Restart Lines over 2 different voltages

3.3v <---RST---> 5V

So if I have an input restart line which is 5v and I use resistors to divide it down to 3.3v, will that mess up the restart on the 5v system?

3.3v <---RST (Expecting 5V input)---> 3.3V

And if I move the project to another system that uses 3.3v would I need an amplifier to 5v so that it can go through the resistor dividers and would that mess up the RST line.

Or is there a simpler way to all of this?

Hi,
What systems are we talking about?

Are asking if you have a logic level 0 to 3.3V and you apply that to a 5V system, will it cause any problems?

You need to check the specs/data of the 5V controller, some will accept 3.3V as a logic HIGH.

Tom… :slight_smile:

No idea what you're doing... but for the vast majority of 5V systems a 3.3V signal will read as high. In case of a 5V Arduino, anything over about 2.5V will read high (that's my real world experience - the spec guarantees a high signal for 0.6*Vcc or 3.0V for a 5V system, the actual changeover point is of course below that and varies between individual chips).

So 3.3V on a RESET pin of a 5V system will most likely not cause it to reset, but do refer to the datasheet of your specific device for the details.

or possibly a couple of (schottky or germanium) diodes (anode to -RST pins), then (assuming the correct pull-up on each pin, by putting a 0V state on the cathodes will reset both devices.

Thanks for all the help, unfortunately I'm designing a circuit to interface with an older retro and proprietary hardware so I don't have much information on it. I do know it's TTL and that a 3.3v signal will read as high but I didn't know a reset signal was the same as an io signal. I know the io signal will read as high that is 3.3v but I thought RESET/RST worked differently than an io signal. If it's the same then it shouldn't be a problem.

Thank you!

In response to your diode suggestion, I'll try both ways. Theres lots of ways to handle the RST signal but it's only needed for the programmer to program the arduino. The retro device does provide an RST signal but I don't see the point in it so in all honesty I may just output a 3.3v high when connected to the tro device and use the RST signal as is when connected to the arduino programmer.

Is that reset signal active high or active low?

Arduino reset signals are active low - i.e. they require a low signal (connect to GND) on the reset pin for a reset, during normal operations it's pulled up to Vcc.

You should also know what Vcc is for that unknown device, as the required input signal levels depend on Vcc, giving the maximum voltage guaranteed read as low, minimum voltage to guaranteed read as high, and the absolute maximum voltage it can handle on the input.

The RESET is active low on it as well and I know it's a TTL system that's compatible with the CMOS output of the microcontroller. So it should all work ok unless I'm misreading your reply.

Sounds like it will work just fine then.