the LoPy.

• This is not a spec or reality. What is the range for each. I would assume it is the same as the ESP32 and the SX1276?

Yes that makes sense and what's there matches what's on the ESP32 design guide.

• Knowing the pin mapping between the module pins and the ESP32 pins would be very helpful.

You are connected to the ESP32 directly, the pin mapping is on the LoPy pin map I linked above already.
Use the GPIOxx labels in the pin diagram to refer to the ESP32 data sheet.