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I have 9 volt battery to get 5 volt using step down converter. I want to enable the step down using a timer IC max input voltage 6 volt. I will use 3.7 v battery for this.
So my question is: Can I join the ground or negative pole of batteries in the same PCB or circuit, I will have problem with signal voltage to enable step down and power off the timer from the MCU 5 volts ????????
You should have no problem interconnecting all the grounds. As a matter of fact since you want a common "off" signal from the uC connecting the grounds would be required anyway.
Yes, you can join the ground of different voltage supplies together. There's no problem with the circuit as drawn.
Remember that voltages are always relative. Tying the negative poles of the two batteries together only ensures that those poles are at the same voltage, which I'm going to call "0V", just because I can. You can call it whatever you want, but I'm going to use zero for simplicity. The "3.7V" only indicates that the positive pole of the battery is 3.7V above the negative one, but doesn't say anything beyond that.
So, the timer will still see 3.7V across its power supply pin and ground pin, the step-down circuit will see 9V across its pins, and the MCU will see 5V across its pins.
You need a self-approving reply relying on incomplete schematics? Try, it might work.
You need a detailed proper answer? Depends on the EN input of the regulator. Some of them are using an open drain solution, with voltage capabilities between supply of the buck and some logic level. In that case, provide you are in the specsheet boundaries, is OK connecting the ground.
BUT different circuits than an open drain requires analysis: does the EN input is a logic input with well determined maximum AND minimum LOGIC levels? Are there bypass diodes in the chip? You might discover are incompatible logic signalling and you need to put something more than just a wire to connect the EN input, ranging from a logic level converter down to just a resistor. But for this, we need to know the architecture of the EN input with respect the whole buck regulator. And with that schematic, we don't.
