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| Why not CMOS? |
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| Siwastaja:
Battery-backed SRAM is still a thing. Thanks to EEPROM and flash, not as much as it used to be, but still. Reason is simple: many applications require real-time clock. To have that, you need to run 32.768kHz crystal and clock at least a few dozen logic gates and registers to keep time, and this will require significant enough power to require a battery cell. Total solution cost and PCB area is already significant. Once you have all that, adding a kilobyte of SRAM comes for almost free, relatively speaking. Both from price and power perspective. This battery-backed RAM is available in many microcontrollers that come with RTC peripheral. Even simple external RTC chips come with a few user accessible registers if not kilobytes of RAM. Writing to flash quickly at powerdown is problematic. Erase cycles take time and power beyond the capability of small electrolytic or MLCC capacitors. Battery-backed SRAM is trouble-free, at least until the battery finally is drained and then it becomes a trouble generator. |
| PlainName:
I have a programmable mains timer which uses a tiny NiCd cell to maintain memory. So long as it's plugged into the mains once a year or something it's fine. Worst case, you shove it in a drawer for so long that the battery actually runs flat, but then you just leave it plugged into the mains for a few hours to recharge, and then you can hide it down the back of the sofa for another year. Flash is OK, but look at Tesla - writing to flash like it was SRAM with the result that cars were borked after not very long at all. You'd think the designers would know what they're doing, but apparently some of them had never heard of flash wearing out. Result: they were made to recall 158,000 cars to fix... |
| Benta:
--- Quote from: Siwastaja on January 29, 2022, 05:29:10 pm ---Battery-backed SRAM is still a thing. Thanks to EEPROM and flash, not as much as it used to be, but still. Reason is simple: many applications require real-time clock. To have that, you need to run 32.768kHz crystal and clock at least a few dozen logic gates and registers to keep time, and this will require significant enough power to require a battery cell. Total solution cost and PCB area is already significant. Once you have all that, adding a kilobyte of SRAM comes for almost free, relatively speaking. Both from price and power perspective. This battery-backed RAM is available in many microcontrollers that come with RTC peripheral. Even simple external RTC chips come with a few user accessible registers if not kilobytes of RAM. --- End quote --- Absolutely true. But a cost factor also came into play: the EU RoHS directive. This banned built-in NiCd cells in RTC/RAM modules (like Dallas used to offer), so a separate cell holder clip is also needed. |
| Rick Law:
--- Quote from: TerraHertz on January 29, 2022, 03:10:20 am ---Reasons: Cost of the battery, and board space for it. Need to implement power-fail data protection (if using standard CMOS RAM, which doesn't include that.) But most importantly, probably the primary reason in corporate minds: CMOS RAM plus easily replacable, no-leak coin cell, does not meet corporate 'planned obsolescence' objectives. They don't want to sell hardware that users can keep operational forever. They want to make and sell stuff that dies after some years, so the user has to buy a new one. Limited lifetime flash memory is perfect for that. Since few users can diagnose and replace dead SMD chips. Especially if it held the firmware as well as config data. --- End quote --- I use a cheapo $15 USD B3603 digitally controlled CC/CV as my bench supply. The thing still "works" except it will no no longer store calibration data. It still "works" only in the sense that if I ignore the build-in display and use external DMM for V/I display. After testing it every way I can think of, my conclusion is that the EEPROM or flash base calibration data storage failed. That is what made me think why not CMOS to begin with. If only the calibration storage hasn't fail... I had a few of them. I am now using another B3603 purchased around the same time. Perhaps "planned obsolescence" is exactly the reason... I could replace the MCU, but then I would need to bring the firmware over. Yeah, not a thing user can easily do. I am keeping that failed one for part. I got my money's worth out of it even with "planned obsolescence", but still, it is such a garbage-creation unnecessary waste. |
| retiredfeline:
--- Quote from: Benta on January 29, 2022, 10:19:56 pm ---This banned built-in NiCd cells in RTC/RAM modules (like Dallas used to offer), so a separate cell holder clip is also needed. --- End quote --- I have some of those ancient things. They are lithium cells and non-rechargeable. |
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