CCS C Software and Maintenance Offers
FAQFAQ   FAQForum Help   FAQOfficial CCS Support   SearchSearch  RegisterRegister 

ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

CCS does not monitor this forum on a regular basis.

Please do not post bug reports on this forum. Send them to CCS Technical Support

M41T80 RTC question

 
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion
View previous topic :: View next topic  
Author Message
Hans Wedemeyer



Joined: 15 Sep 2003
Posts: 226

View user's profile Send private message

M41T80 RTC question
PostPosted: Sat Jul 31, 2004 1:33 pm     Reply with quote

I'm trying to use the M41T80 RTC chip.
So far things are not so good...
Once synchronized (to GPS) it appears to work but reading it once a second it can produee a wrong time.
The clock is a Dallas 32KHz at room temperture (26oC) should be farily stable for at least a few days.
The read error give the same second count. i.e.

20:33:28
20:33:29
20:33:29
20:33:31

It appears to be keeping time. I think this is a read problem.

I tried both methods for reading. and the problem persists.

Does anyone have any idea what the issue could be ?

Are any known issues with this chip ?

Correction: It does not keep time... Just searched a large data file and found this...
RTC -- GPS
18:05:14 -- :14
18:05:15 -- :15
18:05:17 -- :16
18:05:17 -- :17
18:05:19 -- :18
18:05:20 -- :19
18:05:21 -- :20


Last edited by Hans Wedemeyer on Sat Jul 31, 2004 3:05 pm; edited 1 time in total
dyeatman



Joined: 06 Sep 2003
Posts: 1937
Location: Norman, OK

View user's profile Send private message

M40T80
PostPosted: Sat Jul 31, 2004 2:10 pm     Reply with quote

Hans,
The M40T80 is a new one on me. Where is the data sheet? Who makes it?
Ttelmah
Guest







PostPosted: Sat Jul 31, 2004 2:33 pm     Reply with quote

Not a chip I have heard of. You will have to post a link to a data sheet.
However some general comments apply:
1) Most such chips, update at their own 'second'. If you set the time, their internal divider, feeding this counter, is not reset (some have seperate commands to do this). Hence, if this timer is due to expire in 0.1 second (say), and you set the time to 14 seconds, then only just slightly more than 0.1seconds latter, th time will advance to the next second.
2) Some such chips have a 'holding register' for the time, and the contents of the counters are latched into this, on the first part of the read instruction. The time read, is then the moment when the instruction started, even if something else (serial I/O for instance), took place in the meantime, delaying completion of the read.
3) In chips that don't have such a holding register, it is vital to read the chip, when the registers are not updating. Hence it is normal to synchronise the read to a signal from the chip, otherwise there can be a problem if the register values change during the read.

Best Wishes
Hans Wedemeyer



Joined: 15 Sep 2003
Posts: 226

View user's profile Send private message

Sorry wrong chip number it's M41T80 made by ST (SGS)
PostPosted: Sat Jul 31, 2004 3:09 pm     Reply with quote

Ttelmah wrote:
Not a chip I have heard of. You will have to post a link to a data sheet.
However some general comments apply:
1) Most such chips, update at their own 'second'. If you set the time, their internal divider, feeding this counter, is not reset (some have seperate commands to do this). Hence, if this timer is due to expire in 0.1 second (say), and you set the time to 14 seconds, then only just slightly more than 0.1seconds latter, th time will advance to the next second.
2) Some such chips have a 'holding register' for the time, and the contents of the counters are latched into this, on the first part of the read instruction. The time read, is then the moment when the instruction started, even if something else (serial I/O for instance), took place in the meantime, delaying completion of the read.
3) In chips that don't have such a holding register, it is vital to read the chip, when the registers are not updating. Hence it is normal to synchronise the read to a signal from the chip, otherwise there can be a problem if the register values change during the read.

Best Wishes

This one also has a "holding" feature, butthe explanation of it is unclear. It could be read as "the chip stops updating the clock" but I think they mean "the time is latched"...(I hope !) It's a poor data sheet, I've seen better.

In case you need it the M41T80 data sheet is here
http://www.st.com/stonline/books/pdf/docs/9074.pdf


Last edited by Hans Wedemeyer on Sat Jul 31, 2004 3:12 pm; edited 1 time in total
Hans Wedemeyer



Joined: 15 Sep 2003
Posts: 226

View user's profile Send private message

Re: M40T80
PostPosted: Sat Jul 31, 2004 3:10 pm     Reply with quote

dyeatman wrote:
Hans,
The M40T80 is a new one on me. Where is the data sheet? Who makes it?


Sorry about that...

link here to the M41T80

http://www.st.com/stonline/books/pdf/docs/9074.pdf
Guest








PostPosted: Sat Jul 31, 2004 6:01 pm     Reply with quote

Hans,

First let the thing run for a day or over a weekend and then tell us how for out the time is..

I am guessing like Ttelmah said you are reading the register between time changes. That is the clock is updating but you are reading a cached version of the time.

If your application needs accurate time base consider using the frequency out, square wave out. This will allow the processor to keep the time more accurate than you can read through the I2C interface.

Trampas
Guest








PostPosted: Sat Jul 31, 2004 9:02 pm     Reply with quote

Anonymous wrote:
Hans,

First let the thing run for a day or over a weekend and then tell us how for out the time is..

I am guessing like Ttelmah said you are reading the register between time changes. That is the clock is updating but you are reading a cached version of the time.

If your application needs accurate time base consider using the frequency out, square wave out. This will allow the processor to keep the time more accurate than you can read through the I2C interface.

Trampas

Well your last item first.... NO I've tried that. With a whole host of other things going on, there is too much chance of missing an interrupt.
The hardware RTC should do better.
I think Ttelmah is correct, only thing is to figure out where in the code and when to read the chip.

At the moment it's synchronized to the GPS 1PPS Reading the RTC is done just after the 1PPS interrupt (not in) has completed.

The strange effect is it gained time !

Maybe I'll look for a better RTC.... this one costs $1.15

Maybe the price should tell me something :-)
Guest








PostPosted: Sun Aug 01, 2004 2:17 am     Reply with quote

The chip has an IRQ output what you could set for 1/second. I would use this feature to be sure the read is done right after the second increment.
Ttelmah
Guest







Re: M40T80
PostPosted: Sun Aug 01, 2004 3:28 pm     Reply with quote

Hans Wedemeyer wrote:
dyeatman wrote:
Hans,
The M40T80 is a new one on me. Where is the data sheet? Who makes it?


Sorry about that...

link here to the M41T80

http://www.st.com/stonline/books/pdf/docs/9074.pdf

Looking at the data sheet, how are you setting the clock?.
If I wanted to set this, I'd read the GPS time, then add one second to this. Write this into the register map. Then keep writing the 'seconds' value, till the GPS advances to the next second. Every time you write to the chip, the tenths/hundredths counter, is reset, and this will then result in a reasonably accurate 'sync'.
Seperately, another poster has said to use the 1/sec interrupt output to synchronise your read - a _very_ good idea.
The data sheet does say that the clock registers will not update while being read, _but_ it is vital that you use the sequential update mode to read these as a block, if you are not going to see problems (how is your I2C coded....).
Basically, if you read register 1, the value of register 1 will not change during the read. If you seperately read register 1, then 2, then 3, each register will be held during the read, but they can change between the reads. If instead you use the sequential read, all the registers will be held, till the entire block is transferred.

Best Wishes
Hans Wedemeyer



Joined: 15 Sep 2003
Posts: 226

View user's profile Send private message

Re: M40T80
PostPosted: Sun Aug 01, 2004 6:49 pm     Reply with quote

Ttelmah wrote:
Hans Wedemeyer wrote:
dyeatman wrote:
Hans,
The M40T80 is a new one on me. Where is the data sheet? Who makes it?


Sorry about that...

link here to the M41T80

http://www.st.com/stonline/books/pdf/docs/9074.pdf

Looking at the data sheet, how are you setting the clock?.
If I wanted to set this, I'd read the GPS time, then add one second to this. Write this into the register map. Then keep writing the 'seconds' value, till the GPS advances to the next second. Every time you write to the chip, the tenths/hundredths counter, is reset, and this will then result in a reasonably accurate 'sync'.
Seperately, another poster has said to use the 1/sec interrupt output to synchronise your read - a _very_ good idea.
The data sheet does say that the clock registers will not update while being read, _but_ it is vital that you use the sequential update mode to read these as a block, if you are not going to see problems (how is your I2C coded....).
Basically, if you read register 1, the value of register 1 will not change during the read. If you seperately read register 1, then 2, then 3, each register will be held during the read, but they can change between the reads. If instead you use the sequential read, all the registers will be held, till the entire block is transferred.

Best Wishes


As it turns out the problem was reading it too soon after the GPS 1pps. As the RTC is synchronized to gps, it will also update it's registers at the same time as the gps 1pps (at least unitl the Osc. drifts) I added a few uS delay before reading the RTC and that did the trick.

Synchronizing is a breeze if you know the one trick....

Register 0 is the 1/10 and 1/100 register, and it is read only....... !
It MUST be included in the sync. so just write 00 (yes to a read only reg. ) to it followed by the seconds+1 minutes etc. then hold the YEARS until the "next" 1pps then write the years and it's in sync. In fact reading the RTC at the gsp 1pps give .00 for the 1/10 and 1/100 until the osc. starts to drift.

In my code I only read location 0 after the 1pps and then read the rest of the registers after getting the $GPGGA and $GPRMC strings.

Next ... M41T81 and add some corrections based upon temperature....


Thanks for the help.
Ttelmah
Guest







PostPosted: Mon Aug 02, 2004 2:20 am     Reply with quote

I noticed it referenced being able to 'set' the tenth, and hundredth register, 'but only to 0'. It does provide an easy way to do the sync. :-)
Glad it is working.

Best Wishes
Guest








PostPosted: Mon Aug 02, 2004 7:31 am     Reply with quote

Ttelmah wrote:
I noticed it referenced being able to 'set' the tenth, and hundredth register, 'but only to 0'. It does provide an easy way to do the sync. :-)
Glad it is working.

Best Wishes

Correct, however they clearly state wrtting to any other time register will zero the tenth and hundreds... implying it is taken care of ... It's a crappy data sheet, need to much reading between the lines...

from the data sheet...
quote:
A WRITE to any clock register will result in
the Tenths/Hundredths of Seconds being reset to
�00,� and Tenths/Hundredths of Seconds cannot
be written to any value other than �00.�
end quote:

Anyway, thanks for the help. BTW MAXIM are doing a second source for this chip, with a few changes.
hansw
Display posts from previous:   
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group