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power consumption query

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cf_corp

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Hi,
Just a quick query regarding power consumption and microcntrollers.
If i connect a 5v output of a micro controller to an input of the same microcontroller using a wire with virtually no resistance with a 100k resistor in series and no load, what will the power consumption be? thanks
 
I may not understand.

Output ----100k----Input ?

Power Consumption:
A unloaded output will have (almost) no power consumption.
A input (pulled up or down but not at 1/2) will consume no power. Maybe 10uA of current at the most.
A input that is not high or low but in the center will pull power internally.
 
thats correct--- output---100k--- input, When output is made high its 5vdc, how would one calculate the miniscule current? thanks for your quick reply
 
What Ron said, unless you enable the "weak pullup" that a lot of PICs, etc have.

The 100K is a red herring. It dissipates no power because it has no voltage across it. If the output is high, the input is high, and similarly low > low.

Now if you do something externally to the input end of the 100K resistor, such as switching it to Vdd or Vss, then the resistor will draw current...
 
thats correct--- output---100k--- input, When output is made high its 5vdc, how would one calculate the miniscule current? thanks for your quick reply

The only current that might flow is listed on the data sheet as input/output pin leakage current, usually ~1uA for modern CMOS.
 
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Its a PSoC 1 device, cy8c24966 24pxi, Basically i have a cable with a resistance of around 60ohms which is connected from the output and is high at 5v and the input is connected to it and receives the high signal, all it is is that if the wire were to be cut then the microcontrollers input would see no signal on the input side and light up an led connected to another port.
 
In order to detect the missing connection, you would need a Pull-down resistor at the input. You can not reliably detect the state of a "floating" input.

If the connection is intact, the output sources current that flows to ground through the pull-down resistor. If the connection is broken, the pull-down resistor pulls the input to near zero volts, and no current flows anywhere...
 
I understand, I have this system working on the PSoC 1 prototyping board, in the psoc designer it gives the option to select drive modes, i have the input set to pulldown, it may have its own integrated system. as this system currently works very reliably.I know in the past when i used an msp430 i had to use pull down resistors
 
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On the driven output, disable both pull-up and pull-down. Write a permanent high to it. On the receiving input, enable the pull-down. The input will read as a high if the external connection is intact. If the external connection is lost, the pin will be pulled to a low by the pull-down.

While the connection is intact, the power dissipation will be approx (Vdd*Vdd)/Rpulldown. If that is too high for you, don't use the internal ~ 5K pull-down, and substitute your own external pull-down resistor of ~100K.
 
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You can reduce the dissipation in the pull down resistor by setting the output pin low most of the time. Set it high just before testing the input pin. Then set it low again.

If the input follows the state of the output, you have good continuity. If it doesn't then..................
 
You can reduce the dissipation in the pull down resistor by setting the output pin low most of the time. Set it high just before testing the input pin. Then set it low again.
.

You just invented PWM ;)
 
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