Re-Using Crystal Frequency of HCS08 on another board component

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Re-Using Crystal Frequency of HCS08 on another board component

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Thunder
Contributor III

Hi, I'm using the HCS08LG32 with an external 4.9152MHz xtal.  Another component on the board is the ADS1241 (24-bit A/D converter), which also uses a 4.9152MHz crystal.  Is it possible to use the xtal on the LG32 and port the frequency somehow into the ADS1421 ?  My goal here is to eliminate the cost/space for one of the crystals.  If I tried this, would I need to use a buffer ?

Thanks.

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bigmac
Specialist III

Hello,

 

One solution is to use a packaged oscillator, and drive both devices with the output from this.  However, if you wish to retain the use of a simple crystal for other reasons, including cost, the following may be a possibility.

 

It appears that the ADS1241 may have internal biasing arrangements, since an external resistanc is not requird between the Xin and Xout pins.  You can check this by measuring the DC voltage at both pins, without a crystal connected - the pins should be biased in the region of Vdd/2 level.

 

Assuming that internal biasing is present, the idea is to connect the crystal to the MCU in the normal fashion, and then provide capacitive coupling (say 47p or 100p) between the XTAL pin of the MCU and the Xin pin of the ADC device.  The AC coupling is necessary because the voltage swing at the XTAL pin may be less than the full logic swing.  When determining the capacitor value at the XTAL pin, you may need to take into account the input capacitance at the Xin pin, and any additional stray capacitance for the interconnection.  Needless to say, the two devices should be in relatively close proximity to each other to minimize the stray capacitance.

 

With this method, the HGO bit in the ICSC2 register should be set for high gain mode.

 

Regards,

Mac

 

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Thunder
Contributor III

Thanks,  I'll give it a try.

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