Battery back-up and RTC

[mark]

Hi

This is in fact a quick follow-up on Hervé's post: http://www.utasker.com/forum/index.php?topic=738.msg3228#msg3228 (I use external RTC because of higher power consumption of the lpc23xx).

Since I have also been working with the LPC2XXX RTC recently here are my remarks.

1) A decent external RTC like the DS1307 requires about 500nA battery current. This chip also has 56 bytes of battery backed up RAM in it, which can be useful for saving program states etc. so that they can be recovered after a program cycle. A 48mA lithium battery will hold data and run the clock for > 7 years.

2) The LPC2378 (for example) with internal RTC and 2k of battery backed up RAM requires about 20uA (typical), which is about 40x more than the external RTC reference but the 2k battery backed up RAM is a lot in comparison and can be accessed at fully speed when used in normal program operation. A rather larger battery is required to hold the data over long periods of time (eg. about 3.5 years with 1000mA lithium).

3) The LPC2103 (for example) with internal RTC and 12 bytes of battery backed up RAM requires about 8mA (typical), which is still 16x more than the external RTC reference, with less RAM storage space but this is easier to use since it can be accessed at full speed. A 540mA lithium battery will hold data and run the clock for a period of more that 5 years (note that a larger Lithium battery is not much more expensive that a small one but does take up more space).

Personally I find 20uA rather high for this function (eg. LPC2378) but in many cases the larger storage space and saving of external component will still be a good advantage).
I find the 8uA required by the LPC2103 fairly good since it saves an external component without having to use a much more expensive battery, achieving similar back up life (although battery space may be an issue in some designs). The fact that there are some backed up variables that can be accessed at full speed (without sending them back and forth over an I2C bus) makes it easier to use (if the space is adequate).

There are plans for a first uTasker hardware kit based on the LPC2103 - including low power design - making use of its battery backup capability and RTC [> 5 year back-up], a display, external SPI memory, external UART extension and some other things too. 25mW (from 5V supply) at 14MHz is typical operating power consumption when using low power software support - so watch out for the first "uTasker Professor"...;-)

Regards

Mark

[jezc]

Hi Mark,

We've had to use an external I2C RTC solution for our design as well - the LPC2478 has a similarly high backup current to the LPC23xx.

The newer LPC17xx and LPC13xx families (using the Cortex M3 core but the same/very similar peripherals) have much better RTC backup currents (I think it's targetted at less than 1uA for RTC plus 20 bytes of battery backed RAM for the LPC17xx).

I'll be very interested to hear more about this when you've got it ready.

Cheers,
    JezC

[mark]

Hi Jez

Thanks for the feedback - I didn't check the LPC24xx but assumed that it is similar to the LPC23X in terms of battery backup current.

The newer Cortex M3 offerings are certainly interesting with such low battery consumption requirements. I am thinking about first an LPC2103 board (I like the chip - small, low power, good price, lots of features and 3V tolerant). Then an M52100 board (similar low-end offering from Freescale - battery backup current is a more in the LPC23XX league but bit battery backed up SRAM space (16k if I am not mistaken): http://forums.freescale.com/freescale/board/message?board.id=CFCOMM&message.id=6407&query.id=49267#M6407 - not really 3V tolerant but also good price, slightly larger package, but not necessarily more GPIO, but very good peripherals with DMA etc. For very little extra price a M5220 with USB OTG is foot-print compatible...
Maybe an LPC17XX or LPC13XX would be the better way to go...??

Regards

Mark