topic Re: RTC power consumption in LPC Microcontrollers

RTC power consumption

lpcware — Wed, 15 Jun 2016 18:16:02 GMT

Content originally posted in LPCWare by jurrien on Wed Apr 29 06:52:30 MST 2015
In our new LPC1837 design we use a SuperCap (1.5F) to power the RTC.
Based on experience with our similar design, based on the LPC1767, we would expect the clock to keep running for a couple of (3-4) weeks.
When VDD powered off however, the supercap runs empty (< 1.8V) on our LPC1837 within 1 day.
This happens even without the RTC enabled.
Appearently Ibat is much larger than the 3uA stated in the documentation.
Is there a solution to this problem?

Re: RTC power consumption

lpcware — Wed, 15 Jun 2016 18:16:03 GMT

Content originally posted in LPCWare by embd02161991 on Thu Apr 30 16:09:15 MST 2015
Hi Jurrien,

There is an errata IBAT.1 for LPC18xx flash devices. Please check the problem and solution here : http://www.nxp.com/documents/errata_sheet/ES_LPC185X_3X_2X_1X_FLASH.pdf

Thanks
NXP Technical Support

Re: RTC power consumption

lpcware — Wed, 15 Jun 2016 18:16:04 GMT

Content originally posted in LPCWare by jurrien on Tue Jun 23 00:20:45 MST 2015
Thanks for your support and sorry for my late response. As I understand from the errata sheet, I have to 'pull down' two pins; the RTC_ALARM and the SAMPLE pins. I have tried this and performed our rtc experiment again, but it seems to have no impact. Just to verify that I have done the right thing, executing

LPC_CREG->CREG0 |= (3 << 6) | (3 << 12);

just before the RTC is enabled should be the work around, right? Do you have any other suggestions what could cause the battery to be drained so quickly?
Looking forward to your response,
Thanks in advance,

Jurrien

Edit: Its not completely true that it did not change; we think the time the RTC is running is now doubled, thus the the current is indeed changed dramatically. However we are using the same super cap as we have been using with a lpc1767 processor in a different project. With this setup, the RTC runs for a couple of weeks without any outside power. With our new setup however, the super cap is empty after, more or less, two day's. If we make a rough estimation, the current used by the lpc1837 is then 10x larger as the lpc1767, is this a difference we should expect?