NJU6355 RTC woes
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Would anyone here happen to know what the load
capacitance of the accompanying 32.768khz crystal
should be? I've read and reread the datasheet, but
can't seem to find it. Three e-mails to NJR have gone
unanswered.
I've tried a 10.5pf crystal, and it ran fast. On a 12.5pf
crystal it ran much better, but would still drift off one or
two seconds per hour. Am I maybe expecting too much
accuracy from such an inexpensive RTC?
Oh, one more question. Where can one find those 6pf 32.768khz
crystals that are needed for some of Dal-Semi's RTC's? I can't find
these things anywhere! :-(
Cheers,
Steve
capacitance of the accompanying 32.768khz crystal
should be? I've read and reread the datasheet, but
can't seem to find it. Three e-mails to NJR have gone
unanswered.
I've tried a 10.5pf crystal, and it ran fast. On a 12.5pf
crystal it ran much better, but would still drift off one or
two seconds per hour. Am I maybe expecting too much
accuracy from such an inexpensive RTC?
Oh, one more question. Where can one find those 6pf 32.768khz
crystals that are needed for some of Dal-Semi's RTC's? I can't find
these things anywhere! :-(
Cheers,
Steve
Comments
clock. I received it from an engineer at NJR. I can dig it up if you like.
I was surprised to learn from this document, that there is actually a way
to put the nju clocks into a mode which would allow you to calibrate the
frequency, though i think you need a long period counter to accomplish this.
let me know if you want me to find it.
good luck,
steve
At 07:28 AM 10/11/2000 EDT, you wrote:
>Would anyone here happen to know what the load
>capacitance of the accompanying 32.768khz crystal
>should be? I've read and reread the datasheet, but
>can't seem to find it. Three e-mails to NJR have gone
>unanswered.
>
>I've tried a 10.5pf crystal, and it ran fast. On a 12.5pf
>crystal it ran much better, but would still drift off one or
>two seconds per hour. Am I maybe expecting too much
>accuracy from such an inexpensive RTC?
>
>Oh, one more question. Where can one find those 6pf 32.768khz
>crystals that are needed for some of Dal-Semi's RTC's? I can't find
>these things anywhere! :-(
>
>Cheers,
>
>Steve
>
>
>
I'm surprised at how little information there is available online about this
part. Maybe I should stop being so stubborn and make the move to
the Dal-Semi RTC's (if I can ever find a source for those 6pf crystals).
Thanks!
Steve
<< I have a document at work which discusses calibrating this particular
clock. I received it from an engineer at NJR. I can dig it up if you like.
>>
12.5pf
load capacitance like the one you tried. That was the best I could
get
it to run.
Andy
--- In basicstamps@egroups.com, PicProgrammer@a... wrote:
> Would anyone here happen to know what the load
> capacitance of the accompanying 32.768khz crystal
> should be? I've read and reread the datasheet, but
> can't seem to find it. Three e-mails to NJR have gone
> unanswered.
>
> I've tried a 10.5pf crystal, and it ran fast. On a 12.5pf
> crystal it ran much better, but would still drift off one or
> two seconds per hour. Am I maybe expecting too much
> accuracy from such an inexpensive RTC?
>
> Oh, one more question. Where can one find those 6pf 32.768khz
> crystals that are needed for some of Dal-Semi's RTC's? I can't find
> these things anywhere! :-(
>
> Cheers,
>
> Steve
NJR makes three real time clock chips:
NJU6355
NJU6356 -- "stabilized" oscillation <???!!!>
NJU6358. -- alarm and wakeup feature
If you download the data sheet for the NJU6358 you will see a diagram for
the oscillator circuit, including values for Cg and Cd.
http://sdremote1.njr.com/cdrom/index/ce/ce12010.pdf
The description of the circuit is hilarious, translated from Japanese, like
directions on a package of top ramen. It suggests, "It is required to
examinate the matching of the crystal and the oscillation circuit, so that
some kinds of the crystal may be required to connect the external
capacitor." But it doesn't spec a value for the load capacitance. FYI,
the load capacitance cannot be derived directly from Cg and Cd, rather, it
is usually measured by its effect on the frequency of a known crystal. I
have no idea if '6355 circuit is the same. The existance of the NJU6356
makes me wonder. The only difference between that and the '6355 seems to be
the "stabilized" oscillator, but there are _no specs_!. FWIW, in
connection Pierce oscillators "stabilized" can refer to inclusion of the
series resistor to reduce drive power to the crystal. But they can't be
driving the crystal too too hard. The circuit operates at 4 microamps.
Steve Sargeant, if you can't find that calibration paper, I recall that you
sent a copy of it to me at one time when this came up. If you can't find
it, I'll take a look in my disarray of papers. Did you start using the
Dallas DS32KHZ then? -- 4 minutes per year over the -40 to +85 temperature
range
-- Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
>>>Would anyone here happen to know what the load
>>>capacitance of the accompanying 32.768khz crystal
>>>should be? I've read and reread the datasheet, but
>>>can't seem to find it. Three e-mails to NJR have gone
>>>unanswered.
>>>>I've tried a 10.5pf crystal, and it ran fast. On a 12.5pf
>>>crystal it ran much better, but would still drift off one or
>>>two seconds per hour. Am I maybe expecting too much
>>>accuracy from such an inexpensive RTC?
>>>Steve
>>I have a document at work which discusses calibrating this particular
>>clock. I received it from an engineer at NJR. I can dig it up if you
like.
>>I was surprised to learn from this document, that there is actually a way
>>to put the nju clocks into a mode which would allow you to calibrate the
>>frequency, though i think you need a long period counter to accomplish
this.
>>let me know if you want me to find it.
>>steve
>I would _really_ appreciate that Steve, if it's not too much trouble.
>I'm surprised at how little information there is available online about
this
>part. Maybe I should stop being so stubborn and make the move to
>the Dal-Semi RTC's (if I can ever find a source for those 6pf crystals).
>Steve
I have not been able to locate the document yet, but I'm sure i can find
it. I may also have a name and number for an application engineer at NJU. I
spoke with engineer last year and was impressed with his knowledge and
desire to help. He's the one that set me up with this "secret document".
I drove the NJU6355 with the DS32Khz for about two months, and at room
temperature, it was off by only 4 or 5 seconds. of course how can one know
this, my solution was to check the time against a web site that was updated
from a clock found here http://tycho.usno.navy.mil/cgi-bin/timer.pl
Steve
I did find my copy of the NJR "product note" document you (S. Sargent)
faxed to me back in May'99. It states that the load capacitance is 14 pf.
It gives a laborious procedure to add external trims to improve the
stability. As you mentioned, it also tells how to get the chip into test
mode, which is 128 hertz output.
The DS32KHZ is remarkable. I'm suprised that the established crystal
manufacturers like Statek have not followed up with a product like that.
-- Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
---original message--->
>Hi Steve and Tracy,
>I have not been able to locate the document yet, but I'm sure i can find
>it. I may also have a name and number for an application engineer at NJU.
I
>spoke with engineer last year and was impressed with his knowledge and
>desire to help. He's the one that set me up with this "secret document".
>I drove the NJU6355 with the DS32Khz for about two months, and at room
>temperature, it was off by only 4 or 5 seconds. of course how can one know
>this, my solution was to check the time against a web site that was
updated
>from a clock found here http://tycho.usno.navy.mil/cgi-bin/timer.pl
>Steve
Now I know why the 6355 is as inexpensive as it is. ;-)
I'm glad that I didn't buy more than one tube. For short term,
non-critical timing, I suppose I'll continue to use the 6355
with the 12.5pf crystals. But all this talk has persuaded me
to move on to the Dallas parts (1302/1305/1306/1602). On
a happy note, I did finally find a source for those 6pf crystals,
if anyone else is looking. Part number C-002RX 32.768K-E,
available from www.norvell.com, only $0.20 a piece. With the
DalSemi parts as popular as they are, I'm surprised it is so
hard to find the supporting crystals (at least in my usual
haunts - Digikey, Mouser, Jameco, Newark, Allied, etc).
Speaking of searching for parts, can anyone recommend any
online parts searching tools besides www.findchips.com? I use
findchips every day, but was just wondering what else was out there.
Thanks again guys.
Cheers,
Steve
I've always used the 10.5 pf crystals from jameco. Most of my projects are
outside, long term and the NJU clock is usually off by no more than 5
minutes per month.
fwiw
Steve
At 10:56 AM 10/13/2000 EDT, you wrote:
>Thank you for all the input Steve, Tracy, Andy, and Peter.
>Now I know why the 6355 is as inexpensive as it is. ;-)
>I'm glad that I didn't buy more than one tube. For short term,
>non-critical timing, I suppose I'll continue to use the 6355
>with the 12.5pf crystals. But all this talk has persuaded me
>to move on to the Dallas parts (1302/1305/1306/1602). On
>a happy note, I did finally find a source for those 6pf crystals,
>if anyone else is looking. Part number C-002RX 32.768K-E,
>available from www.norvell.com, only $0.20 a piece. With the
>DalSemi parts as popular as they are, I'm surprised it is so
>hard to find the supporting crystals (at least in my usual
>haunts - Digikey, Mouser, Jameco, Newark, Allied, etc).
>
>Speaking of searching for parts, can anyone recommend any
>online parts searching tools besides www.findchips.com? I use
>findchips every day, but was just wondering what else was out there.
>
>Thanks again guys.
>
>Cheers,
>
>Steve
>
>
>
I found an interesting posting on another mailing list:
--snip--
From: Brian Dickens <bdicken@a...>
Date: Mon, 01 May 2000 13:48:26 -0400
Hello Vikki et al.,
I got a clock crystal and programmed the NJU6355 clock. In the first
trial, I ran the clock overnight, reading the time from it every
second. It was about one hour slow after about 13 hours of running. Much
later, yesterday in fact, I ran it again, this time reading it only every 5
minutes to test whether the clock or the crystal was at fault. It still
lost some time in about 10 hours of running but less than a minute. The
clock only seems to be useful if you read it very infrequently or if you
want to use it as a battery monitor - I haven't tried that.
--snip--
In my testing of the 6355 I polled it almost continuously. I'll have to
do some more testing - running it for a couple of hours without polling
it, then running it for the same amount of time while polling continuously.
The plot thickens......... LOL
Steve
<< I've always used the 10.5 pf crystals from jameco. Most of my projects are
outside, long term and the NJU clock is usually off by no more than 5
minutes per month. >>
Is your clock on a breadboard? I believe that could cause some serious
errors due to the stray capacitance?
I've always had my clocks mounted on a pc board
Steve
At 01:30 PM 10/13/2000 EDT, you wrote:
>Steve,
>
>I found an interesting posting on another mailing list:
>
>--snip--
>
>From: Brian Dickens <bdicken@a...>
>Date: Mon, 01 May 2000 13:48:26 -0400
>
>Hello Vikki et al.,
>
>I got a clock crystal and programmed the NJU6355 clock. In the first
>trial, I ran the clock overnight, reading the time from it every
>second. It was about one hour slow after about 13 hours of running. Much
>later, yesterday in fact, I ran it again, this time reading it only every 5
>minutes to test whether the clock or the crystal was at fault. It still
>lost some time in about 10 hours of running but less than a minute. The
>clock only seems to be useful if you read it very infrequently or if you
>want to use it as a battery monitor - I haven't tried that.
>
>--snip--
>
>In my testing of the 6355 I polled it almost continuously. I'll have to
>do some more testing - running it for a couple of hours without polling
>it, then running it for the same amount of time while polling continuously.
>The plot thickens......... LOL
>
>Steve
>
>
><< I've always used the 10.5 pf crystals from jameco. Most of my projects are
> outside, long term and the NJU clock is usually off by no more than 5
> minutes per month. >>
>
>
>
Something else I've thought of which may or may not matter:
I started using this clock when I purchased Scott Edwards DCPB(excellent kit).
This kit uses a pulldown resistor from the CE terminal to ground(around 10
K) and also a 1 kohm
resistor in series with the data pin. I think the 1 kohm resistor is there
in case the NJU data pin is at, say a high state(output) and the stamp pin
was low or visa-versa.
Also, if you like you could send your clock code for us to look at.
I've always had good luck with this chip and think something must be
messing yours up.
good luck,
Steve
>>--snip--
>>From: Brian Dickens <bdicken@a...>
>>>Hello Vikki et al.,
>>>I got a clock crystal and programmed the NJU6355 clock. In the first
>>>trial, I ran the clock overnight, reading the time from it every
>>>second. It was about one hour slow after about 13 hours of running.
Much
>>>later, yesterday in fact, I ran it again, this time reading it only
every 5
>>>minutes to test whether the clock or the crystal was at fault. It
still
>>>lost some time in about 10 hours of running but less than a minute. The
>>>clock only seems to be useful if you read it very infrequently or if you
>>>want to use it as a battery monitor - I haven't tried that.
>--snip--
>In my testing of the 6355 I polled it almost continuously. I'll have to
>do some more testing - running it for a couple of hours without polling
>it, then running it for the same amount of time while polling
continuously.
>The plot thickens......... LOL
>Steve
Brian's observation seemed pretty incredible. Could a company like NJU
really sell a product with a quirk like that and not mention it in the
specs? (you don't have to answer that!).
I think something must have been wrong with Brian's setup. To check, I ran
an NJU6355 for 24 hours, reading out and displaying the time every 1
second. At the end of 23 hours, the time reported by the NJU6355 (compared
to a time server) was 4 seconds fast. That is an error of 46ppm, within
specs of the crystal itself. That is without any special tuning of the Cg
value, just a stock Digikey 12.5pf xtal on a printed circuit board, at room
temperature. So my conclusion is that the chip does _not_ have that
particular "fatal flaw".
I second Steve's comment, never to expect good performance from a clock
chip mounted on one of those white breadboard blocks. There is about 200pf
extra capacitance between adjacent nodes on the block.
-- Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
I had the circuit built up on plain perfboard, wired point
to point. Before I give up on the part, I'm going to build
it on a proper PCB, with the crystal mounted securely.
Now if I can only get Proteus Lite to output some useable
artwork........ [noparse]:([/noparse]
Steve
<< I second Steve's comment, never to expect good performance from a clock
chip mounted on one of those white breadboard blocks. There is about 200pf
extra capacitance between adjacent nodes on the block. >>
That's exactly how mine is set up, CE has a 10K pd and Data a 1K pd.
The circuit and code came right out of Scott's Stamp App #20. If I can
figure out how to get Proteus Lite to print a quality positive, I'm going to
shoot a PCB this weekend and see if that will make a difference over the
perfboard hatchet job I've been using thus far.
I have the crystal attached as closely as possible to the IC, but I got to
wondering if it is necessary to attach (glue?) the crystal case to the board?
So far I've had it more or less flapping in the breeze, to make it easier to
swap out crystals during testing.
Regards,
Steve
<< This kit uses a pulldown resistor from the CE terminal to ground(around 10
K) and also a 1 kohm resistor in series with the data pin. I think the 1
kohm
resistor is there in case the NJU data pin is at, say a high state(output)
and the stamp pin was low or visa-versa. >>
>That's exactly how mine is set up, CE has a 10K pd and Data a 1K pd.
Do you mean that the data pin has a series resistor, not a pull down?
though I don't think that it would effect the timekeeping.
I've been thinking about your mentioning that you were reading the clock at
a quick rate(several times per second). I've never had an application like
this, usually i read the clock at the most once a minute. I just
reprogrammed a project where the clock runs fine on, to simply read the
clock as fast as it can.
I'll let you know if the drift increases.
>The circuit and code came right out of Scott's Stamp App #20. If I can
>figure out how to get Proteus Lite to print a quality positive, I'm going to
>shoot a PCB this weekend and see if that will make a difference over the
>perfboard hatchet job I've been using thus far.
One way to decrease the capacitance from a protoboard would be to mount the
clock in an 8 pin dip socket, which has had the two legs for the oscillator
circuitry cut off. Then the pins don't contact the protoboard.
I doubt though this will make much difference.
Has anyone on this list used a NJU clock in an application where the clock
was read quickly over and over?
Did the clock still track time well?
Steve
>
>I have the crystal attached as closely as possible to the IC, but I got to
>wondering if it is necessary to attach (glue?) the crystal case to the board?
>So far I've had it more or less flapping in the breeze, to make it easier to
>swap out crystals during testing.
>
>Regards,
>
>Steve
>
><< This kit uses a pulldown resistor from the CE terminal to ground(around 10
> K) and also a 1 kohm resistor in series with the data pin. I think the 1
>kohm
>resistor is there in case the NJU data pin is at, say a high state(output)
>and the stamp pin was low or visa-versa. >>
>
>
>
10
>> K) and also a 1 kohm resistor in series with the data pin. I think the 1
>>kohm
>>resistor is there in case the NJU data pin is at, say a high
state(output)
>>and the stamp pin was low or visa-versa. >>
>
> That's exactly how mine is set up, CE has a 10K pd and Data a 1K pd.
> The circuit and code came right out of Scott's Stamp App #20. If I can
> figure out how to get Proteus Lite to print a quality positive, I'm going
to
> shoot a PCB this weekend and see if that will make a difference over the
> perfboard hatchet job I've been using thus far.
>
> I have the crystal attached as closely as possible to the IC, but I got
to
> wondering if it is necessary to attach (glue?) the crystal case to the
board?
> So far I've had it more or less flapping in the breeze, to make it easier
to
> swap out crystals during testing.
Hi Steve,
You shouldn't need to glue down the crystal, although it might be a good
idea in the long run. A sliver of thick double-stick tape works pretty
well.
In your original post you stated that the time was off by one or two
seconds per hour. That is a lot, more than 300ppm, compared with the
specification of around 20-50ppm for tuning fork watch crystals. When the
capacitance is matched, and at room temperature, it should be within 2
seconds per day, given a 20ppm crystal. I've found that adding 10-20 pf
externally brings the time within 1 second per day (at room temperature).
That is extra Cg, from pin 2 to ground.
Have you tried a different crystal, or a different NJU chip from the tube?
Maybe it was off or damaged. Or there is something in the layout or the
power supply.
> Has anyone on this list used a NJU clock in an application where the
clock
> was read quickly over and over?
> Did the clock still track time well?
Yes, I've tried that. The result in terms of time drift on a particular
clock chip is the same whether reading at once per second, once per six
seconds, or once per minute.
-- Tracy
Oooops, sorry Steve. I was typing from (faulty) memory. You're right, I
meant CE has a 10K pulldown, while Data has a 1K series resistor.
<<I've been thinking about your mentioning that you were reading the clock at
a quick rate(several times per second). I've never had an application like
this, usually i read the clock at the most once a minute. I just
reprogrammed a project where the clock runs fine on, to simply read the
clock as fast as it can. >>
The circuit I had built up was little more than an LCD clock - albeit costing
$100 for less functionality than a $1.99 dime store version. ;-) I just
wanted to
test out the date/day/month/etc. rollover of the 6355 before going on and
using
the part in a real (read useful) application. That's when this timing problem
reared
its ugly head.
<<I'll let you know if the drift increases.>>
Thanks Steve. I look forward to hearing of your results. Oh, there's one more
thing I wanted to mention. I'm not sure if this could have anything to do with
the problem, but for simplicity sake I'm powering the clock directly off of
one
of the BS2 pins.
Regards,
Steve
After 14 hrs of reading my clock at 20 times a second, It's still accurate
to within a second.
Powering the clock from a stamp pin is ok, provided you don't use the sleep
or nap instructions.
I would run it off the 5 volt regulator, with an assortment of capacitors
accross the power leads of the clock. maybe a .01, .1 and a 1 microfarad.
I agree with Tracy about the possibility of the clock or crystal not
working properly.
I'm not sure how involved your program is, but I always try to remove all
unneccsary code when trying to track down a problem. Then build it back up
once you get it working.
how about a look at your clock routine?
One potential problem could be the state of the NJU I/O pin during your
program. I think that if this pin is high anytime the CE pin is brought
high, that the clock believes your going to give it new time information,
and may halt the clock.
Steve