ST semi RTC M41T94

My question is why would you need such a chip and not implement the clock function in your micro? Standby current of 1.4mA and active 2mA is not exactly low power either.

Software RTCs as you are suggesting does not keep accurate time in the long run.

After a few weeks the time is off by a few seconds. Months or years, its off by minutes.

Standby current is not the only consideration.

donald

PS: Using the DS1307, I found that you can not read the chip without a battery connected or VBat grounded. The data sheet was not real clear about this, but programming a PIC bit-bang function did not work till I connected a battery.

Using the same watch crystal as the chip, you can get same accuracy.

If you know how much your timing cycle is off, you can adjust it.

Mainly

1) it can be trimmed to 5ppm accuracy, the rest of the system takes 2A peak for the display. There is a possibility of jamming the RTC to a GPS every couple of hours 2) It is SPI protocol, I just hate I2C 3) Not made by Maxim

martin

Only if you have better crystals. Those I am looking at are 20 to

25ppm.

I would consider such a chip if it's standby current is much less than the active current of my target uC of 0.9mA. Otherwise, there is no point in adding the chip.

Then a RC timing loop is good enough.

Have a squint at AN934 ftom ST, it means I can use a cheap xtal, and it can be auto calibrated by the 1PPS from a GPS when I make the boards up

You left out IMHO :) and that would mean a pot in the circuit, nope

martin

With the GPS PPS, there is absolutely no reason to have external clock chip. I guess you are operating outside bean counters (cost managers).

Depending on the uC. Some don't need external components at all, with internal clocking. Others (my current target) require one R and one C for clocking.

Sadly, I'm the bean counter.

This is a portable thing, GPS will only be used to calbrate once, after a 24 hour burn in and not fitted on the final unit, which as you say, would negate the need for a RTC.

martin

... snip ...

Hah. Now we know your opinion is worthless. Try it.

--
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: 
            Try the download section.

Sure, I withdraw my opinion then. I got the wrong impression that GPS is available for synchronizations every couple of hours, which is not the case after the OP clarify it. My opinion is only as good as known fact at the time.

FWIW this is what I'm working on

martin

NXP have a couple of SPI RTC's, but they do not seem to be widely stocked yet. The PCF2123 sounds interesting, but it is now 3 months since their press release, and still no data ?!

-jg