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RTC problems (N8 board) [message #5188]

Sun, 16 September 2018 11:05

wtim55
Messages: 13
Registered: September 2018
Location: Sao Paulo SP, BR

Junior Member

Hello everyone,
Has anyone had problems with the RTC in N8 board ?
I've grounded the crystal case, but without success.
it continues to run quickly when the board is turned on.
Thank you for any help.

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Re: RTC problems (N8 board) [message #5206 is a reply to message #5188]

Thu, 20 September 2018 15:42

Wayne W
Messages: 385
Registered: October 2015
Location: Fallbrook, California, US...

Senior Member

wtim55 wrote on Sun, 16 September 2018 11:05

Hello everyone,
Has anyone had problems with the RTC in N8 board ?
I've grounded the crystal case, but without success.
it continues to run quickly when the board is turned on.
Thank you for any help.

I have not had any problem with the RTC running fast on either of my two N8 boards.

I have heard that you need to be careful that the 32.768KHz crystal has 6pf load capacitance.

-Wayne

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Re: RTC problems (N8 board) [message #5207 is a reply to message #5206]

Thu, 20 September 2018 16:53

wtim55
Messages: 13
Registered: September 2018
Location: Sao Paulo SP, BR

Junior Member

Thank you, Wayne

The funny thing is that I have two different types of crystal
and on the Zeta board they work normally.
The problem only occurs on the N8 board.
But I do not know if the crystals I use have 6pF load capacitance.
Probably not.
I'll try to find the proper crystals.

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Re: RTC problems (N8 board) [message #5208 is a reply to message #5207]

Thu, 20 September 2018 18:56

plasmo
Messages: 881
Registered: March 2017
Location: New Mexico, USA

Senior Member

Does the RTC runs equally fast when powered off? The reason I ask is because there is an exclusion zone under the DS1302 footprint that's limited to VCC & Ground only. If there are fast toggling signals running inside the exclusion zone, the RTC will run fast when board is powered up but runs normally when board is powered off. If RTC runs equally fast with or without power, then the problem is not active signals inside the exclusion zone.
Bill

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Re: RTC problems (N8 board) [message #5209 is a reply to message #5208]

Thu, 20 September 2018 20:31

wtim55
Messages: 13
Registered: September 2018
Location: Sao Paulo SP, BR

Junior Member

Hi Bill,
The RTC runs fast with the board on. When the board is off, the RTC runs normally.
On the N8 board, the CPU oscillator is next to the RTC and under the DS1302 there are several signal lines.
I have checked the datasheet of DS1302 and it is in accordance with what you said.
I'll try another crystal as Wayne suggested, but if it does not work I think nothing can be done about it.
Thank you

Jorge

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Re: RTC problems (N8 board) [message #5210 is a reply to message #5209]

Thu, 20 September 2018 20:54

plasmo
Messages: 881
Registered: March 2017
Location: New Mexico, USA

Senior Member

Is the DS1302 socketed? Double socket it to putting some distance between the signals and the chip. Another possibility is making a ground shield with a thin copper pc board that fits under the DS1302 chip and soldered the copper foil to the ground.
Bill

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Re: RTC problems (N8 board) [message #5214 is a reply to message #5209]

Fri, 21 September 2018 10:12

jcoffman
Messages: 332
Registered: October 2015

Senior Member

Up until this year, across many boards, I always saw the DS1302 RTC's maintaining time accurately. However, on the experimental SBC386EX board, I have noticed the same thing: the RTC keeps time with VCC power off (runs from the SuperCap); but the RTC runs noticeably fast when VCC power is on. Whether this is a function of the crystal (I've used 2 of them), or of the DS1302 of newer manufacture, I do not know.

--John

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Re: RTC problems (N8 board) [message #5215 is a reply to message #5214]

Fri, 21 September 2018 10:21

Andrew B
Messages: 467
Registered: October 2015
Location: Near Redmond, WA

Senior Member
Administrator

I suppose a swap of an 'old' and 'new' DS1302 between 2 boards might answer the question?

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Re: RTC problems (N8 board) [message #5216 is a reply to message #5210]

Fri, 21 September 2018 12:44

wtim55
Messages: 13
Registered: September 2018
Location: Sao Paulo SP, BR

Junior Member

I made a shield with aluminum foil soldered to ground and placed under the DS1302 without success.
I also put an extra socket to increase the distance of the DS1302 from the board but keep running fast.
I made the swap with the DS1302 that was on the Zeta board and the problem continued.
So, I'm buying crystals with the specification that Wayne said.
When they arrive I will make the exchange and I will put the results.
Thanks everyone for the reply

Jorge

[Updated on: Fri, 21 September 2018 12:47]

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Re: RTC problems (N8 board) [message #5217 is a reply to message #5215]

Fri, 21 September 2018 12:59

jcoffman
Messages: 332
Registered: October 2015

Senior Member

Swapped DS1302's; RTC time gain does not move -- the two DS1302's are from the same manufacturing lot.

Both crystals are from Digikey, but ordered at different times. The crystal on the misbehaving board is a Citizen "tuning fork", 6pf, 10ppm. I am not totally sure, but I believe the earlier lot of 32Khz crystals were Epson, 6pf, ?? ppm.

My first SBCv1 used a 12pf crystal, unknown mfgr. The clock on it lost a little time both on & off.

Right now, the Citizen crystal appears to be the culprit.

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Re: RTC problems (N8 board) [message #5218 is a reply to message #5217]

Fri, 21 September 2018 13:12

plasmo
Messages: 881
Registered: March 2017
Location: New Mexico, USA

Senior Member

I ran into the active-signal-in-the-exclusion-zone problem with the first version of Z280RC. My solution was cut and jumper the active signal away from the exclusion zone. That fixed the problem. The 2nd revision of Z280RC rerouted the signal on the pc board and RTC works fine.
Bill

https://www.retrobrewcomputers.org/doku.php?id=builderpages: plasmo:z280rc:z280rc_r0_ec

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Re: RTC problems (N8 board) [message #5362 is a reply to message #5188]

Fri, 19 October 2018 19:37

wtim55
Messages: 13
Registered: September 2018
Location: Sao Paulo SP, BR

Junior Member

After trying xtal from other sources without success, I decided to try the DS1302N.
And to my surprise, it works fine. Strange because the difference should only be in the temperature range.
RTC is OK now

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