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Lorenzo
Joined: 23 Apr 2004
Posts: 14
Location: Pescara (Italy)
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| MCLR sinks much current |
 Posted: Thu Jun 17, 2004 1:09 am |
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I connect a 47k resistor directly to MCLR pin of a 16LF648A.
The power supply is 3V but the voltage on MCLR pin is 1.8V.
With 10k the voltage is 2V.
Why? |
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Ttelmah
Guest
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| Re: MCLR sinks much current |
| Posted: Thu Jun 17, 2004 4:19 am |
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| Lorenzo wrote: | I connect a 47k resistor directly to MCLR pin of a 16LF648A.
The power supply is 3V but the voltage on MCLR pin is 1.8V.
With 10k the voltage is 2V.
Why? |
Generally, I'd say that 47K, is too high a resistance, to guarantee to pull the MCLR pin into the 'high' state. There was a problem in the past with using high value resistors on this pin (when building systems to support external programming), and on the higher voltage chips, MicroChip advised reducing the value of the resistor used here to 22K.
The 'key', is that the pin draws more current in the 'indeterminate' state, than it draws when it is actually 'high'. This is covered in the note at the bottom of table 17.4, where relating to the current drawn on the MCLR pin, the comment 'The leakage current on the MCLR pin, is strongly dependant on the applied voltage level', is made.
I have seen exactly this behaviour, where if you pull the pin up with (say) a 10K resistor, it goes nearly to the +ve supply rail, and you can then increase the resistor value, and it stays there. However if you try the larger value to begin with, the pin fails to pull through the transition level, and sits drawing more current than normal...
Best Wishes |
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Haplo
Joined: 06 Sep 2003
Posts: 659
Location: Sydney, Australia
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| Posted: Thu Jun 17, 2004 5:45 pm |
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| Any implications with having a 22k pull-up with a diode in series on the MCLR line? |
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