LMH1983 cutting TOF rate in half

Dave Winder15947

Other Parts Discussed in Thread: LMH1983

I am seeing that the TOF2 and TOF3 frame rates are cut in half when I use auto align. For example, when programming PLL3 fro 23.98 Hz output, with reg 0x13 set to 0x00, I get 12 Hz TOF3 rate. If I program reg 0x13 to 0x20 i get the proper 23.98. Is this a known issue? I thought i read elsewhere on here that Always Align could lead to increased jitter.

TIA Dave

over 9 years ago

0 Arvind Sridhar over 9 years ago

TI__Mastermind 24205 points

Hi Dave,

The apps team responsible for this device has been contacted and will reply shortly.

Regards

Arvind Sridhar

0 Nasser Mohammadi over 9 years ago

TI__Guru* 88905 points

Hi Dave,

Please make sure you initiate TOF3 INIT after you set reg 0x13 to either 0x00 or 0x20. I would expect TOF3 to be at 23.98 in bother cases. If you see otherwise, please send me your register dump so we can review and look deeper into this.

Regards,,nasser

0 Dave Winder15947 over 9 years ago in reply to Nasser Mohammadi

Intellectual 485 points

Hi Nasser

OK, I set the INIT bit and that gives me full speed in Auto Align mode.

Next question is: what is the criteria to choose between auto align and always align? Under what conditions to I select one versus the other?

Thanks

0 Nasser Mohammadi over 9 years ago in reply to Dave Winder15947

TI__Guru* 88905 points

Hi Dave,

I believe Always Align is more likely to have more output jitter on the output vs Align when mis-align. This all depends on how much jitter you have on your HSync signal.

Regards,,nasser

0 Dave Winder15947 over 9 years ago in reply to Nasser Mohammadi

Intellectual 485 points

Hi Nasser

Thanks for that.

New Question: when trying to generate a 23.9 frame timing while receiving a 29.9 reference, the Fout3 signal is half the expected rate. Is the LMH1983 capable of operating in this mode? If so, what is the correct method to achieve this?

0 Nasser Mohammadi over 9 years ago in reply to Dave Winder15947

TI__Guru* 88905 points

Hi Dave,

TOF3 would go in and out of alignment and this is as expected. In this case, 27MHz PLL1 and PLL3 are genlocked to the incoming Hsync.

Regards,,nasser

0 Dave Winder15947 over 9 years ago in reply to Nasser Mohammadi

Intellectual 485 points

Hi Nasser

What about in Never Align? We tried that and still it is half rate. It seems the output rate must match the reference rate always. yes?

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LMH1983 cutting TOF rate in half