Is this a good design?

Hi,

I have a circuit to do the controlling and those major parts included a MCU, CMOS SRAM (CY62128B), ROM, power management IC and a rechargeable battery to back up the CMOS SRAM once the main supply is off.

The SRAM should be working fine when the main supply is on since those pull-ups/pull-downs to the SRAM pins are functioning. But when the main suppply is off, the battery is only provide the supply to the SRAM only, hence all the parts are off and SRAM pins are left floating at this moment, would this stand a chance to corrupt the data in the SRAM when all pins left floating?

The idea to use a battery to back up the SRAM is to do data retention. Thank you.

If the circuit's function is to double post to sci.electronics.design, congratulations, it works great!

You need to check the RAM's data sheet. It is never a good idea to let pins float. Even if you held the enable and W/R stable and the others wouldn't matter (that's what the data sheet or the manufacturer's application engineers should say) any drifting pin on a digital device can cause the stage behind it to cross-conduct a little. IOW, it can leak current and deplete your battery faster than you had calculated.

Your SRAM pins aren't truely "floating" in this case, they are connected to the bus and other chips that are powered down. So there will be some "off-state leakage" on each pin pulling it to ground. This is common in this application and usually works just fine. But make sure you read the data sheet to ensure that your power down sequence is correct to prevent corruption when the power switchover takes place.

Dave :)

The clean way to do this is to get a reliable powerdown reset signal, as from a standard microprocessor reset chip (you need one anyhow) and use it to force the sram chip-select pin inactive. And one easy way to do that is to use a cmos logic gate that's also powered by the battery. Your ram chip has two chip selects, so you can probably connect the por signal directly to one of them.

A lithium battery may well outlive a rechargable one here. I have checked lithium coin cells that have been in the field for 10 years, doing uP backup, and they seem to have about the same mAH capacity left as new ones do. Most rechargables die in a few years.

Welcome.

John

The lithium backup battery in my Yaesu FT726 lasted something like 15 years. Since replacing it would have meant disassembling the radio, I pulled a jumper and wired in a 3 x AA Alkaline pack. That is still working after 5 years.

Tam

Hello John,

Agree. But there seems to be a huge quality and durability difference between NiCd. The ones in the HP4191 here in the lab lasted well over 25 years. The ones in the old Dolch logic analyzer really blew me away. It is also at least 25 years old, bought it used and the battery is in a fancy enclosure potted up with some electronics in there. Since I mostly do analog designs I hadn't used it in about four years. Then a digital debug came along, fired it up, the message "Non-Volatile Memory Test Passed" appeared and all my old settings from four years ago popped up. I was impressed.