Author Topic: IC digital output pins and voltage divider?  (Read 1155 times)

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Offline ApokalypzTopic starter

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IC digital output pins and voltage divider?
« on: September 12, 2017, 05:47:36 am »
    This may be a stupid question, so intelligence be warned. I've just never really understood how integrated circuits like microcontrollers can drive LEDs without the drawbacks of the LEDs being in parallel. How exactly can each output pin source or sink current and not affect the other pins?

    My knowledge about how voltage dividers work makes me think the load on one pin would greatly affect the other pins.
 

Online Ice-Tea

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Re: IC digital output pins and voltage divider?
« Reply #1 on: September 12, 2017, 06:32:48 am »
Consider the outputs of ICs as being in parallel. A power supply rail -> some resistance -> a switch -> your LED. And in parallel for all IOs.
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Offline Ian.M

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Re: IC digital output pins and voltage divider?
« Reply #2 on: September 12, 2017, 06:49:11 am »
In general, when driving a LED from a digital output, one should *ALWAYS* use a series resistor so the LEDs are never paralleled.   The only exceptions to the need for a series resistor are when the logic supply voltage is only fractionally more than the LED Vf, and the output resistance of the logic is sufficient to limit the LED current, or special purpose chips that have a constant current output characteristic.  In both cases parallel LEDs on the same output would be problematic.
 

Online Ice-Tea

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Re: IC digital output pins and voltage divider?
« Reply #3 on: September 12, 2017, 06:54:36 am »
^
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This. I didn't read your question careful enough.

Offline danadak

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Re: IC digital output pins and voltage divider?
« Reply #4 on: September 12, 2017, 10:31:44 am »
There are two current flows one has to worry about for UPs driving
loads. The individual pin limits/capability, and the sum of all loads
affecting the internal power rail of the UP. The latter, in most UP's,
is speced, usually a total for a port or total for chip or both.


Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 


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