RM46L852: Bug or misinterpretation in the value of the capacitors for the crystal

Francisco Vera

Part Number: RM46L852
Other Parts Discussed in Thread: LAUNCHXL2-RM46, TMDXRM46HDK

Hello, my name is Francisco, engineer of the R+D+i Department of the Rother Industries & Technology company.

I would want to report a bug or misinterpretation in the value of the capacitors for the crystal of the LAUNCHXL2-RM46 and TMDXRM46HDK development Kits. In the boom of the LAUNCHXL2-RM46 appears the ABLS-16.000MHZ-B4-T crystal, its manufacturer recommends capacitors of 18pF, but the Texas designers of this kit use capacitors of 33pF. Which is the reason? Is it a bug?

On the other hand, in the boom of the TMDXRM46HDK development appears the 7A-16.000MAAJ-T crystal, its manufacturer recommends capacitors of 18pF, but the Texas designers of this kit use capacitors of 10pF. Which is the reason? Is it a bug?

I had thought to use the AA-16.000MALE-T crystal (crystal dedicated for automotive application) and the capacitor values recommended by its manufactures, in other words, 12 pF. Would be correct?

Best regards,

Francisco.

over 2 years ago

+1 QJ Wang over 2 years ago

TI__Guru**** 197336 points

Francisco Vera said:
Which is the reason? Is it a bug?

C1 = C2 = (CLOAD – 2pF) * 2 = (18-2)*2=32pF where 2pF is the stray capacitance in the oscillator circuit. Stray capacitance is a function of trace lengths, PCB construction, and microcontroller pin design. CLOAD is manufacturer’s specified crystal load capacitance.

It is a bug on HDK.

Francisco Vera said:
12 pF. Would be correct?

If CLOAD=12pF, the C1=C2=(12-2)*2=20pF, assume the stray capacitance is 2pF.

0 Francisco Vera over 2 years ago in reply to QJ Wang

Prodigy 120 points

Hi QJ Wang,

I need you to be more explanatory.

Firstly, I need you to explain to me where the formula that you have used to calculate the capacitors to assign to the crystal comes from and based on what requirements must be taken into account.

Secondly, the values of the capacitors for the TMDXRM46HDK development do not comply with the formula that you have told me. Why?

Best regards,

Francisco.

+1 QJ Wang over 2 years ago in reply to Francisco Vera

TI__Guru**** 197336 points

Francisco Vera said:
Firstly, I need you to explain to me where the formula that you have used to calculate the capacitors to assign to the crystal comes from and based on what requirements must be taken into account.

https://www.ti.com/lit/an/swra372c/swra372c.pdf?ts=1683004128123

Francisco Vera said:
Secondly, the values of the capacitors for the TMDXRM46HDK development do not comply with the formula that you have told me. Why?

It is a bug on HDK. If the actual capacitance, c1*c2/(c1+c2), is too high, the crystal will oscillate more slowly. If the actual capacitance is too low, it will oscillate faster.

0 Francisco Vera over 2 years ago in reply to QJ Wang

Prodigy 120 points

Hi QJ Wang

Thanks for your reply. Thanks to the document that you have provided me, I already understand this aspect.

Best regards,

Francisco.

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RM46L852: Bug or misinterpretation in the value of the capacitors for the crystal