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LMX1204: SYSREF Repeater mode and TICS Pro

Mark Guastaferro
Genius 11645 points

Part Number: LMX1204

I need a little help clarifying the use of the SYSREF Global Delay in Repeater mode. Based on my interpretation of the data sheet, I should be able to use the delay in repeater mode. The data sheet recommends that SYSREF_DELAY_BYPASS should be in Bypass mode when operating in Repeater mode, but it's not clear why. Later in the data sheet, section 6.3.6.3.2.2 discusses how to use the delay gen in Repeater mode.

Assuming I can use delays in Repeater mode by making sure the STSREF_DELAY_BYPASS is set  = 2 (engage in all modes), TICS Pro isn't clear this is correct. On the Main page, when Repeater mode is selected, the SYSREF Global Delay section goes grey as if its not usable.  I can't set the SYSREF_DELAY_BYPASS = 2 on the main page as there is only a toggle box that sets it to 0 or 1. I can set it using the User Control page or the Raw Register page, but the SYSREF global delay and individual channel IQ selection are still greyed out.  

This is a snap shot of the main page with Repeater mode and SYSREF_DELAY_BYPASS = 2. I can change the values in the grey boxes, but I'm confused why they are grey in this mode. Am I misunderstanding something about possible configurations? 

  • 0
    TI__Mastermind 34270 points

    The problem is that SYSREF_DELAY_BYPASS should have been represented as a combobox in the profile, but instead it's a checkbox. Setting SYSREF_DELAY_BYPASS to 1 in repeater mode doesn't affect anything. Setting it to 2, on the other hand, should bring all the delaygen options online - but of course, if we truncate the values by placing a checkbox where a combobox is required, and the UI update logic never sees "2"...

    In other words this is a bug. I'll pass it along to the LMX1204 profile maintainer. For now, assume that if you've set SYSREF_DELAY_BYPASS to 2, and you see the value 2 in the register map page, all the SYSREF individual delays and the interpolator remain active.

  • 0 in reply to Derek Payne
    TI__Genius 11645 points

    Hi Derek,

    Thanks that helps a lot. 

    Why does the SYSREF_DELAY_BYPASS default to Bypass mode when selecting Repeater Mode when SYSREF_DELAY_BYPASS set = 0? Is this a convenience as the expected way most users use Repeater Mode, or is there a reason to avoid using delays in Repeater Mode?

  • 0 in reply to Mark Guastaferro
    TI__Mastermind 34270 points

    The field for SYSREF_DELAY_BYPASS is in R72 – it's actually a set of test modes that we thought could be valuable for customer use. It’d be better if we split this into two SYSREF_FORCE_BYPASS and SYSREF_FORCE_DELAY bits. The default mode is “force bits off” which allows the default behavior to occur. If both bits are set to 0, the delaygen engages in SYSREF generator mode and disengages in repeater mode.

    The reason is because the interpolator divider has a specific phase offset relationship with the SYSREF divider after every SYNC event, so in generator mode it’s easy to control and predict the delay; but in repeater mode, if you aren’t counting CLKIN cycles relative to the SYSREFREQ input, it’s not always clear what the “absolute” delay is going to be from SYSREFREQ pins to SYSREFOUT pins. You can still use it, and as long as you’re aware of the phase of the interpolator divider it’s not a problem, but it’s hard to observe the phase of the interpolator divider, and we never characterized the “SYNC -> First Delayed SYSREF” timing, so it would all be experimentally discovered and adjusted.