Pro Mini Power Issues

Hi all

Hope someone can help as a little lost as to what is going on.

So I built a small outside box with a Pro Mini, and NRF2401 2.4Ghz transmitter and a BME280 Temperature/Humidity sensor.

As its a pro mini there is no USB so only have the header pins to connect to, am using a serial to TTL converter to dump the code to the Arduino.

That all works great.

As this is going to be a battery based outside unit, I removed the power led, the voltage regulator and am using the low power library.

When its all powered up using the laptop and the 5 connections, VCC, GND, TX, RX and reset it all works great.

If I remove the laptop and just supply power to VCC and Ground nothing happens, if I reconnect the reset pin , so have VCC, GND and reset connected to the laptop it works again.

Any ideas why I cant get it working with just VCC and GND?

All help greatfully appreciated

What is the voltage of the battery?

Its all around 3.3v, as I`m using the 3.3v Arduino, the power for the serial converter is also 3.3v

Are you connecting Reset or DTR?
How confident are you that you haven't damaged the board with your desoldering attempts? If the Pro Mini doesn't work with VCC and GND connected (which it should) but does work with VCC, GND and Reset connected then it implies that the reset line is being held low when it isn't connected to the USB-Serial adapter.
Try connecting VCC and GND to the USB-Serial adapter and then measuring the voltage on the Reset pin. What voltage do you get?

You haven't said which Pro Mini you are using - there are 5V 16MHz and 3.3V 8MHz units.

And picture of your wiring will be helpful as well.

Hi all

Appologies, on closer inspection of the pro mini board its not a reset pin but marked GRN, and if its connected to the Serial TTL adaptor on that board it would be the reset, so I assume its the same on the pro mini.

Its the 3.3v 8Mhz version, and I measured the voltage on the GRN pin with just VCC and GND and the reading is Zero volts

I'm really confused. There's no "reset" pin on the FTDI header of the pro mini, at least not on the schematic I have. Nor is there any GRN pin. There might be a RTS pin on the USB adapter, but that's not reset. In any case, it still ought to work with Vcc and GND.

All the Pro Minis I have come accross have a DTR pin next to the TXD.

This DTR pin is connected through a capacitor to the RESET pin on the ATmega.

A pulse on the DTR pin causes the ATmega to reset.

A permanent connection, logic high or logic low, on the DTR pin should not prevent the ATmega from running.

Standard programming interface beahviour.

Ozric13:
Hi all

Appologies, on closer inspection of the pro mini board its not a reset pin but marked GRN, and if its connected to the Serial TTL adaptor on that board it would be the reset, so I assume its the same on the pro mini.

Its the 3.3v 8Mhz version, and I measured the voltage on the GRN pin with just VCC and GND and the reading is Zero volts

The 'GRN' pin is the DTR pin. Is that the one that you connect to Reset on the USB-Serial adapter?

Hi BJ Henry

Yes that is correct, my friend said it all needs to be grounded which made no sense to me as its meant to be a battery driven project.

What I did discover though is that if I momentarily short the DTR pin to GND, it all starts working fine.

Not an ideal solution, but as its now running and the battery should be good for at least a year I`m not too worried now.

But I am still curious why it was necessary, maybe I did damage the board a little when modding it.

I`ll try it on another pro mini and see if its the same.

Cheers

There’s no need to ground the DTR (the ‘GRN’) pin at all, just leave it unconnected if you aren’t actively using it to program the board.

I understand that, but by touching it to ground for one second, my arduino runs normally.

Its very odd

Ozric13:
I understand that, but by touching it (DTR) to ground for one second, my arduino runs normally.

Its very odd

Not odd at all, expected and normal behaviour.

Pressing the reset button should have the same effect.

The reset signal is explicitly pinned out on the Pro Mini - you could add a delay circuit to this to ensure
correct reset is asserted on power up. Perhaps a MAX803 or similar supply monitor/reset generator?

If you connect only power and ground to a pro mini, or any Arduino, it should boot up and run. It should not be necessary to do a reset. As MarkT said, the Reset pin of the 328 is pinned out on the pro mini, to two pins it looks like, and if you measure the voltage on either of those pins it should be high. It has an external 10k pullup resistor (R2). So I think it's still a mystery.