Need to set an output pin without it going LOW during initalization

Question

For a control system I am using a relay module that controls a larger relay for high-current electrical stuff, as part of a battery protection circuit for my solar plant. This module switches 120 VAC for the larger one. The larger one is set up to latch the power off until a button is pressed manually.

I have several Adafruit relays, (single, dual, quad) and they all seem to be active-LOW. I am using one of them for this project.

My problem is that I can't seem to prevent my Arduino Nano code from briefly setting the output pin LOW in setup, setting in motion the cascade involving latching the power off.

If at all possible with this hardware, I would like to prevent the need to manually reset the latch when uploading code changes.

Here is how I am initializing that pin in my setup code:

pinMode(12,OUTPUT); digitalWrite(12,HIGH);

All in one line. The LOW pulse is just long enough to trigger the other relay.

I know I can probably find parts like an inverter chip or maybe a different relay, but really want this working yesterday.

So my question is: Is there any way to prevent it from pulling it LOW during initialization? Or, is there a different approach I should take in relation to this relay output?

Thanks in advance

Answer 1

Arduino sketches are actually (somewhat constrained) C programs. As such, what is true for C is usually true for sketches. In C, a segment of code usually ends with a ";". It matters not if there are two (or more) of these on a single line. The computer will execute the segment before the first ";" first and the segment after the first ";" second.

In this case you are setting a register inside the Atmel processor on the Nano to the "out" direction. So it will force the associated pin to what ever the data in already set up in that pin's data register. Then you are setting that pin's data register to high or 1.

Instead, try setting the data register to 1 before setting the direction register to "out". Do this by switching the order of the code segments.

In case this does not work. Or in case you find the relays acting randomly on power up, consider the relay driver inputs to be high impedance and susceptible to any noise prior to the Nano taking control of the relay driver control pins. In such a case, consider adding pull up resistors small enough to ensure the inputs of the relay drivers are held at a known voltage but high enough such that the Atmel processor in the Nano can easily over power the pull up resistor when necessary.