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LFXT accuracy on FRAM-based experimenter boards

Peter Bigot
Expert 2505 points

I'm doing some calibration tests on the external 32768 Hz crystal on various MSP430 experimenter boards, using one of the timer peripherals sourced from ACLK, which in turn is sourced from LFXT1, capturing on a 1PPS signal from a GPS system and measuring the counts observed within one second.

On most boards (2xx, 4xx, and 5xx/6xx), if I set the crystal capacitance correctly in the clock peripheral I get within the 20ppm accuracy of the crystals provided with the board, meaning it's mostly 32768 counts per second with an occasional 32769.

However, two rev 1.1 EXP430FR5739 ("Fraunchpad") boards reliably measure 32774 Hz, or a 183 ppm error.  An experimental Wolverine chip (FR5969) in a rev 1.0 TS430RGZ48C measures 32770 or sometimes 32771, which is better but still not what I was hoping for.

The CS and CS_A peripherals don't provide a facility to set internal capacitance for LFXT1 as previous clock peripherals did.  The user's guide suggests the correct capacitors must be installed on the board itself.

Do these two boards (EXP430FR5739 and TS430RGZ48C) have the wrong crystal capacitance, or am I being unreasonable in expecting to get the same sort of watch-crystal accuracy on the FRAM series chips as I had in previous families?

  • 0
    Guru 58965 points

    I think the PCB of those boards have the pads for capacitors but you need to get the proper capacitors and solder them on the board yourself. Did you look at your boards? Are they there?

  • 0 in reply to old_cow_yellow
    Expert 2505 points

    old_cow_yellow said:
    I think the PCB of those boards have the pads for capacitors but you need to get the proper capacitors and solder them on the board yourself. Did you look at your boards? Are they there?

    No, they aren't.  That'd probably be it; the schematic for the Fraunchpad does have 12pF capacitors specified but they're DNP.  Came with a crystal, but not with the capacitors necessary to make the crystal work properly.  As a software guy I'm not going to try to source or install SMD capacitors of that size; I could barely get the SMD crystal on there properly.

    The Launchpad doesn't have capacitors populated either but BC2 allows me to set the MCU so they aren't necessary, and I'd mistakenly assumed the Fraunchpad was intended to be a comparably complete evaluation kit.  (I know the RGZ isn't: the thing has only one LED populated and no buttons, so it's pretty nearly useless for my needs.)

  • 0 in reply to Peter Bigot
    Guru 227245 points

    Well, the FR devices do not have internal capacitors. On the LaunchPad, it is necessary to configure the internal capacitors or add external ones. It should have been obvious that there are no internal capacitors (no controls in the clock system) on the FR when trying to configure them too.

    I agree that soldring small SMD capacitors (or even buying them) is a bit difficult for a normal user. However, the MSP experimenter boards aren't meant as complete products for end users. But as a starting point for hardware and software development engineers. And those usually have no problems with SMD capacitors. Or at least shouldn't.

  • 0 in reply to Jens-Michael Gross
    Expert 2505 points

    The Launchpad comes with a user-installable crystal and works correctly when it's installed.  The Fraunchpad comes with a user-installable crystal and doesn't work correctly when it's installed.  An inconsistency that naturally led me to assume the capacitors were already on board, or were otherwise unnecessary, because if they were necessary they should have been provided along with the crystal, no?

    At any rate, question answered and moving on.

  • 0 in reply to Peter Bigot
    Guru 227245 points

    Peter Bigot said:
    An inconsistency that naturally led me to assume the capacitors were already on board, or were otherwise unnecessary, because if they were necessary they should have been provided along with the crystal, no?

    Well, I have about 5000 ceramic capacitors of any standard capacitance at hand but no crystals. Caps are standard parts, assumed to be present in every lab.
    And comparing LaunchPad and FraunchPad is like comparing different generations of mspgcc :)

    However, I agree that there should be some additional note, especially since the exact capacitance needed for the provided crystal is unknown but vital for correct crystal operation. Now it's not sure either that any the four selectable capacitances on LaunchPad's G2 MSPs is suitable for the provided crystal at all.
    Something to be added to the documentation...

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