Arduino Mega Serial Input Impedence

Hey everyone,

I'm looking to see if anyone would know a rough input impedance for Serial3 (Digital pins 14 and 15) on the Mega or how I could measure it myself.

I don't think the TX pin 14 is an issue for me, since it should be setup as an output with low impedance. My biggest concern is knowing what the input RX pin 15 would draw from a communicating device. This should be a high value, but how high would it be?

I've tried to measure this pin whenever the UART is setup for communication, but the RX pin defaults to an output of 5V where I was expecting 0V. I planned to drive the pin up with a 5V supply and measuring how much current is drawn in order to calculate the resistance. Because of this unexpected logic level, I tried dropping the pin down to ground, then measuring the current draw out from the pin (which ended up around 1mA). I don't think this is the correct method to doing this, which is why I'm asking here.

The overall end goal is to have a beaglebone communicate with the mega. I will be creating a interface between the two in order to shift the voltage levels between 3.3V and 5V. I plan to follow a schematic similar to this image. If I can measure the input impedance on both the RX pin of the mega and the RX pin on the beaglebone, I should be able to increase the resistor values to trim power loses within the interface while keeping a reliable connection and voltage drops to a minimum.

Thanks for any help I can get.

All the inputs are CMOS gate inputs, with essentially infinite input impedance.

1 megaohm. The high is from the 16u2 and the 1k between the 16u2 and the 2560.