resistor in series to micro inputs
resistor in series to micro inputs
(OP)
i have seen lot of people put like 100k resistors in series on digital inputs to a micro.
why is that? how does affect the digital signal.
why is that? how does affect the digital signal.





RE: resistor in series to micro inputs
The resistors are normally not needed on "inter-IC" signals, but are used on signals that come from outside the board. Or sometimes as level shifters when you go from a higher level system to a TTL or other lower level system.
100 k is a lot of resistance, though. It should not be used on high speed signals since the resulting filter (pin and trace capacitance) will have a cut off frequency in the 100 - 1000 kHz region. A 100 k resistor will reduce (yes, reduce, not enhance) internal EMI immunity since the input is no longer tied to Vee or Vcc as effectively as without the resistor. A paradox is that it enhances external EMI immunity and that the immunity can be further enhanced by adding capacitance between input pin and Vee.
A more normal resistor value would be 10 kohms. See also "10 kohm disease".
RE: resistor in series to micro inputs
Meant to ask this one and never got around to it... you mentioned no need for "inter-IC" signal resistors. At what point do you consider them necessary?
For example, if I wanted to connect multiple latch chips to a micro's data bus, at what point would you suggest inserting resistors to limit the initial current rush?
RE: resistor in series to micro inputs
RE: resistor in series to micro inputs
Be that as it may, I've seen arguments for and against using resistors between chips. It seems the only prudent thing to do, but I was wondering others thoughts on the matter. Maybe they're making chips stronger these days and the resistors are no longer required...
RE: resistor in series to micro inputs
Best operation is achieved with a fast risetime input signal. Adding a resistor will, as skoggsgurra said, create a low pass filter, delaying the point of switching and adding a delay of uncertain length.
Low value resistors (100 ohm - 4700 ohm) in series with inputs from the external world are a good idea as stated in earlier posts.
RE: resistor in series to micro inputs
RE: resistor in series to micro inputs
Some automated production board testers (ICT) simply measure the I/O diodes inside the chips to determine if the chip is ok. They cannot tell the difference between an output diode and the next stages input diode. Adding inter IC resistors in these lines helps the tester detect the presence of these protection diodes and therefore the assumed condition of the chip.