how to calculate the input and out put impedance for a micro controller , for the purpose of modeling in LT spice ,
for example RX610 family from Renesas
For example the lumped characteristics can be modeled for a trace , if i know the o/p impedance of the controller and hence the rigging effect can be simulated and debugged accordingly
The input impedance could likely be assumed to be very high, >1Mohm. The output impedance could be estimated by dividing the supply voltage by the maximum drive current of the I/O. However, if you are intending to drive a real transmission line of some sort, an external buffer would probably be best.
The output impedance of a uC GPIO (like any general-purpose digital gate) is not necessarily well-designed for driving transmission lines, and the output impedance is generally not specified. They are designed to be used in circuits where all the interconnect is short enough to be considered lumped.
If you want to simulate the worst-case ringing, assume a 0 ohm output.
If you want to design a compensation circuit (say with a series resistor termination) you may have to design it experimentally. Build the circuit and adjust the resistor value until the ringing is reduced sufficiently (hopefully without degrading the transition time or logic levels too much).
The output impedance is not constant. It is low when the output is sitting at the rail and higher in between (in the middle it is more like two opposing constant current sources).
To reduce ringing, series 33 ohms on a 4-layer board with ground planes and 10 mil traces is about right, but you can experiment.
If you need a well-matched driver for long controlled impedance lines I would suggest drivers and receivers designed for the task, and with proper termination resistors .
