(Topic ID: 223281)

Nvram vs cmos in system11

By Tuukka

9 months ago

Topic Stats

  • 7 posts
  • 4 Pinsiders participating
  • Latest reply 9 months ago by Tuukka
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    #1 9 months ago

    I was wondering how long the FRAM type Nvram will last in typical pinball conditions. The FRAM is specified for a trillion, or 10^12 reads/writes which seems plenty enough.

    Currently my only game at home is system 11b Bad Cats, but I think WPC doesn't differ very much from it in RAM access - I will check this when I visit my gameroom with WPC games.

    Measured from the RAM /CS pin, System 11b CPU accesses its RAM about 100k per second. Considering FM1608 is specified for 10^12 reads/writes, this means in system 11b the FRAM will last 10^12 / 100000 = 10 million seconds. This means 2777 hours, or 116 24h days.

    Of course, if you only keep the game poẃered on for 1 hour/day then the FRAM lifetime would be over 7 years.

    Now with the CMOS RAM: Backup battery drain current on original System11b board was 3.6 uA with RAM chip in place, and 2.8 uA with RAM chip removed. Replacing the leaky Schottky diode D1(1N5817) with a 1N60 reduced leak current and consumption dropped to about 1 uA with RAM in place.

    With this 1uA consumption, expected lifetime of 2000 mAh AA batteries is over 200 years And you dont have any lifetime limitation on access cycles. Just stay clear from known to leak batteries (e.g. Duracell) and replace at the "best before" date.

    #2 9 months ago

    This reads more as your favoring batteries. The real question I have to ask is, what does it matter? Unless the machine is routed most will never hit the write limitations of nvram. For operators, the cost of nvram is moot. For owners, the cost of nvram is moot if it fails. For all of us we are more concerned with the game failing in general because they so often do.

    Also if nvram fails the game ceases to work, but is easy to fix. If batteries leak the game may still work, but it becomes a question of when it fails. Fixing it is beyond the scope of all but the most skilled. Yes, most of us can fix the issues preventing the machine from working, but once masking is damaged or traces have been eaten you won't see 99 percent of collectors being able to repair it back to new.

    #3 9 months ago

    Yes I admit that replacing a failed nvram is an easy job compared to cleaning the damage after leaking batteries. Thanks for the thought!

    #4 9 months ago

    I've seen many many batteries leak before the best before date.

    #5 9 months ago

    Cypress guarantees 100 trillion read/write on the fm16w08 device. An endurance cycle is counted every time the /CE is strobed. You could check the frequency of the chip enable pin then figure out how long it would take to reach 100 trillion cycles.

    I check Stern Hot Hand NVRAM /CE and it average 16khz in attract mode
    100,000,000,000,000 / 16,000 = 6,250,000,000 seconds worth of endurance.
    31,536,000 seconds in a year
    6,250,000,000 / 31,536,000 = 198 years of endurance if powered on 24/7

    I checked Dr Dude (Sys 11C) NVRAM's /CE and it cycled at roughly 130khz in attract mode

    100,000,000,000,000 / 130,000 = 769,230,769 seconds worth of endurance.
    31,536,000 seconds in a year
    769,230,769 / 31,536,000 = 24.39 years straight if powered on 24/7

    I suspect the chips will even last beyond their rated endurance. I questioned Cypress about why the older datasheets showed less endurance than the current datasheet of the same part number. They told me the part did not change but they are now able to guarantee the longer endurance. So the older parts would have had the same endurance as new ones even tho the datasheet at the time did not say so.

    Also considering that there are thousands of FM1608 used pulls out there in operation and I have never heard of one failing so i'd say they are pretty robust and the endurance shouldn't be much of a worry to anyone.

    Also an interesting snippet from the data sheet.
    Internally, a F-RAM operates with a read and restore mechanism. Therefore, each read and write cycle involves a change of state. The memory architecture is based on an array of rows and columns. Each read or write access causes an endurance cycle for an entire row. In the FM16W08, a row is 64 bits wide. Every 8-byte boundary marks the beginning of a new row. Endurance can be optimized by ensuring frequently accessed data is located in different rows. Regardless, F-RAM offers substantially higher write endurance than other nonvolatile memories. The rated endurance limit of 1014 cycles will allow 150,000 accesses per second to the same row for over 20 years.
    recent revisions of NVRAM.Weebly 6116 and 5101 modules have the address bits organized in way that should spread out row access to increase endurance.

    #6 9 months ago

    Hmm, your Sys11C /CE rate seems about the same as what I measured on Sys11B - about 1 x 10^5 per second.

    According to data sheet, FM1608 is specified for 10^12 cycles.
    10^12 cycles / 10^5 cps = 10^7, or 10 million seconds.
    10 million seconds is 2777 hours, or 116 days.

    #7 9 months ago

    OK, I see that latest data sheet specifies a 10^14, or 100 trillion cycles lifetime. My calculations were based on the older data of 10^12 cycles.

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