# Input/output impedance of microcontroller

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

• Isn't this information supposed to be in the datasheet? Commented Mar 22, 2016 at 15:54
• according to my knowledge , there is no direct information in most of the data sheets Commented Mar 23, 2016 at 9:49

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.

• thanks for the answer , the o/p impedance can be calculated as per your answer , but is there some evidence for the i/p impedance can some hints be taken from the datasheet somewhere ? Commented Mar 23, 2016 at 9:43
• Your estimation of output impedance would be way off, the error will probably be more that 100%. Simulation using such values won't make any sense. Commented Mar 23, 2016 at 9:55
• dimtri , then what will be the correct way to do so? Commented Mar 23, 2016 at 19:06
• Yes @DmitryGrigoryev, I'm curious as well. Your SE profile indicates that you should probably know better than me. Commented Mar 23, 2016 at 19:07

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).

• yes, i am actually not driving a transmission line :P , its the traces which will act as transmission lines , is my aim of simulation . Commented Mar 23, 2016 at 9:48

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 .