18M2 reset pin?

[nickwest]

I'm subsitituting an 18M2 in place of an 18X in a thermostat circuit. As such, the circuit no longer needs the reset switch or its associated resistor, and I gain an extra input should I ever need it in the future.

At present however, this pin is unused (C.5 on the 18M2, or leg 4 of the chip). My question is, should I pull it high or low? On the 18X it was high, but if I understand correctly the 18M2 is completely different from the 18X internally, and just conveniently has the same pinouts? Does this mean I can just pull the unused C.5 pin to ground?

Thanks all.

 

[hippy]

You can leave the circuit as it is for the 18X, have the 18M2 C.5 floating, have it pulled high or have it pulled down. Recommended practice would be to have it pulled high or low but there's usually no issue if left floating.

The internals are no different, just that the pin to reset circuitry in the chip is disconnected so it doesn't cause a reset and can be used as a standard input. Having a pull-up and a short to 0V button ( as per 18X reset ) won't cause problems.

 

[pete20r2]

Hey Nick,
can you please give more information on your thermostat circuit, is it to control a thermostat in a hot water heater. I ask because I was planning on using a picaxe to move a thermostat on a hot water heater at different times of the day.

 

[nickwest]

Thanks for the clarification Hippy - I'll pull it down to ground, which also allows me to delete the reset circuit from the PCB.

pete20r2 - I said "thermostat" but it's really a glorified indoor/outdoor thermometer to work in a vehicle. As for your water heater controller, how do you plan for the picaxe to know what time it is?

The temperature part of my project is easy, I use ds18b20 temp sensors with the picaxe's "readtemp" command. Waterproof versions of this sensor can be found from various ebay merchants, although be careful as I've found the pinouts on their included plugs to be wrong. I have since graduated to using waterproof sensors from another merchant in China which come with bare wires to solder direct to the board. Let me know if there are any other details I can help with.

 

[westaust55]

If you continue to use a resistor such as 10kOhms for the pull down on the spare pin then there is a good likelihood that you can later add another signal without the need remove the resistor first.