Origin of the "CMOS" battery - Page 1
Forum section: electronics / general

author: ledtester
author_rank: Super Contributor
timestamp: February 25, 2025, 07:59:39 pm
content: Does anytime have any memories of why the coin cell on PC motherboards was called the "CMOS" battery?

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2025, 08:09:14 pm
content: When uneducated PC users (more likely PC magazines) started referring to the battery backed cmos memory storage for BIOS parameters as just 'CMOS'?

author: edpalmer42
author_rank: Super Contributor
timestamp: February 25, 2025, 08:35:31 pm
content: The first PCs didn't have any form of real-time clock.Later, when the clock was added, it required a battery. The clock was in a single chip and happened to include a small number of bytes of system-accessible, rewriteable memory that would persist over a power cycle. This memory was implemented in a technology called CMOS (Complimentary Metal Oxide Semiconductor) which was famous for having extremely low battery drain - in the nanoamp range. So, a simple coin cell was used to run the clock and keep the memory alive.Over time, the real-time clock function was kind of forgotten - even though it's still there - and it was just called 'the CMOS battery'.

author: ledtester
author_rank: Super Contributor
timestamp: February 25, 2025, 09:21:05 pm
content: What chip technology was used in the early PCs, ie. Altairs, PETs, Apple II, IBM PC?

author: oPossum
author_rank: Super Contributor
timestamp: February 25, 2025, 09:27:46 pm
content: Mostly NMOS and TTL.A notable exception would be CMOS SRAM (an alternative to NMOS DRAM).

author: Benta
author_rank: Super Contributor
timestamp: February 25, 2025, 11:18:37 pm
content: Those button cells were (and are) a real PITA. They always conked out at the wrong moment. And then you had to disassemble the PC.I remember the best solution ever, which was the Amstrad PC. It had a small lid on the top concealing two normal AA cells for the BIOS/RTC.Swapped in 10 seconds.But that was back when the UK actually produced things.

author: tom66
author_rank: Super Contributor
timestamp: February 25, 2025, 11:49:45 pm
content: I reckon it came about because people referred to the memory that the BIOS used as 'CMOS memory', which was probably true when the 'BIOS' was a separate device rather than just an SPI flash chip. Such a chip would indeed have a small region of memory that would be implemented using CMOS technology.Nowadays it is all EFI and I'm pretty sure the SPI flash that boots the system is just a data store. Taking the battery out only resets the real time clock. Most laptops don't even have 'CMOS batteries' any more, they just use the main battery all the time, there's usually enough charge to keep the clock going even if it can't support system boot.

author: brucehoult
author_rank: Super Contributor
timestamp: February 26, 2025, 02:15:44 am
content: Quote from: ledtester on February 25, 2025, 09:21:05 pmVery early microprocessors such as the 4004, 8008 used PMOS. The 8080, 6800, 6502, z80, 6809, 8086, 68000 used the faster switching NMOS. In the early 80s there was a switch to lower power CMOS e.g. the 65C02 in 1983 and the Z84C00 (CMOS Z80) in 1985. The 80C86 was available in 1982, and the MC68HC000 was in 1985.As far as computers, the Apple //c semi-portable computer was perhaps the first to use the 65C02, in early 1984. The 65C02 was back-ported into the "Enhanced //e" more traditional PC the next year. The IBM PC/AT also in 1984, four months after the Apple //c, used the 80286 which was CMOS from the start.Also the Epson HX-20 portable in 1982 used a CMOS version of the 6800, so it wasn't compatible with any consumer PC line.Macintoshes didn't get CMOS until the Mac Portable in 1989. The 68000, 68020, 68030 were all NMOS with the 68040 being finally CMOS.

author: Ranayna
author_rank: Super Contributor
timestamp: February 26, 2025, 08:17:03 am
content: Quote from: tom66 on February 25, 2025, 11:49:45 pmInteresting thing though, is that taking out the battery from a decently modern mainboard (Socket AM4 with B550 Chipset) does indeed reset the UEFI settings.So, has this been implemented as a "compatibility" behaviour, or are the settings really still stored in volatile memory?Notebooks are generally different there, for security reasons. They are easily stolen, and at least some things like UEFI passwords are not lost, not even on a deliberate reset. And yes, modern notebooks do not have a CMOS battery anymore. Which can be annoying if the main battery is dead.

author: woofy
author_rank: Frequent Contributor
timestamp: February 26, 2025, 09:00:54 am
content: Quote from: Benta on February 25, 2025, 11:18:37 pmThose AA cells didn't last long. I had both a PC1512 and the PC1640 and I was changing the batteries every few months. Terrible things.

author: Psi
author_rank: Super Contributor
timestamp: February 26, 2025, 10:13:30 am
content: Quote from: ledtester on February 25, 2025, 07:59:39 pmThe number of people who knew what CMOS stood for was a very small fraction of the number of people that changed BIOS settings on computers.The bios menus of motherboards always said "CMOS Setup Utility" or something to that effect as the title.So people who didn't know what CMOS stood for thought it just meant bios and so started to use the words BIOS and CMOS interchangeably. So CMOS battery just means BIOS battery.

author: elektryk
author_rank: Regular Contributor
timestamp: February 26, 2025, 12:07:16 pm
content: Even worse than batteries were RTCs with battery integrated.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 26, 2025, 12:26:06 pm
content: Quote from: elektryk on February 26, 2025, 12:07:16 pmYeah the infamous Dallas non-volatile stuff. The definition of planned obsolescence.I once replaced one of these on a motherboard.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: February 26, 2025, 05:42:16 pm
content: QuoteSame, and the replacement died too a couple years later so I found some hack to literally hack the top of that thing to access the dead battery wires and solder on some leads to a socket for an external battery.On this topic, at work one of our groups has a very expensive Lecroy 8 analog + digital scope (circa 2018) that I borrowed last week to help troubleshoot something when 4 channels is not enough. They had not used it for a while and the RTC battery had died so it would not boot up. This thing runs some version of windows for a OS. I could not get into the bios to set it as they had a password on thatNobody in the department knew anything. I eventually found out what the PW was on some obscure website but it took several attempts before it actually booted up. I didn't dare shut it off for 3 days until I was done using it. Handed it back to them with the story. They did crack it open and found the CR2232 coin cell someplace. A very $ piece of gear bricked for a $5 battery.

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2025, 06:07:35 pm
content: Quote from: brucehoult on February 26, 2025, 02:15:44 amEr, no. Intel struggled mightily trying to get CMOS to work well, when numerous others were already in high volume. The 80286 was NMOS, and that was used in the PC/AT. AMD and Harris both made CMOS equivalents of the 80286. It was that which lead Intel to associate itself with AMD, on a "you can get access to the x86 architecture without us suing you, if you give us access to your CMOS processes." basis. The 80386 was also NMOS, as Intel were still struggling at that point. By the next round the whole industry had mastered CMOS.CMOS took a long time to get established. The earliest stuff worked well for low speed, low power applications, but there was a nasty power spike during switching, when both the N and P MOSFETs were turned on for a moment. As the clock speed was cranked up the percentage of time both transistors were on increased, until the power consumption was worse than NMOS. Then some pioneers, like Mitel, worked out ways to implement isolation barriers, and get better control over the N and P transistors, and as that knowledge spread CMOS became the only sane way to build anything for even the highest speed applications.

author: abeyer
author_rank: Frequent Contributor
timestamp: February 26, 2025, 06:18:56 pm
content: Quote from: brucehoult on February 26, 2025, 02:15:44 amDon't think that's correct. I'm pretty sure at least by the 68020 (maybe even the 68010, though that generation wasn't used in any macs to my knowledge) they were on the same process that the 68HC000 used.

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2025, 06:26:25 pm
content: Quote from: abeyer on February 26, 2025, 06:18:56 pmMemory says the 68020 was the first 68k family part to start out in HCMOS. The other's were reworked later, like the 68HC000, depending whether the sales volumes justified the upgrade or not. A quick Google seems to confirm that, although material on the web can be pretty vague about the processes used for a lot of old parts.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 26, 2025, 07:11:26 pm
content: Quote from: Psi on February 26, 2025, 10:13:30 amYeah. Technical terminology is full of cases like this. It only takes one idiot who has absolutely no idea whatsoever what they are talking about, but can't resist opening their mouth, at the exact right moment, in right place, to introduce mis-terminology for decades to come.Engineers of course hate it when management defines that totally incorrect terminology must be used "because it's always been that way". Especially in the start when literally no one agrees with the claimed "popularity". Later that argument becomes true, however.Other typical example is "LiPo"/"Li-Polymer". Such thing does not exist, and never really existed commercially, yet a few actual serious battery manufacturers use this term, because some manager thinks it must be used.And to be fair, I only blame the initial idiots. After the wrong term istrulyin widespread use, the damage is already done. It takes progressively more and more courage to invent a new term, because it causes a risk of unnecessary new confusion; the cure often is much worse than the disease. And on the other hand, the original mistake might be so well contained (being sand-boxed in totally different context) that it doesn't cause any actual confusion, just irritates pedants (0.001% of population) like myself to no end.

author: edavid
author_rank: Super Contributor
timestamp: February 26, 2025, 07:15:51 pm
content: A less angry way of looking at it is that this is one way that we form new words in English. CMOS is a new word that was invented to mean "nonvolatile settings on a PC". It happens to be the same as an existing word, but lots of English words have multiple meanings.How do you form new technical words in Finnish?

author: thm_w
author_rank: Super Contributor
timestamp: February 27, 2025, 12:56:13 am
content: Quote from: Ranayna on February 26, 2025, 08:17:03 amYou can't say for all cases, but, since the board has RTC IC it will have some memory associated with that. So that memory would be cleared if voltage was lost. On your board the pin header might be called "CLRTC". I checked on a B450 here and the RTC wasISL1208. Which only contains a tiny amount of SRAM, not enough for settings.I believe the settings are stored in the chipset, or maybe some in the EEPROM itself. Can't seem to find a datasheet for B450/550 but here is one for Intel PCH which discusses CMOS RAM in some depth:https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/9-series-chipset-pch-datasheet.pdfSome things will be exempt from being reset during a bios reset as well (security keys, etc, see below).https://www.howtogeek.com/180798/where-is-the-bios-stored/https://www.reddit.com/r/ECE/comments/4n4uom/where_are_the_default_bios_parameters_stored_if_i/https://superuser.com/questions/1344466/why-does-bios-use-battery-backed-memory-instead-of-storing-settings-in-eeprom

author: amyk
author_rank: Super Contributor
timestamp: March 01, 2025, 06:59:20 am
content: The IBM PC/AT was the first to introduce it, and used a Motorola MC146818 RTC that contained 64 bytes of battery-backed RAM. Itsdatasheetrepeatedly refers to it as having "CMOS RAM", and that's where the term originated. Later models maintained compatibility but expanded the capacity to 128 and then 256 bytes.I'm not familiar with the more recent AMD platforms, but on Intel ones, the RTC/CMOS has always been part of the low-speed Southbridge/PCH.Here's the latest Intel 800 series PCH datasheet section on the RTC - it's still compatible with the MC146818:https://edc.intel.com/content/www/us/en/design/products/platforms/details/arrow-lake-s/800-series-chipset-family-platform-controller-hub-pch-datasheet-volume/real-time-clock-rtc/

author: brucehoult
author_rank: Super Contributor
timestamp: March 01, 2025, 09:48:17 am
content: Quote from: edavid on February 26, 2025, 07:15:51 pmThose of us from another tradition call this the "PRAM".Zap your PRAM, man! cmd-option-P-R

author: coppice
author_rank: Super Contributor
timestamp: March 01, 2025, 12:30:43 pm
content: Quote from: amyk on March 01, 2025, 06:59:20 amThat's because the PC is so old it dates from a time when the only reasons you used CMOS were radiation hardness, wide operating voltage range, or being able to run something from a small battery.

author: tom66
author_rank: Super Contributor
timestamp: March 01, 2025, 01:27:31 pm
content: It might well be the case that the CMOS RAM is only used to store a magic 'key' that when reset causes the EFI to erase the existing settings in flash/EEPROM or load some factory ones. So the total amount of memory doesn't really matter.

author: coppice
author_rank: Super Contributor
timestamp: March 01, 2025, 01:35:22 pm
content: The CMOS clock and memory themselves have never been much trouble. However, the batteries can be a PITA. All sorts of quirks happen as those batteries age, even when they still measure close to 3V. Some machines keep resetting the contents of the CMOS RAM. Some won't let the machine shut down properly, and it just restarts after a few seconds, like you have initiated a reboot. Usually the clock still keeps good time, which fools people into thinking the battery must be OK. You replace the battery and the machine is rock solid again.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2025, 11:43:11 am
content: It wasn't always a coin cell. I remember having a PC which used rechargeable NiCad cells. There were two or three (I don't remember) in heat string, with tabs tack welded to the cells and soldered to wires which plugged into a header on the motherboard.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 04, 2025, 01:10:24 pm
content: Quote from: Zero999 on March 04, 2025, 11:43:11 amThe first compact Macs had a 4.5V battery that was accessible from the outside. I don't remember if it was Lithium or Alkaline. Unfortunately, it was a kind of battery that was relatively hard to find when it needed replacement.

author: artag
author_rank: Super Contributor
timestamp: March 04, 2025, 01:38:40 pm
content: Quote from: Zero999 on March 04, 2025, 11:43:11 amAlso the non-rechargeable lithium thionyl chloride cell, often branded 'Tadiran' and encapsulated in a plastic box a bit bigger than an AA.

author: artag
author_rank: Super Contributor
timestamp: March 04, 2025, 01:42:13 pm
content: Quote from: PwrElectronics on February 26, 2025, 05:42:16 pmNot only the Dallas chip. I have a Datron 7.5 digit multimeter with an external cell, now very dead. Consequently all the calibration data is lost. I think earlier Datrons were better, using a true nonvolatile for the calibration but a battery for the user memories.

author: artag
author_rank: Super Contributor
timestamp: March 04, 2025, 01:45:42 pm
content: Quote from: woofy on February 26, 2025, 09:00:54 amInteresting. I didn't have an Amstrad but would have expected the AA cells to have given far longer life than a coin cell and cheaper than a Tadiran call. If they didn't last long it suggests something badly wrong (perhaps in the design) with the memory protection circuits. Perhaps why they had to use AA cells in the first place.