Microcontroller – Measuring High-Impedance Pin Characteristics

high-impedancemicrocontrollerpullupvoltage

I understand that when an IC pin is in a high-impedance state, the pin is not connected to anything internal. It will be at the voltage to whatever it is being pulled from the outside.

I am just curious as to whether anyone has measured the value of voltage when the pin is in the high-impedance state.

Suppose, for example, an IC works at 5 V. A pin is in a high-impedance state at a particular instant. This pin is pulled up to 3.3 V since this signal is interfaced with an MCU. Now, let's remove the pull-up to 3.3 V on this pin. What would be the voltage reading? Just curious to understand this scenario.

Best Answer

High impedance pins on an IC may be a varied range of voltage anywhere between the negative and positive supply rails.

Without outside interference the voltage level will be determined by what is connected to the pin circuit in the IC chip. In the case of some tri-stated output signal there are going to be OFF transistors that are present to pull this output high or low when the output is actually active. When in the off state these transistors will still have some leakage current, however small. The ratio of the leakage on the upper side and that on the lower side creates the effect of a high impedance voltage divider which ultimately determines the open circuit voltage on the open pin. Input pins can behave similarly due to leakage either due to that in the input protection diodes or to the input transistor gate leakages to the supply rails.

There are a number of external influences which can offset the open circuit voltage found on pins due to the internal leakages. These can include:

  1. Probing with an oscilloscope which can change the node impedance on the IC pin in relation to the scope probe impedance which may be in the 1M, 10M or in some cases even 100M ohms. In many cases the probe impedance may be orders of magnitude less than the impedance of the open IC pin and totally override any native leakage voltage on the open pin.
  2. Probing with a digital multimeter. See #1 above but the probe impedance may differ from that of an oscilloscope.
  3. Circuit board traces or copper pours near the open IC pin may induce current into the high impedance pin and as a result change the pin voltage. The closer the adjacent copper is to the pin the more pronounced this effect can be. In addition to that the amount of current flowing through the adjacent copper can have a proportional amount of effect on an open pin.
  4. Often open pins on ICs are connected to short traces which route out to a test point to facilitate board level testing in manufacturing. These short copper runs can intensify the effects mentioned in #3 above.
  5. Stray capacitance and including the native capacitance of the pin of the IC package and the internal circuit node capacitance from on-chip will tend to hold charge to maintain the voltage level on an open pin for a relatively long period of time due to the very high impedance of the IC pin. The voltage on said capacitance may be set at the time the IC pin was last actively driven.

It is very important to keep in mind that there can be quite a variation of leakage current and impedance of pins from IC to IC of the same part number. So be careful not to draw conclusions by looking at a few parts from a particular manufacturing lot.

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