AM3358 CLKOUT2 - is it safe to use?

Chris Kessell

Other Parts Discussed in Thread: AM3358

Hi,

We've just completed the design of a new product based around the AM3358, and are using its CLKOUT2 pin to act as a 4MHz clock source for another IC. The intention is to configure CLKOUT2 to be driven by the LCD pixel clock (which we've set to 24MHz), dividing it by 6 to give the 4MHz output.

However, our attention has been drawn to warnings in the AM3358 data sheet (section 4.2.4) and TRM (section 8.1.6.12) discouraging the use of CLKOUT2 due to "unpredictable jitter performance".

Can anyone at TI comment on this? Is our proposed use of CLKOUT2 at 4MHz likely to cause us problems?

Thanks,

Chris

over 12 years ago

0 Biser Gatchev-XID over 12 years ago

TI__Guru**** 393215 points

Hi Chris,

It would depend on what the clock requirements are on the receive side.

0 Chris Kessell over 12 years ago in reply to Biser Gatchev-XID

Intellectual 260 points

Hi Biser,

Unfortunately the only clock specification given in the IC's datasheet is an accuracy of ±0.1%.

Do you think CLKOUT2 would comply with that?

Chris

0 Biser Gatchev-XID over 12 years ago in reply to Chris Kessell

TI__Guru**** 393215 points

I can't say, but I can refer you to this post, which discusses CLKOUT jitter performance in some detail:

http://e2e.ti.com/support/arm/sitara_arm/f/791/t/323480.aspx

0 Chris Kessell over 12 years ago in reply to Biser Gatchev-XID

Intellectual 260 points

Hi Biser,

Many thanks for the link. That relates to the CLKOUT1 output, do you know if there have been any similar analysis, measurements or advice on optimising the jitter performance of the CLKOUT2 output?

Regards,

Chris

0 Biser Gatchev-XID over 12 years ago in reply to Chris Kessell

TI__Guru**** 393215 points

Hi Chris,

Got some feedback from the factory team:

The note in the Datasheet (4.2.4) is correct. If jitter performance is a key requirement, it’s important that the CLKOUT be sourced from PER_PLL as it is the only ADPLL_LJ (low-jitter) PLL instantiation on AM335x. Unfortunately, since USB is also sourced from PER_PLL and requires a fixed 960MHz clock, it may limit the number of possible output frequencies on the CLKOUT pins.

Again, we never characterized this value so you would need to test it for suitability in your design.

0 Chris Kessell over 12 years ago in reply to Biser Gatchev-XID

Intellectual 260 points

Hi Biser,

Many thanks for the feedback. We will see if we can use the suggested PLL in our application.

Regards,

Chris

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AM3358 CLKOUT2 - is it safe to use?