Baby O Compatibility With Pololu USB-Serial

It says:
“The TX and RX lines use 3.3-volt levels, but the USB adapter input lines will tolerate up to 5.5 volts. Therefore, a direct connection to a microcontroller’s I/O lines is usually possible.”

So… is it safe to connect the TX and RX directly to my Baby O? When I connect only GND, TX and RX as specified, (and the USB is plugged into my PC) it makes the Baby-O red light shine dimly. I havent left it plugged in long enough to attempt comm with it, I didn’t want to blow anything up.

Mike Clemens


There are probably multiple things you are doing wrong. First off, you’re quoting something from the USB adapter documentation when your concern is about your Baby O. Like many microcontrollers, the ATmega328 has limits on its I/O lines that are a function of its supply voltage, and in general, it’s bad to go above Vcc/Vdd or below ground. So, connecting ground and a 3.3V line to an unpowered microcontroller is not good. (In this case, you’re probably not going to destroy anything, but you should still know it’s a bad practice.)

More important, it sounds like you are connecting the TX line to the wrong line. The red LED is on the Baby O’s PD1/TX line, which needs to connect to the USB adapter’s RX line.

- Jan

Hi Jan

I have RX/TX and GND hooked up correctly. I can transceive successfully between the Pololu USB serial adapter and Baby-O.
The red light does glow dimly on the Baby O in this configuration with a blank Baby-O.

My question is, is this safe for these devices long term because of the 3.3V, 5V issue?

I have
RX -> TX
TX -> RX
Common GND

I posted the bit from the USB adapter page because thats where it says “Therefore, a direct connection to a microcontroller’s I/O lines is usually possible.”

I was hoping to hear a reply like “Yes, no problem, you can wire the USB adapter TX and RX directly to the Baby-O RX and TX. It’s safe.”

Your line of questioning is kind of equivalent to you posting that your kid just drank a gallon of milk and isn’t looking so good, but you heard milk is fine for kids; you get a reply that you probably have a problem with how much milk he drank, and you reply that you were just hoping for a reply like, “Yes, it’s safe for kids to drink milk”.

From the perspective of the 3.3V, 5V issue, there shouldn’t be a problem, and that’s the point of the passage you quoted: although the adapter is a 3.3V device, it can handle 5V. From the AVR’s perspective, it’s running at 5V but seeing signals that don’t go as high as you might like. Since its threshold for a high signal is less than what the adapter outputs, you should be okay, but you will have less margin for noise than if both devices were 5V devices. It should be “safe” in the sense that the two devices should not break (but don’t forget my comments from yesterday about one part being off violating some specs); there might be more to consider (e.g. wire lengths, noisiness of the environment) if you want “safe” to mean a robust design.

The red LED might be on from a little bit of current coming from a pull-up on the CP2102’s receive line. Does it light up the same way if you connect just ground and the adapter RX to Baby O TX (i.e. remove the adapter TX to Baby O RX connection)?

- Jan

Thanks for the reply.
Indeed the red light still glows dimly if only gnd and ( BabyO: TX -> USB2S: RX ) are connected.

Alright I have a better understanding of the pitfalls associated with 5V / 3V. I wont worry about destroying these parts.

best wishes
Mike Clemens