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arunb
Joined: 08 Sep 2003 Posts: 492 Location: India
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The lifetime of DS1307 NVRAM |
Posted: Sat Feb 16, 2013 3:50 am |
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Hi,
What is the life time of the NVRAM in a DS1307 ?? Is this an EEPROM or a FLASH based RAM.
I notice that the values are retained even when the +5V supply is switched OFF and with the lithium battery removed.
Is there any data on how many times data can be written or read to it ??
Also are the time/date values stored an EEPROM or a memory similar to NVRAM ??
The reason I need to know this is that I am planning to use the DS1307 NVRAM to save some data in case of power failure. Instead of doing this on the PICs EEPROM (which I know has limited write cycles)
thanks
a |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19369
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Posted: Sat Feb 16, 2013 4:27 am |
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EEPROM, and flash are basically exactly the same. Flash is a type of EEPROM. However the term 'EEPROM', is reserved (normally) for memories that have a cell size of 1. Having an erase transistor for every cell, as opposed to a block of cells. This makes them more expensive to make, and bulkier. All memories of this type have limited erase cycles. The actual life depends on the exact cell structure and chemistry. Historically the structure used for EEPROM, was a relatively 'early' form, which had a lower erase life than most modern types.
The memory in the DS1307, is 'non volatile SRAM'. The form used actually does require voltage to hold it's contents, but the voltage needed is tiny, and the capacitances inside the chip and on the board will hold the contents often for days. I remember a guy from Dallas demonstrating that without a battery, and with no supply, normally it'd hold it's contents, even overnight. As such, there is no limit on the write life, but you do need to maintain the battery if you are to be _sure_ that the contents won't be lost.
Best Wishes |
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arunb
Joined: 08 Sep 2003 Posts: 492 Location: India
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RE: |
Posted: Sun Feb 17, 2013 4:31 am |
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Thanks Ttelmah for clearing that up.
thanks
a |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19369
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Posted: Sun Feb 17, 2013 4:57 am |
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I was lucky enough to have a guy from Dallas come and show me the chips when they were launched (admitting how long ago that was would be scarey...). We asked exactly that question about whether the battery was needed.....
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ezflyr
Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA
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Posted: Sun Feb 17, 2013 9:04 am |
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Hi,
I think the premise of the original question is flawed. The Dallas 1307 datasheet refers to the memory on the chip as "Battery Backed SRAM", and does not use the term 'non-volatile' at all. SRAM, or static RAM, is memory that retains its contents so long as power is applied - remove the power, and lose the memory contents! As Ttelmah pointed out, the memory loss is not always instantaneous, but it will go away sooner rather than later....
You need a battery to maintain the DS1307 memory!
John |
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bkamen
Joined: 07 Jan 2004 Posts: 1611 Location: Central Illinois, USA
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Posted: Sun Feb 17, 2013 11:14 am |
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ezflyr wrote: | Hi,
You need a battery to maintain the DS1307 memory!
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I've also seen super-capacitors used for more temporary storage where battery replacement is undesired.
So the tradeoff is that the super cap will keep storage for maybe a week - (depending on capacitance value) and then poof out...
but you'll never have to replace the capacitor.
So if the device is mostly going to be on with temporary power outages and can have the time set easily, a super-cap might be a better choice.
It's up to you. :D
-Ben _________________ Dazed and confused? I don't think so. Just "plain lost" will do. :D |
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ezflyr
Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA
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Posted: Sun Feb 17, 2013 6:12 pm |
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Hi Ben,
Yeah, I probably should have said something more generic like "external power source !
John |
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bkamen
Joined: 07 Jan 2004 Posts: 1611 Location: Central Illinois, USA
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Posted: Sun Feb 17, 2013 7:04 pm |
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No worries.
I just wanted to toss that idea out there... make sure it was covered.
If you had said, "external power source" - I could just see the next question of, "like what?"
So we covered both bases. Score!
Cheers,
-Ben _________________ Dazed and confused? I don't think so. Just "plain lost" will do. :D |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19369
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Posted: Mon Feb 18, 2013 2:01 am |
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ezflyr wrote: | Hi,
I think the premise of the original question is flawed. The Dallas 1307 datasheet refers to the memory on the chip as "Battery Backed SRAM", and does not use the term 'non-volatile' at all. SRAM, or static RAM, is memory that retains its contents so long as power is applied - remove the power, and lose the memory contents! As Ttelmah pointed out, the memory loss is not always instantaneous, but it will go away sooner rather than later....
You need a battery to maintain the DS1307 memory!
John |
The key here is which data sheet you have.
The original Dallas sheet, does not mention battery anywhere in relation to the SRAM. It just calls it '56 byte nonvolatile RAM for data storage'. The later Dallas sheet instead calls it 'NV SRAM'. Only the latest sheet has the line in the next column '56-Byte, Battery-Backed, General-Purpose RAM
with Unlimited Writes'. Now, this later line was obviously added because given the existence now of RAM with attached EEPROM, which automatically keeps itself backed up by writing to the EEPROM, and is quite often called 'NV SRAM', there was a potential confusion.
The RAM keeps it's contents down to about 0.5v, and once you drop below a couple of volts, the oscillator stops, so the holding current drops to the order of just 10nA. Even 1uF, will deliver this for over a couple of minutes (which is why on things like the PC, you can change the battery without losing the configuration data). You'll find quite a few bits of kit with messages like this "when changing the battery, insert the new one within xxx seconds, to avoid loss of data". Provided you don't want/need the clock, you can extend the 'hold time' massively on things like a super capacitor, by switching off the oscillator yourself.
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arunb
Joined: 08 Sep 2003 Posts: 492 Location: India
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re |
Posted: Tue Feb 19, 2013 6:58 am |
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Quite right, the Dallas specs mentions it as battery backed NVRAM.
For my project it does not matter as I use the rtc as a battery backed stopwatch.
thanks
a |
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