MSP432P4111: RTC on Launchpad has low precision

I tried earlier to compensate with coefficients, but had had bad results. I need to dig further to find out: is it a LF crystal problem or RTC_C module problem. By the way, this is an XMS version of the chip installed on Launchpad, so maybe this cannot be reproduced in the final silicon.

Alexey

The code is representing RTC settings only and not actual program execution. RTC readings is actually called by a timer.

RTC interrupt:

void RTC_C_IRQHandler(void)
{
    RTC_C->CTL0 = (RTC_C->CTL0 & ~(RTC_C_CTL0_KEY_MASK |  RTC_C_CTL0_RDYIFG |  RTC_C_CTL0_TEVIFG)) | RTC_C_KEY;
    RTC_C->CTL0 = RTC_C->CTL0 & ~(RTC_C_CTL0_KEY_MASK);
}

RTC config:

PJ->SEL0 |= BIT0 | BIT1;
CS->CTL2 |= CS_CTL2_LFXT_EN | CS_CTL2_LFXTDRIVE_0; // LFXT on

CS->CTL1 &= ~CS_CTL1_SELA__REFOCLK; // use LFXT

RTC_C->CTL0 = RTC_C_KEY | RTC_C_CTL0_RDYIE | RTC_C_CTL0_TEVIE;
RTC_C->CTL13 = RTC_C_CTL13_HOLD | RTC_C_CTL13_MODE | RTC_C_CTL13_BCD | RTC_C_CTL13_TEV_0;
RTC_C->CTL13 = RTC_C->CTL13 & ~(RTC_C_CTL13_HOLD);
RTC_C->CTL0 = RTC_C->CTL0 & ~(RTC_C_CTL0_KEY_MASK);

You may want to think about if that's indeed your problem. Those crystals have very low pullability.

I would look into why a 9pf crystal requires 22pf to run on spec.

The situation is still unknown due to RTC/crystal gives good accuracy (~2ppm) for as long as 14 hours and then is loosing 5 secs in 1 hour.

Another crystal - RTC is loosing 5 sec/hour. Looks like something happen in the MSP432P4111 chip.

Ok, all works fine at least down to 1.61V with this crystal drive settings:
CS->CTL2 |= CS_CTL2_LFXT_EN | CS_CTL2_LFXTDRIVE_3;

Regards,
Alexey

I didn't view the oscillation through the oscilloscope, but such problem persist with different verified (on other chipset) crystal. Practically, in specified voltage range the RTC is loosing ~5 sec/hour. In other voltages crystal works exceptionally precise (just in sync with compensated crystal after 96 hours).

Amit,

I was interested in electrical characteristics of this chip in specific ultra-low power conditions. Right now part of the project is "growing" based on MSP432, but in the end I plan to change the chip to more integrated solution like CC1352. For now CC1352 is not suitable for that by many aspects, but I hope all specific requirements will be fulfilled within a year.

But for now, look what CC1352 official issue have: "Engineering samples revision A (rev. 1.0) of CC13x2 devices do not work with SmartRF Studio. This can be seen when starting Packet TX. It will fail with the command status = 0xbad0".

I understand, that there should be a "window" to change characteristics of the chip to bring new capabilities, but this stops me from buying the Launchpad, as such capabilities/specifications/characterizations they don't have for now. But I declared such requirements early and I keep in touch with TI's roadmap and behave accordingly. :)

Alex

Looks like the problem disappeared after some week of chip power-on in these conditions.