The lifetime of DS1307 NVRAM

arunb · Joined: 08 Sep 2003 Posts: 492 Location: India

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

Ttelmah · Joined: 11 Mar 2010 Posts: 19335

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

arunb · Joined: 08 Sep 2003 Posts: 492 Location: India

Thanks Ttelmah for clearing that up.

thanks
a

Ttelmah · Joined: 11 Mar 2010 Posts: 19335

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.....

Best Wishes

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

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

bkamen · Posted: Sun Feb 17, 2013 11:14 am

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi Ben,

Yeah, I probably should have said something more generic like "external power source Very Happy

!

John

bkamen · Posted: Sun Feb 17, 2013 7:04 pm

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

Ttelmah · Joined: 11 Mar 2010 Posts: 19335

arunb · Joined: 08 Sep 2003 Posts: 492 Location: India

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