TMS320F28335-Q1: External crystal interface circuit related

Hello Krishna,

Series dampening resistor (RD) is required only to ensure that the crystal is not overdriven to save the crystal from being damaged or degraded in the long term.  To determine if RD is required, need to infer what the power is being delivered to the crystal and this can be in the form of ESR*(I_RMS)^2.  To determine the RMS current flowing through the crystal, you can measure the differential voltage across the 1Kohm resistor that is already in series with the crystal using a differential active probe (active probes have very low parasitic impedance and would have no impact to the overall impedance in the crystal circuit, normal scope probes will not do).  Waveform across the resistor is sinusoid so once the peak differential voltage is determined across the resistor, you can derive IRMS by simply dividing the resistance with the quotient of the term 1Kohm/(Vpeak*0.707).  You can then calculate the power absorbed by the crystal using the equation ESR*(I_RMS)^2.  If the result exceeds the drive level (DL) of the crystal per specification, you need to adjust the 1K resistor.  You can also lower the resistance value if the power is too low in order to ensure proper start up at all times.

Typically, this characterization is performed with the crystal vendor as they have the equipment to fully characterize the crystal with your board but if you have access to differential active probes, you can follow the steps described above.

Regards,

Joseph

Dear Joseph,

I will do that measurement. However at present my board is working since 3 months. NO issue. But i came to know (during derating analysis), Series Damping resistor shall be placed on X2 line. That's what i asked below queries. Please reply to my 4 queries.

My 4 queries: 

1) Whether i have to place series damping resistor 1kohm between "X1 pin & crystal" or "X2 pin & crystal" for this TI micro controller?

2) If it has to be placed on X2 pin side, then how its working as i placed on X1 pin side?

3) Whether series damping resistor is already there in TI Micro controller inside?

4) What is the Maximum Drive current from TI Micro controller side. How much current it will pass on X1 & X2 lines?

Hi Krishna,

Krishna Kishore Edara said:

) Whether i have to place series damping resistor 1kohm between "X1 pin & crystal" or "X2 pin & crystal" for this TI micro controller?

JC: Need to place the series damping resistance to X2 (XTAL out pin).  X1 side of the Pierce Crystal Oscillator circuit is a high impedance path.  Adding the series resistance to X1 will not be effective in dampening the DL to the crystal.

Krishna Kishore Edara said:

2) If it has to be placed on X2 pin side, then how its working as i placed on X1 pin side?

JC: See above.  The series resistance you have right now will not be effective in dampening the power to the crystal.

Krishna Kishore Edara said:

3) Whether series damping resistor is already there in TI Micro controller inside?

JC: No, there is no series dampening resistor internal to the microcontroller

Krishna Kishore Edara said:

4) What is the Maximum Drive current from TI Micro controller side. How much current it will pass on X1 & X2 lines?

JC: The microcontroller is capable to drive up to 1mW power to the external crystal

Regards,

Joseph

Dear Joseph,

Thank u so much. I got clarity. i have one more query. As i mentioned above, 

Crystal details & My circuit what i implemented: 

a. Max Drive level is 100uW, 

b. CL=12pF, 

c. C1, C2=18pF by considering 3pF stray capacitance.

d. I want to consider Derating stress factor: Power consumption by crystal <= 25%. So i should limit to 25uW by placing series Damping resistor on X2 line.

e. Operating temperature of Micro controller & Crystal in my design: -40 to 125degC.

Query 1: So, how much Series Damping resistor is good to limit crystal power consumption to Max 25uW? (22ohms to 100ohms - which is best value by considering my equipment Operating temperature -40 to 71degC)?

Hi Krishna,

Please see my response earlier on how to determine power delivered to external crystal:

Joseph Casuga said:

Series dampening resistor (RD) is required only to ensure that the crystal is not overdriven to save the crystal from being damaged or degraded in the long term.  To determine if RD is required, need to infer what the power is being delivered to the crystal and this can be in the form of ESR*(I_RMS)^2.  To determine the RMS current flowing through the crystal, you can measure the differential voltage across the 1Kohm resistor that is already in series with the crystal using a differential active probe (active probes have very low parasitic impedance and would have no impact to the overall impedance in the crystal circuit, normal scope probes will not do).  Waveform across the resistor is sinusoid so once the peak differential voltage is determined across the resistor, you can derive IRMS by simply dividing the resistance with the quotient of the term 1Kohm/(Vpeak*0.707).  You can then calculate the power absorbed by the crystal using the equation ESR*(I_RMS)^2.  If the result exceeds the drive level (DL) of the crystal per specification, you need to adjust the 1K resistor.  You can also lower the resistance value if the power is too low in order to ensure proper start up at all times.

Regards,

Joseph

Dear Joseph,

I understood what you told & i gone through literature to calculate Series Damping Resistor.

As i told in all above mails, resistor i placed on X1 line not on X2 line. You clarified this won't work for reducing Crystal Driving level, because on X1 line is high impedance line. 

What was existing PCB with me is Engineering unit. Now i am going to build final PCB with this update (Series damping resistor on X2 line).

I already worked out theoretically to calculate series resistor. for mentioned crystal details mentioned in my prev reply. Its coming around 470 - 670ohms.

But this resistor (470-670ohms) and CL1 (16-18pF) act like low pass filter and that cut off frequency will come around 2MHz. So its reducing X2 output at crystal pin to half of voltage.

So that is the reason, i am requesting TI team to suggest what value of series resistor is good (because we don't have clarity on inside Crystal oscillator of TI Micro controller. So, please suggest 

Hi Krishna,

With the 470 to 670ohm series resistor, you are observing a 2MHz shift from the expected crystal output of 22.1184MHz, bringing it down to 20.1184MHz?  This is a big shift.  Seems like the load capacitances and other components (series resistances) are modifying the resonant frequency of the intended crystal.  If you remove the dampening resistor or any series resistor that is connected to the crystal in your board, does the frequency go back up to target of 22.1184MHz?

Regards,

Joseph

Hi Joseph,

Sorry for the delay. It will work fine without Series Damping Resistor. But Drive level will be more. Thats what i raised this Tech support.

Here one clarification: Frequency is not shifting to 20MHz from 22.1184MHz. Series Damping resistor (470- 670ohms) & CL1 (16-18pF) forming like Low pass filter which Cut off frequency is around 2MHz. So 22.1184MHz amplitude will attenuate and it won't be 22.1184MHz. 

So, please suggest proper Series Damping resistor.

Hi Krishna,

Ok, understood.  To get to a proper size of the dampening resistor, need to measure the peak voltage across different Rd values so we can perform calculations for RMS current and how much power being dissipated by the crystal.  Looks like you already have Rd of ~470 ohms.  Can you provide me the peak voltage across the 470 ohm resistor.  Can you also connect an Rd of 1kohms and provide me the peak voltage measured across the resistor?  We should be able to determine power consumed by the XTAL with the 2 different Rd values and fine tune it further for your DL target.

Thanks,

Joseph

Hello Krishna,

I will be out of the office for a couple of days, but if you follow the steps in calculating the RMS current and crystal power dissipation, you should be able to derive Rd that should sit below the max DL of the crystal you are planning to use.  I will respond to your queries when i get back.

Regards,

Joseph