LMK03318: Questions on output filtering/power, STATUS pins, unused clock input pins, VCO calibration, and crystal selection

Hi Larry,

1) I think that would be okay as long as they're still connected to 1.8V. (Hao, can you confirm?)

2) You may use the information on page 9 of the datasheet to estimate how much power you could save by disabling outputs.

4) Leave PRIREF pins floating and makes sure to disable the buffer by PRIBUFSEL = 0x2.

5) I recommend calibrating the VCO.

Hi Hao,

Please help answer the remaining questions.

Regards,
Jennifer

Hello Hao,

Thanks for your help and for Jennifer's also. I have two followup questions below to make sure I understand your reply and Jennifer's reply. 

I see that Jennifer was asking for your input also on question number 1. What are your thoughts for question #1 about leaving off output power filters for unused buffers?

Also please provide me with some clarification for my question #4. The reply was that I need to disable the buffer by PRIBUFSEL = 0x2. I was under the impression that I could use the LMK03318 directly for my design without doing any register changes. I have the GPIO pins set to ROM option 45. I have the HW_SW_CNTRL pin tied high. I have the REFSEL pin set to Vim. The 25MHz crystal is connected to the secondary reference input clock pins. What is the problem with using the LMK03318 as I have the hardware pins set if I do not disable the buffer by PRIBUFSEL = 0x2.?

Regards,

Larry

Hi Hao,

Thank you for the additional followup. Just to make sure I understand for the LMK03318, I will set the REFSEL pin to the Vim voltage. Is that correct?

Regards,

Larry

Hello Hao,

I am sorry that I did not reply back to you sooner, but I was on vacation last week.

I think I need more information to understand how the REFSEL pin works when ROM Option 45 is set on the GPIO pins. In looking at Table 3 the options are based on the settings of register bits R50.1 and R50.0. When using ROM Option 45 how are bits R50.1 and R50.0 configured?

Is there a document that explains all of the register settings for the various ROM Options?

Regards,
Larry

Hello Hao,

Thank you for the explanation about REFSEL in the ROM options table and pointing me to the relevant entries in Table 3. I still do not understand the primary vs. secondary PLL reference selection. If I look at the relevant entries in Table 3, it seems to me that I should set REFSEL to Vim, as I suggested previously in this discussion. But you told me to set REFSEL low. It seems from the relevant entries in Table 3 that setting REFSEL low will cause the PLL to select the primary PLL reference. Please provide me with an explanation to relieve my confusion. 

Regards,

Larry

Hi Hao,

Please recall from the earlier parts of this conversation that for the clock reference I'm only using a 25MHz crystal connected to the SECREF_P and SECREF_N pins. I have nothing connected to the PRIREF_P and PRIREF_N pins, they are floating. 

Thanks,

Larry

Thanks Julian,

That makes sense to me about the REFSEL pin.

I would like to ask a followup question concerning the reply to my original question #5 on the topic of VCO calibration. The reply was that it is recommended, which makes sense, but I would like to have a more detailed explanation please of the consequences of not performing the VCO calibration. Is it really necessary to meet the phase noise or any of the other requirements of the TMS320C6678ACYP? If so, which ones? ls it reasonable to expect that our inital models that are run at room temperature conditions would work? Do we need it for factory volume production? Please elaborate on this topic. 

Regards,

Larry