Does any1 know what the purpose of the 1Mohm resistor across the crystal?
Does any1 know what the purpose of the 1Mohm resistor across the crystal?
"I think fish is nice, but then I think that rain is wet, so who am I to judge?" - Douglas Adams
http://www.tla.co.nz/xtal1.htmlOriginally Posted by Kamikaze47
From above link: "...Many microcontrollers have this resistor internally and may have different values for different frequency operations. If so, then the external resistor is not needed."
Pretty much every PIC data sheet i've looked at indicates that this resistor is internal to the PIC. (e.g. PIC18F4550 data sheet pg. 24).
So I guess all of the circuit diagrams that indicate to add this resistor externally for a PIC would be incorrect (or in the very least, redundant).
"I think fish is nice, but then I think that rain is wet, so who am I to judge?" - Douglas Adams
As far as I know, that's entirely correct.
You switch config settings (i.e. XT, HS, etc) to basically change resistors inside the PIC for the different drives levels required for external crystals.
Higher speed crystals/frequencies need more juice to switch logic levels, higher slew rates, which require more power.
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