# GPIO pin input impedence?

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#### muzzamil8

##### Newbie level 6
Normally what is the input impedance on any general purpose i/o pin of the microcontroller?

#### alexan_e

Assuming no internal pullup the input is at least a few hundred Kohms.
The input current is described in the datasheet so you can download a few models and check the datasheet.

Alex

#### muzzamil8

##### Newbie level 6
So If I add 27ohms series resistor to the GPIO pin and this pin is input, there will be no sink current problem? Actually this signal is high frequency coming from external debugger into the microcontroller. so if I calculate current, is it correct formula: I = 5 volts/(27 + few hundred kilo ohms)?

#### alexan_e

yes, if the pin is set as input.

#### muzzamil8

##### Newbie level 6
ANd normally what is the output impedence of the pin?

#### alexan_e

Can we talk about the specific mcu?
Which one are you using?

#### muzzamil8

##### Newbie level 6
I am using LM3S6965 TI microcontroller. It is not clearly mentioned about input and output impedence. Actually I have to interface JTAG signals TDI,TCK,TMS,TDO signals. I am thinking of adding 27ohms series resistors for signal integrity. So i am confused about current sinking and sourcing. TDI,TMS and TCK will sink current as they are input pins and TDO will source current as it is output pin. So thats why I am asking about input and output impedance.

#### alexan_e

yes the datasheet is not very clear, for output it gives the current specification as 2/4/8ma but there is no graph of the voltage vs current, it only says that the high output specification is for 2.4v and for low output 0.4v , you can calculate the impedance based on the voltage drop for the given current.

#### FvM

##### Super Moderator
Staff member
The Stellaris M3 JTAG inputs have pull-ups with a specified resistance range TDO is a 2mA drive strength output. It don't see a problem with 27 ohm series resistors it also doesn't change the signal speed much as the output impedance will be considerably higher.

muzzamil8

Points: 2