Battery backup?

edited June 2009 in WTPA1 Mods
So is there a reasonable way to keep the samples on the SRAM?


  • Hey! Here's an answer I gave somebody via email to a similar question:

    1. What would it take to buffer the SRAM, so it would keep the sample-content after switching off the power-supply?

    You'd need to keep power applied to the the SRAM -- it isn't a difficult circuit, some applications do this with a small coin cell battery, and you could isolate the power going to the SRAM with a schottky diode. However, that SRAM chip draws 10mA even when idle, so it would eat a small battery really quickly. To really keep that SRAM active for any long period of time (days, for instance) would really take a lot of stored energy (big batteries).

    The RAM data retention voltage is around 2.0v, so a 3v or greater battery supply would be able to do it, but it'd need to have some mAh -- a coin cell couldn't swing it for very long at all. A supercap couldn't for much than a couple hours either. You might end up using some rechargeable NiMH cells, but no matter how you swing it I bet you'd have a hard time getting samples to keep for any longer than a week. WTPA wasn't really designed to have permanent memory.
  • Interesting. I would only be interested in an overnight thing so maybe I would add a switch to keep the memory. Could you possibly schem it up?
  • Sure. Keep bugging me about it. I can do it this weekend.
  • Surely if its just overnight, then you may as well leave it plugged into a mains adapter?

    Just my 2 bits worth.. Paul
  • Hi Todd,

    Just wondering why you chose the Cypress SRAM chip to do the sampling in the WTPA? I ask this for 2 reasons:

    There is discussion on the VSE forum about RAM expansion. I suggested about expanding the sample RAM by simple bank expansion (via chip stacking), having already done this type of memory expansion to my Casio SK-8 and Alesis MMT8 sequencer.

    The trouble is, the Cypress chip doesn't seem to conform to the standard SRAM pin layout convention, so I can't really use any of the other popular standard layout SRAM chip out there easily for the expansion (BSI BS62LV4006, Hitachi HM628512A etc).

    That's still OK though - we could just use the same cypress chip and stack them.

    The other reason I ask is that using those other standard SRAM chips I mentioned, you could implement memory battery backup successfully, as they have a standby current drain of less than 2uA - which a button cell lithuim battery could operate for at least a year.

    I haven't come across that cypress chip before in my travels - it's the first one I've seen laid out like that.

    Would this be possible in future board revisions, or is it too big of a re-design of the board to do it? It would be neat to have battery backup.

    The other implication is that, with using the standard SRAM configuration, it shares this same configuration with EPROM chips (eg ST Microelectronics M27C4001) which means you could hypothetically burn some favourite samples to EPROM with an EPROM burner and plug them in as a sample cartridge.

    Just dreaming, hehe!

    Cheers, Graham
  • Hi Graham,
    I chose that Cypress SRAM because it was cheap, fast, high capacity, 5v, and available from a ton of suppliers.

    I didn't know about those Hitachi or BSI chips! I got the datasheets on them and they look to have serveral advantages -- the low power is one, however the fact that they are available in a DIP package is probably more important. I'm tired of soldering those SOJ packages for people.

    It looks like they aren't quite as fast as the Cypress (not a big deal for WTPA) and they're more expensive, and more importantly they're both a pain in the butt to source. I can turn up about 1000 of the BSI chips at Newark/Farnell but that's pretty much it, and I can't find the Hitachi chips anywhere (that doesn't suck).

    Where would you recommend getting them? It's definitely worth considering replacing the RAM in the next revision, for power concerns, but also just to make assembly easier.
  • Hi Todd,

    Jameco also sell the BSI and the Hitachi, but don't mention bulk quantities. I think the Hitaci is old stock, Jameco seem to prefer to stock the BSI. You can ask them if they'll do 1000+:

    Regarding other standard layout and low power (but still SMD) SRAM, there are other places like mouser, who do large orders:

    Also Digikey:

    Regarding the Cypress chip, I notice that Samsung also have an equivalent chip with the same layout and high speed - and also high power drain. I suspect the power drain is a trade off for the high speed. I think that these are now the current generation SRAM chips by the look of it.

    I hope you go with the BSI DIP package in the next revision, too, and that you can get a bulk supplier. It's so much easier using sockets, especially when you're selling kits and trying to troubleshoot for customers, as compared to what you have to do now ( which way did you solder in the SMD SRAM chip? Oh, THAT way...). And also for the low power/battery backup option.

    Cheers, Graham
  • Hey Graham (or anybody),
    So I'm starting to redesign WTPA for the next version, and I'm looking for a list of "standard pinout 4Mbit SRAM" chips. If you have a line on one of these, that'd be great.
    I've contacted Brilliance Semi directly and will be curious to see how that works out, but I'd love to look elsewhere in the meantime.

    Frankly, if you have a lead on ANY 4Mbit RAM in a DIP package, I'm interested. It's mostly the DIP part that I'm interested in and not the compatibility.

  • Todd, I'll ask around town. I know a few guys who have been doing this DIY thing for a while; maybe they can help. I'll get back to ya.
  • Cool, thanks. Having the option to use a DIP package would solve some problems. If I can make it work out and not suck and not have to buy a boatload of parts (I just asked BSI for a quote for 1000 which is about 800 more than I want, but if they're cheap enough I may do it anyway). Farnell in the UK has these chips, too, and it may make more sense to just buy a grip of them there.
  • Hi Todd,

    Sorry for being slow in getting back, just spent 2 weeks with the whole family being sick. It's nice to be able to keep my stomach contents again (ewwwww!!!)

    It seems that a far as DIP packages go, the only ones out there I can find are the BSI and the Hitachi. There are also the NVRAM chips in DIP packages (Texas instruments BQ4015Y and also ST microelectronics equivalent) but they are way too expensive (about $20) so I don't think they would be an economical contender, despite them being more widespread than the normal SRAM. They are, however, standard SRAM layout, and already have a backup battery built into them, so I guess in the event of a post-nuclear holocaust where people are living underground and the sky is a blood red and all the worlds's manufacturers of SRAM are wiped out then you could use them as last resort :D I suppose you could justify them a bit more if you said, "well, to add a battery to the WTPA kit is going to cost this much per unit, the battery holder this much, the diode and any other components this much" and it might reduce the difference between them and the normal BSI SRAM.

    You would have to check one first to see if it actually worked in the WTPA before going down that NVRAM route. I tried them in my Alesis MMT8 sequencer memory expansion project and the machine would not work at all with them - locking up, crashing, garbled screen. Pull them out and drop the BSI's in - perfect. Yet they performed perfectly in my Casio SK8 expansion. Weird!

    Cheers, Graham
  • Thanks for getting back, and glad you're feeling better.
    These are totally in line with my findings, as well. Farnell UK has a bunch of BSI chips and the price is right, but nowehere else does. This makes me hesitant to use them. I can't find the Hitachis for cheap enough, and the NVRAM is really overkill for WTPA.
    Cypress makes a low power SRAM in an SOIC package which is really cheap -- but again I can only find it (for cheap) at one distributor. It's better in 3 ways -- package is easier to solder, price, and quiescent current -- but it still isn't DIP.
    I think what I will likely do is change the footprint of the SRAM to allow the use of some "normal" RAM like the current model, and then add a parallel footprint so that it is possible to use one of the BSI or Hitachi chips INSTEAD (the pinouts are different but the read/write sequences are the same). Then I'll change the "TMB solders the SRAM" purchasing option to "upgrade to an expensive DIP chip and another socket" option. It'll probably cost the same.
    I don't relish the annoying PCB routing that this will require, but it seems the best option as it helps keep WTPA from getting too rare chip-specific, allows people who aren't confident with SMT soldering to bail out, allows WTPA to be more repairable, and keeps me from spending hours doing tedious work which I can't test before shipping.

    What do you think?
  • edited August 2009
    Hi Todd,

    I found another SRAM in a DIP package, an AS6C4008-55PCN from Alliance:

    These look like a direct substitute for the BSI, and are also from Farnell. Maybe this could bolster the total DIP chip supply numbers for the WTPA.

    I agree with you on the new board layout strategy. That would pretty well future-proof the board for all RAM types, and enable the battery backup system when the "upgrade to an expensive DIP chip and another socket" option is chosen - you will have to also inform people that if they want battery backup then they MUST choose this option - not available on the "DIY solder RAM in" option, as that chip is the high drain one.

    From your perspective it's going to save you a hell of a lot of soldering time once implemented - and these kits are only going to sell faster in this new release with all the extra options etc. so it looks like the only way you CAN go - unless you can train a hoarde of monkeys in the basement to do your soldering for you!! So it's definitely worth the hassle of reconfiguring the board for the 2 chips. Also, check to see if mounting the DIP chip on the reverse side of the board would make the reconfiguring easier - just a thought.

    Try one of your present boards out with the BSI chip - just hotwire a socket to the cypress chip tracks with short wires - for the reasons I mentioned with my NVRAM troubles experience. I'd hate to see you go to all that redesigning to find out that the DIP chips just for some inexplicable reason refuse to work in the WTPA board. These chips are significantly slower than the cypress ones, which may not matter, but you'd want to try one out to be sure.

    Itching to see it all work! I'm definitely getting one of these new ones!

    Cheers, graham
  • Hot damn. Graham, you are in line for a whole box of Narrat1ve coffee mugs!

    So I did some poking around and I found a source in the states (Future) for those Alliance chips, and they have thousands, and they're cheap.
    Thanks so much for all this legwork. If these chips work out, I'll probably just move to DIP-only and lose the Cypress part entirely.
    The memory access is still fairly slow, which may necessitate changing some bus timing, but it isn't a big deal.

    Thanks. You rule,
  • Hey, that's great news! Hope the speed issue works out easy, I'm really looking forward to these new DIP boards (and I'm sure you are too, heheh!)

    Cheers, graham
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