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Are there any non-obvious considerations in using an external oscillator to drive the clock pin of the LPC210X (instead of a crystal using the internal oscillator)? |
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External Clock Source
Started by ●March 8, 2004
Reply by ●March 8, 20042004-03-08
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It capacitively couples at a low level. It is not a direct connect from the oscillator to the pin on the LPC. Regards -Bill Knight R O SoftWare On Mon, 8 Mar 2004 13:00:18 -0500, James Dabbs wrote: Are there any non-obvious considerations in using an external oscillator to drive the clock pin of the LPC210X (instead of a crystal using the internal oscillator)? Yahoo! Groups Links |
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Reply by ●March 8, 20042004-03-08
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> It capacitively couples at a low level. It is not a direct > connect from the oscillator to the pin on the LPC. So, if I need to clock the LPC from an external oscillator, what should I do? In this case, it's actually an sine waye from a UHSO, but I was planning to square it up for this chip. |
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Reply by ●March 8, 20042004-03-08
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--- In , "James Dabbs" <jdabbs@t...> wrote: > > It capacitively couples at a low level. It is not a direct > > connect from the oscillator to the pin on the LPC. > > So, if I need to clock the LPC from an external oscillator, what should > I do? In this case, it's actually an sine waye from a UHSO, but I was > planning to square it up for this chip. I use an 3.3V oscillator IC and use a resistor divider (330 Ohms from the oscillator to a common point, 75 Ohms from the common point to ground). I then run a 100pf cap from that sommon point to the X1 input of the processor. Robert |
Reply by ●March 9, 20042004-03-09
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--- In , "James Dabbs" <jdabbs@t...> wrote: > > It capacitively couples at a low level. It is not a direct > > connect from the oscillator to the pin on the LPC. > > So, if I need to clock the LPC from an external oscillator, what > should I do? In this case, it's actually an sine waye from a UHSO, > but I was planning to square it up for this chip. Check out page 39 in the LPC210x User Manual (download from Philips website). It shows you exactly how to do it. Basically, you need to put a 100pF capacitor between your oscillator and X1. The oscillator should have an amplitude of at least 200mV and no more than 1.8v. -bob |
