Author Topic: TDS 684C NVRam Backup  (Read 13648 times)

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Offline Mosaic

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Re: TDS 684C NVRam Backup
« Reply #50 on: April 22, 2017, 09:28:08 pm »
Can u be more specific as to why it isn't a drop in replacement and if the fix is doable?"
 

Offline Armadillo

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Re: TDS 684C NVRam Backup
« Reply #51 on: April 22, 2017, 09:44:06 pm »
Can u be more specific as to why it isn't a drop in replacement and if the fix is doable?"

Just compare the address spaces of the two [page 8]
 

Offline deepskyridge

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Re: TDS 684C NVRam Backup
« Reply #52 on: June 03, 2017, 04:06:26 pm »
Sorry Guys, I checked my DS1486 backup from the original scope IC and noticed that the first 50 bytes are actually "FF". The bytes that will varies automatically actually start at byte 51.
Now how do you tell TL866 to start programming from bytes 51 onwards?

Edit: So, that's the reason why TL866 does not have a device named DS1486.

Did anyone figure out if you could use a DS1486 in a TL866A. I have a TDS744A i need to backup.

Thanks
Gary
 

Offline Polarue

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Re: TDS 684C NVRam Backup
« Reply #53 on: November 17, 2018, 04:31:26 pm »
I have looked at the datasheet for the ds1486-120 nvram in my TDS640A and it occurred to me that it might just be possible to add a common rechargable lithion ion battery to the Vcc of this chip in order to maintain the voltage of the Vcc to about 3.5v, when the main power of the scope is off.  When the scope is powered, a simple blocking diode from the Vcc to the new battery would regulate the voltage down from the nominal Vcc of 5.0v to 4.0v by placing two diodes in series and then one additional diode returning to the same Vcc leg of the chip(32) to yield about 3.3-3.5v when the scope is powered down but safely charging while powered up.

The above scenario is made possible as the integrated LTC cell in the chip only switches on if the Vcc drops to about 3.0v. 

Forget the whole "replacing the chip idea" and just bypass the LTC cell entirely. That battery will never again be relied upon if the Vcc never drops to 3.0v.

The new external cell will not allow the voltage to drop if the unit is switched on  a few times a year to top it up with charge and it is cheap and easy to replace every 5 years.

Am I missing something here like too much current drain on the board if the Vcc is powered by the lithium external when the scope is off.  If so, a small circuit could remedy this yes?  Is this a great idea for these TDS scopes or not? :-+
« Last Edit: November 17, 2018, 09:24:20 pm by Polarue »
 

Online james_s

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Re: TDS 684C NVRam Backup
« Reply #54 on: November 17, 2018, 06:09:10 pm »
That's not going to work, the other parts on the board will draw a lot more power than you think and quickly drain the battery. Also those Dallas chips shut down and switch over to internal battery when Vcc drops below something like 4.5V.
 

Offline Polarue

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Re: TDS 684C NVRam Backup
« Reply #55 on: November 17, 2018, 08:21:25 pm »
Are you guessing, James?  Where did you get that info. regarding the chip?  See below.

"As VCC falls below approximately 3.0V, a power switching circuit turns the internal lithium energy source on to maintain the
clock and timer data functionality. It is also required to ensure that during this time (battery-backup
mode), that the voltage present at INTA and INTB (INTB) never exceeds VBAT. During power-up, when
VCC rises above VBAT, the power-switching circuit connects external VCC and disconnects the internal
lithium energy source. Normal operation can resume after VCC exceeds 4.5V for a period of 200ms."

The above is from the datasheet for the ds1486-120, so maybe you misdirected yourself, unless I misunderstood the above?

Secondly, have you measured the current draw as the battery can be much larger than the one in the chip! 

Third, could not a circuit be built to block the current to the other parts of the board with a transistor driven and triggered by the board's own 5v (tying the emitter and base of npn together) and then it would block the 3.5v from the external battery to the rest of the board when the unit is off? Of course this would mean lifting Vcc pin.  :-+

If the details and proper component selection are worked out, this would be a heck of a lot easier than replacing the chip and further, the chips are costing more than I paid for my scope these days. |O
« Last Edit: November 17, 2018, 09:48:09 pm by Polarue »
 

Online james_s

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Re: TDS 684C NVRam Backup
« Reply #56 on: November 18, 2018, 04:22:25 am »
I was thinking of the related DS1220 datasheet, but I realized looking at it now that the voltage threshold I was thinking of was for the write protect, not for the backup battery.

"The DS1220AB provides full functional capability for V CC greater than 4.75 volts and write protects by
4.5V. The DS1220AD provides full functional capability for V CC greater than 4.5 volts and write protects
by 4.25V."


Yes you could hack up the board to isolate the Vcc pin to the Dallas chip but why would you want to? It's easy to cut the battery out of a Dallas chip and attach an external 3V battery. That then makes use of the built in changeover circuit and requires no modification to the original circuit. On top of that it's easier to modify the Dallas chip than the PCB in the scope. It's a superior solution all around.
 

Offline Polarue

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Re: TDS 684C NVRam Backup
« Reply #57 on: November 18, 2018, 12:24:40 pm »
Easy?  I have not read that anyone has actually succeeded in doing what you describe to the dallas chip.  I have seen the xrays and also a pic of a partial removal of some epoxy etc, but never a completed mod while the chip is on the board.  There is risk, if you remove the chip from heat, although some have done it. High probability of damaging the data.

From my experience, there is great risk in carving up the working chip in the scope.  I would rather make up a circuit with battery and add it with minimal disturbance to the board and the chip.  Just lifting the pin 32 and connecting the  battery circuit to the pin and its pad seems way less risky, along with the fact that no programming is required either.  This way the chip is not at risk to the same extent, YMMV.
« Last Edit: November 18, 2018, 12:31:09 pm by Polarue »
 

Online james_s

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Re: TDS 684C NVRam Backup
« Reply #58 on: November 18, 2018, 04:35:15 pm »
Not in a scope, but I have replaced the battery in a Dallas chip before and it was quite easy, in my case it was one that was used on a vintage PC motherboard.

IIRC the Dallas chips in the TDS600 series don't store the calibration data anyway, just the options, and you can enable any of those you need via GPIB. The TDS300 series is another story.
 

Offline Jwalling

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Re: TDS 684C NVRam Backup
« Reply #59 on: November 19, 2018, 10:39:49 am »
Not in a scope, but I have replaced the battery in a Dallas chip before and it was quite easy, in my case it was one that was used on a vintage PC motherboard.

IIRC the Dallas chips in the TDS600 series don't store the calibration data anyway, just the options, and you can enable any of those you need via GPIB. The TDS300 series is another story.

SPC constants are stored in the DS1486 as well.
Jay

System error. Strike any user to continue.
 

Offline Polarue

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Re: TDS 684C NVRam Backup
« Reply #60 on: November 19, 2018, 12:36:11 pm »
Did you do it while it was still on the board?  Do you have any pictures to share? Exactly what tools did you use so as to not damage the rest of the chip etc.  This would be most appreciated. I do not want to reinvent the wheel.   :scared:
 


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