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ST semi RTC M41T94

Started by Martin Griffith September 8, 2008
I'm waiting for a couple of M41T94 real time clock chips to arrive, so
I'm scribbling some 8052 C code in advance. It's an SPI ic.

Any gotchas on this RTC?
Thanks

martin
On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote:
> I'm waiting for a couple of M41T94 real time clock chips to arrive, so > I'm scribbling some 8052 C code in advance. It's an SPI ic. >
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.
linnix wrote:
> On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so >> I'm scribbling some 8052 C code in advance. It's an SPI ic. >> > > 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.
On Sep 9, 9:26 am, donald <don...@notinmyinbox.com> wrote:
> linnix wrote: > > On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: > >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so > >> I'm scribbling some 8052 C code in advance. It's an SPI ic. > > > 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. >
Using the same watch crystal as the chip, you can get same accuracy.
> After a few weeks the time is off by a few seconds.
If you know how much your timing cycle is off, you can adjust it.
> 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.
On Tue, 9 Sep 2008 09:08:12 -0700 (PDT), in comp.arch.embedded linnix
<me@linnix.info-for.us> wrote:

>On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so >> I'm scribbling some 8052 C code in advance. It's an SPI ic. >> > >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.
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
On Sep 9, 10:33 am, Martin Griffith <mart_in_med...@yah00.es> wrote:
> On Tue, 9 Sep 2008 09:08:12 -0700 (PDT), in comp.arch.embedded linnix > > <m...@linnix.info-for.us> wrote: > >On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: > >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so > >> I'm scribbling some 8052 C code in advance. It's an SPI ic. > > >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. > > Mainly > 1) it can be trimmed to 5ppm accuracy,
Only if you have better crystals. Those I am looking at are 20 to 25ppm.
> the rest of the system takes 2A peak for the display.
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.
> There is a possibility of jamming the RTC to a GPS every couple of hours
Then a RC timing loop is good enough.
> 2) It is SPI protocol, I just hate I2C > 3) Not made by Maxim > > martin
On Tue, 9 Sep 2008 10:43:25 -0700 (PDT), in comp.arch.embedded linnix
<me@linnix.info-for.us> wrote:

>On Sep 9, 10:33 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> On Tue, 9 Sep 2008 09:08:12 -0700 (PDT), in comp.arch.embedded linnix >> >> <m...@linnix.info-for.us> wrote: >> >On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so >> >> I'm scribbling some 8052 C code in advance. It's an SPI ic. >> >> >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. >> >> Mainly >> 1) it can be trimmed to 5ppm accuracy, > >Only if you have better crystals. Those I am looking at are 20 to >25ppm. >
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
>> the rest of the system takes 2A peak for the display. > >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. > >> There is a possibility of jamming the RTC to a GPS every couple of hours > >Then a RC timing loop is good enough.
You left out IMHO :) and that would mean a pot in the circuit, nope
> >> 2) It is SPI protocol, I just hate I2C >> 3) Not made by Maxim >> >> martin
martin
On Sep 9, 10:51 am, Martin Griffith <mart_in_med...@yah00.es> wrote:
> On Tue, 9 Sep 2008 10:43:25 -0700 (PDT), in comp.arch.embedded linnix > > > > <m...@linnix.info-for.us> wrote: > >On Sep 9, 10:33 am, Martin Griffith <mart_in_med...@yah00.es> wrote: > >> On Tue, 9 Sep 2008 09:08:12 -0700 (PDT), in comp.arch.embedded linnix > > >> <m...@linnix.info-for.us> wrote: > >> >On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: > >> >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so > >> >> I'm scribbling some 8052 C code in advance. It's an SPI ic. > > >> >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. > > >> Mainly > >> 1) it can be trimmed to 5ppm accuracy, > > >Only if you have better crystals. Those I am looking at are 20 to > >25ppm. > > 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
With the GPS PPS, there is absolutely no reason to have external clock chip. I guess you are operating outside bean counters (cost managers).
> when I make the > boards up>> the rest of the system takes 2A peak for the display. > > >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. > > >> There is a possibility of jamming the RTC to a GPS every couple of hours > > >Then a RC timing loop is good enough. > > You left out IMHO :) > and that would mean a pot in the circuit, nope
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.
> > > > >> 2) It is SPI protocol, I just hate I2C > >> 3) Not made by Maxim > > >> martin > > martin
On Tue, 9 Sep 2008 10:58:03 -0700 (PDT), in comp.arch.embedded linnix
<me@linnix.info-for.us> wrote:

>On Sep 9, 10:51 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> On Tue, 9 Sep 2008 10:43:25 -0700 (PDT), in comp.arch.embedded linnix >> >> >> >> <m...@linnix.info-for.us> wrote: >> >On Sep 9, 10:33 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> >> On Tue, 9 Sep 2008 09:08:12 -0700 (PDT), in comp.arch.embedded linnix >> >> >> <m...@linnix.info-for.us> wrote: >> >> >On Sep 8, 7:19 am, Martin Griffith <mart_in_med...@yah00.es> wrote: >> >> >> I'm waiting for a couple of M41T94 real time clock chips to arrive, so >> >> >> I'm scribbling some 8052 C code in advance. It's an SPI ic. >> >> >> >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. >> >> >> Mainly >> >> 1) it can be trimmed to 5ppm accuracy, >> >> >Only if you have better crystals. Those I am looking at are 20 to >> >25ppm. >> >> 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 > >With the GPS PPS, there is absolutely no reason to have external clock >chip. I guess you are operating outside bean counters (cost >managers). >
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.
>> when I make the >> boards up>> the rest of the system takes 2A peak for the display. >> >> >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. >> >> >> There is a possibility of jamming the RTC to a GPS every couple of hours >> >> >Then a RC timing loop is good enough. >> >> You left out IMHO :) >> and that would mean a pot in the circuit, nope > >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. > >> >> >> >> >> 2) It is SPI protocol, I just hate I2C >> >> 3) Not made by Maxim >> >> >> martin >> >> martin
martin
linnix wrote:
> Martin Griffith <mart_in_med...@yah00.es> wrote: >
... snip ...
> >> 1) it can be trimmed to 5ppm accuracy, > > Only if you have better crystals. Those I am looking at are 20 to > 25ppm. > >> the rest of the system takes 2A peak for the display. > > 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. > >> There is a possibility of jamming the RTC to a GPS every couple >> of hours > > Then a RC timing loop is good enough.
Hah. Now we know your opinion is worthless. Try it. -- [mail]: Chuck F (cbfalconer at maineline dot net) [page]: <http://cbfalconer.home.att.net> Try the download section.