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GPIO Sinking Resistance

edwardsbobg

I am powering an LED / current limiting resistor by sinking current into an IO pin. Where can I find the resistance of the IOs when sinking current?

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  • 0

    Hey I'm a team member of edwardsbobg and wanted to followup on this question.  We're running an RGB LED and sinking the current into the nrf52832 using 100, 470, and 100 Ohm resistors respectively.  For simplicity let's just look at the red - it's powered with 3.3V, drops 1.92V over the LED, then drops 0.517V over the 100 Ohm resistor.  Thus the voltage at the pin is 0.863V.  The voltage drop over the resistor calculates to a current of 5mA.  

    The 0.863V was surprising to me because I figured the GPIO to have close to zero resistance and thus the voltage would be close to 0V.  But this voltage is not negligible, which to me suggests I need to factor in this voltage when calculating the external resistor value.  Do people know why this 0.863V would be higher than 0.4V?

  • +1 in reply to HertzSoGood

    you don't say what drive strength you are using on the GPIO which is fairly important. If it's standard drive then it's as I wrote above, not GPIO resistance but current limitation, in fact 5mA is higher than the max in the spec sheet for SD. If it's HD then you're just below the minimum in the spec sheet but still possible. So again the likely reason for the voltage being higher than 0.4V is because the pins have current limiters. The only way a current limiter can work on a pin driven in the way you are driving them is to 'raise' the low voltage to the point the current falls to the clamped current limit. 

  • 0 in reply to RK

    oh and one other calculation you can do - check the spec sheet for the current at which the max output for a 'low' is. I seem to remember they spec it at 0.5mA but I'm not sure. You can work that back to the max static resistance the pin could have. Personally I've always had lows be very close to zero on all the Nordic products I've worked with, but it is within spec to have a static resistance on the pin. If you really want to know, put in a higher value resistor and short out the LED and see what measurements you get at a low current like 0.5mA, that will tell you if the GPIO does have noticeable static resistance. 

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  • 0 in reply to RK

    oh and one other calculation you can do - check the spec sheet for the current at which the max output for a 'low' is. I seem to remember they spec it at 0.5mA but I'm not sure. You can work that back to the max static resistance the pin could have. Personally I've always had lows be very close to zero on all the Nordic products I've worked with, but it is within spec to have a static resistance on the pin. If you really want to know, put in a higher value resistor and short out the LED and see what measurements you get at a low current like 0.5mA, that will tell you if the GPIO does have noticeable static resistance. 

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