External Quartz Crystal for Piccolo TMS320F28069

Masoud Zamani

Other Parts Discussed in Thread: TMS320F28069

Hello,

I would like to use External Quartz Crystal for clock generation of my design using TMS320F28069.

However, as shown in the following picture, TI recommends 15 pF for each capacitor (when 10 MHz clock is required).

On the other hand, in the " Hardware Design Guidelines for TMS320F28xx and TMS320F28xxx DSCs" it is mentioned C1 and C2 are calculated as Cload = (C1XC2)/C1+C2

The Crystal I am using has 20 pF load capacitance (i.e. C(Load)=20), therefore using the above equation I should use C1 = C2 = 40 pF.

I will appreciate if you let me know shall I use 40 pF for C1 and C2, or I should use the datasheet recommended value for these two capacitors which is 15 pF.

Thanks in advance for any help.

Best regards,

Masoud.

over 12 years ago

0 Brett Larimore over 12 years ago

TI__Guru 57255 points

Masoud,

Table 2-14 in the datasheet is an estimate. In reality, the proper choice of load capacitors is dependent on the load capacitance requirement of the crystal you choose as the "Hardware Design Guideline" appnote suggests. As a result, based on your crystal I would estimate C1 and C2 needs to be approximately between 32-36pF depending on layout (this assumes the parasitic trace capacitance is 4pF to 2pF respectively).

In general, TI recommends that customers have the resonator/crystal vendor characterize the operation of their device with the microcontroller. The resonator/crystal vendor has the equipment and expertise to tune the tank circuit. The vendor can also advise the customer regarding the proper tank component values that will ensure start-up and stability over the entire operating range.

Thank you,
Brett

0 Masoud Zamani over 12 years ago in reply to Brett Larimore

Intellectual 830 points

Dear Brett,

Thanks a lot for your comprehensive reply.

best regards.

Masoud.

0 Angela Brock over 9 years ago in reply to Brett Larimore

Intellectual 480 points

Hi, I too am designing with a 10MHz external crystal. My crystal has C(load) of 18pF so my C1=C2 should =36pF? Or slightly less than that? Also, should I still follow the recommended R value for 10MHz of 470 Ohm?

Thanks,

Angela

0 Adam Dunhoft over 9 years ago in reply to Angela Brock

TI__Expert 7885 points

Hello Angela,

You can use the following to calculate CL1 and CL2:

Also, I would recommend adding the footprint for the resistor in your design and determining via experimentation if it will be required. See note 3 under figure 6-12 in the datasheet.

Best Regards,

Adam Dunhoft

0 Angela Brock over 9 years ago in reply to Adam Dunhoft

Intellectual 480 points

Thanks Adam, but isn't that 2*15 or 30pF?

0 Adam Dunhoft over 9 years ago in reply to Angela Brock

TI__Expert 7885 points

Angela,

You are correct. I forgot to multiply by two. :)

CL1=CL2=15pF will give you a CL of 10.5pF. For CL=18pF, CL1=CL2=30pF.

Best Regards,
Adam Dunhoft

C2000™︎ microcontrollers

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External Quartz Crystal for Piccolo TMS320F28069