The HC12 transparent serial transceiver has a SET pin which when low puts the HC12 into command mode. If this pin is held low at power up the serial interface is in a default configuration so known baud etc. Then you can set up whatever parameters you need. Now if the SET pin is changed to a high level it will use the new settings.

So I need a circuit to hold SET low for a period of time (maybe 1 second) to allow the controller to set the desired parameters. A simple way would be a capacitor to 0V and a resistor to VCC. This means that there will be a slow rise time. Will the HC12 be OK with this?

 

I would strongly recommend a gate with Schmitt-Trigger inputs (e. g. 74xx17) which will change the slowly rising signal after the RC element int a defined digital transition.

 

Hi Albert,
Are you operating the HC12 in a frequency channel hopping mode, or are the parameters set just one time.?

The method I use for a selected mode, is to ground the SET pin with a jumper to 0V, use the micro to program the mode, then remove the SET to 0V to link.
The HC12 will remember the setting parameters on power Off/On sequences.

E

 

Hi Albert,
Are you operating the HC12 in a frequency channel hopping mode, or are the parameters set just one time.?

The method I use for a selected mode, is to ground the SET pin with a jumper to 0V, use the micro to program the mode, then remove the SET to 0V to link.
The HC12 will remember the setting parameters on power Off/On sequences.

E

Set just one time, but I am worried about the settings being corrupted and then you don't know what settings to use to change them back again.
Pulling the SET pin low when power is applied sets default parameters. This doesn't happen if the pin is pulled low with power applied - I have the SET pin wired to a uC pin.

I think I have found a solution using an IC test clip. Before power is applied fit the clip to the uC with a link between SET pin and 0V pins. When power is applied the firmware can check this processor pin and know that the link is fitted.

 

I need a circuit to hold SET low for a period of time (maybe 1 second)
I think I have found a solution using an IC test clip.

If that doesn't work try this timer circuit for a definitive 1.1 second open drain low output when first powered ON.
The Set pin on the HC12 has an internal 10K pullup.

 

It will be good if you use the RC network to generate the delay, then use a Schmitt-trigger buffer to produce a clean transition. For an HC-12, you can use a 74HC14 if your supply is 5 V. If your MCU/HC-12 system is 3.3 V, you can use the 74LVC1G14 instead.