How to use a tristate buffer when negative voltage can appear on its output?

Question

This is a followup to Injecting a UART TX output signal to an audio input pin, without messing up the audio?

The preferred solution for TX was a tristate buffer, which makes sense, until we recognize that the buffer's output can see negative voltages (audio). The high-Z is not high-Z when there's a negative voltage, it conducts (probably across internal body diodes).

Is there a clever solution to this? I don't have dual voltage rails, and haven't found an analog switch or buffer that can handle a negative voltage on its output, unless I have a negative rail. I'd rather not add a negative voltage rail just for this, or a relay, if it can be avoided.

The "dirty" approach is to put a large-ish resistor at the output of the buffer, hopefully much larger than the output resistance of the audio source, which will minimize current flow on the negative wave, and perhaps distortion. I'm not crazy about this, as it also drops the UART TX voltage some due to the series resistor and the audio input circuit forming a divider. A workaround might be to output a 5V TX and let the system divide this to 3.3V.

Is there a clever way to solve this problem robustly?

Answer 1

The idle state of a raw MCU UART signal is high. If the intended receiver has an internal pull up resistor to either 3.3 V or 5 V, it will be sufficient to pull the line down to GND in the active TX state.

A Schottky diode in series with a NMOSFET can pull down the line to around 450 mV, which should be seen as low by the receiver, the high level is defined by the supply voltage of the receiver and not relevant here.

As long as you don't send data, the line can be fed with AC audio signals up to 20 V without much distortion. There is just the capacitive load of the diode and the not conducting MOSFET, something between 100 pF and 1 nF.

Since you didn't specify the source impedance of the audio signal, it is hard to say if this capacitance is a problem here.

If the AC signal has a very low source impedance, say below 100 ohm, you may introduce problems if you accidentially send data while the audio signal is present.

To detect a receiver, you can use an R/C low pass filter connected to the line. If you measure a DC voltage above 1 V, a receiver is connected. A bipolar AC signal would sum up to 0V behind the filter.

^{simulate this circuit – Schematic created using CircuitLab}