How can i know the Microcontroller impedence.

[lockman_akim]

im designing the loadboard for testing new microcontroller, the impedence should be maintain 50ohm whether the PCB track or component that i used. Yes for sure i can take look at the datasheet if it state the impedence. But if the impedence is not stated in the datasheet? how can i know the impedence ? what test i should do?it is possible to know the impedence of each pin of the DUT.

 

[biff44]

generally speaking, microcontrollers operate at low frequencies (say below 20 mbps data rates) and do not have impedance matched inputs/outputs. The inputs look like high impedance loads with a small capacitance across them. So there is no requirement for a controlled impedance line leading up to those pins.

If you need to run a very long line to the micrcontroller (like 5 meters), sometimes a differential line is used. In that case you use two conductors, balanced, and drive it with a line driver and receive with a line receiver. In that case, you typically use a 100 ohm differential line, with actual 100 ohm chip resistors as terminating impedances.

It is only when you start using very high data rates and special logic families that you need impedance matched traces.

 

[FvM]

The objective of your question is very unclear. Presumed, you are designing an adapter for dynamic tests, the setup has to consider the microcontroller's specified test conditions rather than pin impedances. If the test system is based on 50 ohm impedance matching, buffer amplifiers may be needed to achieve this.

 

[lockman_akim]

yes..if it low speed, impedence is not really critical thing.. but with the new product, sometime it reach 50mbps. so how can i know the impedence of the uC?

 

[FvM]

so how can i know the impedence of the uC?

There isn't such a thing. Neither for "the uC" nor individual pins. Output pins have nonlinear CV characteristics. IBIS files are a standarized description format for it, that are available for some devices from manufacturers. For a given voltage swing and load, you can derive an eqivalent output impedance from IBIS files or empirically measured CV characteristics. They can be e.g. considered for source side impedance matching.

But unless an uC has specific high speed pins, that are specified with a source or load impedance, I don't see how "pin impedance" should play a role in test adapter design. As said, you should rather refer to the specified test conditions, that are rarely based on impedance matching.

As an exception of pin types, that are specified with impedance parameters, you have e.g. on-chip USB or ethernet interfaces.

 

[lockman_akim]

The objective of your question is very unclear. Presumed, you are designing an adapter for dynamic tests, the setup has to consider the microcontroller's specified test conditions rather than pin impedances. If the test system is based on 50 ohm impedance matching, buffer amplifiers may be needed to achieve this.

actually im just worried if the Device Under Test (microcontroller) that going to be test have different impedence value then PCB. So if it different value, it will course signal reflection right? so if posible, how can i measure the DUT impedence? should i do some testing at lab or just calculate some value from the datasheet? One more thing FVM, if the impedence is not match to the PCB, how can i solve the problem? using buffer amplifiers? honestly i dont know what is buffer amplifiers and how it work..hope you can xplain..

 

[FvM]

You didn't tell about the nature of your intended tests. Generally, the situation will be quite different for pin stimulation and pin probing. You also have to consider the impedance of the connected test equipment.

As I already mentioned, I would expect the uP output pins loaded according to the manufacturers datasheet specification, which hasn't to do with impedance matching.

honestly i dont know what is buffer amplifiers and how it work

Buffer amplifier - Wikipedia, the free encyclopedia