LPC1778 Spread Spectrum Clock input?

[adamgarrison]

We are having emissions failures using the LPC1778 and I am looking at using a SSCG (Spread Spectrum clock generator) to help combat this.

Will the LPC1778 work with an SSCG, have they tried it?

If so, do they recommend a certain SSCG to use?

Reading the datasheet the Xtal1 pin can only handle max 1.95(or 1.85V) on it and most SSCG are outputting 3.3V. I can use a series cap and a cap to gnd to get the amplitude down, but wondering if this will affect he SS clock coming out of the SSCG? With 100pf in series and 100pf to GND that should give me 1.65V in.

Thank you,

Adam

[kerryzhou]

Hi Adam Garrison,

    Yes, the LPC1778 XTAL1 and XTAL2 pin max voltage is 1.95V.

    Actually, as I know, a lot of the generator tool can adjust the output clock amplitude, I have used the GRATTEN DDS function generator before, it can adjust the amplitude.

    If you add the cap to get the amplitude down, did you also check the frequency? Is it still meet your demand, you can check it with the oscilloscope, make sure the clock is meet the LPC1778 datasheet's demand.

Wish it helps you!

Have a great day,
Kerry

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[adamgarrison]

What dither was used and what is the maximum dither I can use?  In the datasheet, they say the IRC is trimmed at 1% but don’t recommend using it if you are using USB (I am guessing because it can drift to far outside clock frequency?)

[kerryzhou]

Hi Adam Garrison,

   Dither is determined by your source, IRC is the 12MHZ internal RC oscillator, normally, the external crystal should lower than 1% accuracy to keep higher accuracy, so I suggest you use the external oscillator smaller than 1%, more smaller, more preciser. I don't know why you use the external SSCG? You can refer to the demo board to use external crystal directly.

Wish it helps you!

Have a great day,
Kerry

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