What is  CMOS  ? Or RTC static NVRAM       

In the dark ages of PC  30+ years old  had this chip or a Dallas;   Later this function moved deep inside the glue logic chip)
  • The PC Realtime clock in your PC goes dead, can't keep time or date for sure in BIOS screens called REAL TIME.
  • The PC beeps or shows CRC errors, in some case the dead battery even makes the BIOS screends dead.
  •  The laptop with the AC power cord and the BIG battery removed, causes, CRC errors or BIOS to fail in any way  (time/date wrong, in BIOS or the BIOS screens acting dead,)
  • I keep getting F2  this happens if BIOS seen TIME or Date wrong or sees CRC wrong, there are no 2 BIOS what work this same here , when the silly battery called COIN cell goes dead.
  • Windows keeps showing Certificate errors. (wrong  date is this battery bad)
  • If the BIOS thinks  password is set and never was used!, check this  battery first. (we use a voltmeter) 2.9 to 3.3vdc is good less is BAD.
To be accurate the BIOS passwords today not stored in NVRAM (was moved first to EEPROM password chip, then later moved to TPM chip 2011? that is not hackable by DESIGN)
It is true pulling all batteries and this coin cell and AC power ,  on relic PCs will reset BIOS passwords. (but never on modern PCs 2011 and newer) (as to do OLD PC's with  Jumper clips marked CMOS_clr, PW-CLR. and the like, not seen TODAY.)
The battery (COIN) seen on all modern PCs, lasts for about 7 years (there is no magic elf inside counting days) The spec life is 5 to 7 years, (per Duracell and Eveready or the Pink bunny rules)!
The very very old PCs had many kinds of battery.
 This battery keeps the NVRAM above (or in the Southbridge) from forgetting all data (a very important job)
Warning: Do not flash your BIOS thinking the vague CRC  errors are caused by BIOS FIRMWARE, no you are wrong thinking that. Buy a battery now and end your pain. (the firmware can stay good for 100+ years by design , ok?)
BIOS only fails after some Nicompoop , flashed the BIOS and did it wrong as most do it seem. (see the 10 ways to do it wrong in the flash link above)

NVRAM coin cell,  as see in most PC made today, finding it on most laptop can be a horror.

The is no need to read my full page on NVRAM , just measure your battery or replace it now. (below 2.9v is a bad battery)
You are here because, the PC BIOS screen,  can't hold time of day (or MM/DD/YY) or BIOS can't save configs,. or BIOS may show  checksum errors in CMOS. (or all 3)
The NVRAM holds RTC = real time clock, data and all BIOS configurations , if the battery dies,  so does NVRAM data !. (NV means non volatile but only if the battery is good.)
NVRAM is CMOS RAM plus a battery.
Replace the RTC Coin Cell battery now, or it will get worse. (end story)
Some old Toshiba laptops used  NiMh rechargeable battery (rare PC's these are)

Warning the below is raw electronics technology. (If really interested in how it works and even PCs as old as 20 to 30 years worked, read on.)

For the curious how things work,, see below: (Do not read this, it's pure electronics and some history at  the end... )

The correct words for this , is BIOS/RTC NVRAM  (or CMOS for short)
We call this battery backed up NVRAM using Static RAM, non-volatile RAM, the coin cell creates that magic. (NV  magic) SRAM + COIN = NVRAM. (SRAM = Static RAM)
This device is seen inside all PCs for 36 years, 1981 to NOW.
Do not read this page below, tech facts., just splurge and spend $2 on a new coin cell battery 2032s are common. (below is Electronics real) The RTC battery is sold at Walmart in the watch department. (show the lady your battery)

Its purpose is  2 fold, one is to store all BIOS settings and yours,  like for example which device boots first?, and 2nd to store the  time of day (&Calendar), this memory section is for the  RTC chip real time clock chip stores TIME in bank 2.
The reasons it's called CMOS is because, it is! (the transistors are CMOS static RAM) It uses almost no current to run and to hold data in standby.  (super super low current in standby)
Complimentary MOS transistors, This is also called a flip-flop or a bi-stable multi-vibrator. (or the basic binary memory CELL) CMOS uses floating gates. (see them)? If VDD drops too low, the gate thresholds are hit and it fails.
The transistor M1 to M4 repeats 2048 times  256x8 memory array.  In standby (PC off) the M5/6 are off, and the gates all hold there charges unless VDD falls to low.
The beauty of this cell is statically it uses almost zero power (Idd drain near zero) that magic is how the battery can last for 7 long years.  The battery drains faster  if all power to the PC is dead.  PCs never fully turns off so the coin cell lasts 7 years for  sure.
This  memory cell stores is 1 BIT.  It uses as little as 0.5uA in standby current Idd , allowing the coin cell to last a long time, PC turned off. (PC turned on it lasts longer)
A nice mosfet transfer curve is below:n (Drain current ID, and VGS (gate voltage )
As you can see if the gate was at 5v charge, some where below 2vdc the flip will go nuts. (toggle in a random way) The below is the typical MOS transfer characteristics.( a large one)
This is not our transistor ,ours is very tiny and has very unique voltage response. In fact is designed to run with weak VDD. (dain) with a weak Gate voltage VGs)
The beauty of this device,  is it can run with Vdd as low as 2.0vdc( on average), (Vdd is coin cell voltage , minus 1 diode drop) the second beauty is the super low current in standby,  really nothing beats this.
The battery has  diode (figure 3 above) that loses 0.5v so 3.3v battery is really  2.8v to the  RAM chip, or if the battery is at 2.9v  the RAM sees only 2.4v and this is very very close to failure point of RAM.
When the PC is on  VDD is 5.0v  on our SouthBridge Chip, intel. ICH9/10 are common on 10 year old PCs with dead battery.
Checkout this super low VDD version SRAM from Philips Semi. (Vdd min,  1.0v !) (unlike my PC)
If you let the coin cell go bad or below 2.9v, the memory above CELL "forgets every thing it ever knew", and the BIOS ROM and the RTC goes nuts, (scrambles) loss of time, data   and CRC errors abound.
Do not panic, the new battery installed go to BIOS page1, and click set factory BIOS defaults, and you are a GOOD to GO.!
 The BIOS may tell you BIOS corruption because the NVRAM is now corrupted.  (DO NOT FLASH YOUR BIOS FOR THIS SILLY COMMON FAILURE OR RISK BRICKNG YOUR PC)
This means the battery is dying fast, below 2.9vdc on the battery , means it's on the CLASSIC  cliff of total death. Put in a new coin cell, end this minor horror.
This can happen every 5 years, buying a top makers COIN is a smart idea.  (avoid no name batteries as seen on fleabay for $1 , like the brand "SOM Ting WONG")
Buy the battery with the cute bunny rabbit? or Duracell (avoid NOS, new old stock on any battery) I like to get mine  at Walmart due to fast change over in stock ! (read FRESH) most batteries have no date codes sadly, so is a crap shoot on fleabay !

CMOS RAM using  tiny chip standby power.
That lets the COIN last for 5-7 years  (some can go 10 years, but is not ever for sure){ others from China, with no famous makers name , can fail in months}
RTC = Real time clock, it works just like your wrist watch clock chip, I runs and stores the time of day,M/D/Year in the NVRAM.
In modern times we don't use  a simple single chip NVRAM now, we use a huge glue chip, below. It's still there, just hidden deeper, like in  the huge Intel ICH10 chip or newer...

THE Intel, ICH8 BGA chip, Glue logic complex.  I/O Controller Hub 8 family, All my laptops work just like this only each year newer more powerful chips and USB 3 added.  AKA the SOUTH BRIDGE CHIP.
This chip has the RTC modules inside and the NVRAM bank inside called BIOS CMOS NVRAM, there too and  a coin cell battery outside it, yes Intel even uses the word COIN too and the word GLue logic,.
All parts here are inside the South bridge but not the battery.
I have real schematics of old intel mobo here. So one can see just exactly how they are wired up.
The Intel chip VCCRTC seen here has voltage spec of 2.0v to 3.465vdc.  We all know less that 2.9v is a bad battery. See my marks in red, my HP has the same setup exactly see those low Vf drop diodes.
The D2 diode will have like 0.5v Vf drop so 2.5vdc is Vbatt  min.
The 2 Diodes are current steering devices,  the lower diode prevent the PC power rails from blowing up the coin cell ,its not rechargeable at all .
Diode keeps the COIN cell from draining dead , on the VCCSUS3_3 buss as it surely would fast.
The RTCRST pin resets the ICH8 if the coin is removed, but all other power needs to be removed first. (Main batteries, and PSU /Power packs turned off.) and let stand awhile to discharge all CAP.'s.
The BIOS ROM connects directly to this same chip via direct way (relic PCs) or SPI serial way as all new PC's do. 
This CHIP saves the BIOS settings to the NVRAM inside the ICH8.  As you can see the battery runs the RTC clock all the time if the PC has no power (no main battery ,no AC power)
The Schottly diodes have 0.5vdc voltage drop (Vforward biased)  Intel warns us never short these pins or ICH blows up;. The 1k resistor below protects ICH. (South Bridge chip from damage)

Intel  ICH8  (

Intel told you , "Never short these pins, Vbat, or VCCRTC ever. " or this bad act will wreck the nice ICH chip . (a common occurance with hackers try hard to reset the BIOS PASSWORD then kills the chip  DEAD, way to go hacker !
No pins on this  page can be shorted.  (remove the coin cell, and the Laptop battery and the line cord and then push power, button, now  it is reset)  (same on desktop no huge battery there)
The exeception of course, is if you  see 2pin jumper clip, with silk screened painted words, CMOS_CLr, or PW_clr  those are fully safe, infact there to prevent the hacker dude from wrecking the southbridge chip. (today those jumpers are illegal by law)
See power timing here) the this ICH8 above,

Desktop MOBO. The cell here is an industry standard part. Childs play easy to replace.

Some laptops use same socket above, very common others do not.
The use the pig tailed coin cell,  if you removed the covering you'd have seen that fact. on the device below.

 Below is a Laptop COIN cell , in shrinkwrap, other laptops use same socket as see directly above. 
The new battery measure 3.3vdc., below 2.9v is DANGER ZONE of death soon. (on the cliff of deeth)
If you are savvy you can solder new  battery to the 2 wires below (red to +) using this 3vdc  cell  with tabs.
The cell below has solder taps inside, so you can buy those too. (google it)
Learn that the  Pink Bunny's never lies, but Pink Elephants do ( LOL)! Duracell shows same 7 year rule.  (china no name, knock off batteries, well good luck there, marked "Sum Ting Wong")

The laptop has this coin plugged in to any where on the MOBO, varies by all laptops made , exact placement.
If savvy and own soldering tools one can turn the below into the above. (easy peasy lemon squeezy.)

The laptop coin cell can be pure hell to  get to , some are easy other the whole laptops needs to be dissembled, totally. (a royal pain some are); Desktops are a cake walk easy.
  • Under the keyboard , keyboard out.
  • Under a lid on the bottom (aka hatches)
  • Under the lid, on the bottom for ram,  but ram pulled to reach it. see mine below photo.
  • Under or near the HDD bay lid off.  and HDD removed?
  • Under the blue tooth hatch.
  • Under the Wifi lid hatch.
  • Now the bad one,  remove the screen, top of case and mobo, and only now can you reach it. (sux no?) on the bottom of the PCB MOBO. NO 2 PCs are the same unless the ARE.

Relic NVRAM battery  madness, it's really just a battery and for 36 years,  endless types were used most over 10 years old are dead, bad, shorted or even leaking acids.
Legacy PCs can be a true  horror, for sure that 3 fat batteries below and the DALLAS. (my tektronic scopes used this Dallas and was just replaced after 20 years of service)
The modern PC uses a coin cell and is huge buys (quanty discounts) the he coin can be 25 cents,  the Dallas is now MAXIM( is RTC chip+NVRAM 128bytes+ battery , so is not a battery alone at all and is very expensive $14+ship,  if fresh date coded, )
The Dallas chip is not really a battery neat, but has one potted inside, (potted means back filled with epoxy or some filler) Seen some fail at 15 years old, (and expected)
I see scalpers selling 1993 Dallas DS1287 in amazon on ebay, and are no good, this old, ! do not buy NOS "new old stock"  this old. OK? If it were me, I'd go for Cute trick below !
  Some folks (even me) do this, to the chip to make the coin cell modifcation,  and endless future replacements. (or use this cute trick)
The Dallas is also seen on 1990 Compaq 386N and other super old 1990 PCs. (best forgotten)
I see bad guys selling fake DS1287 out of China, and some with 20 year old date codes real. In fact on fleabay I see one seller selling 28 year old dallas chip. (pure junk that)
The real ones are made now by  Maxim Integrated Products(2010)

The Dallas (Maxim now) data sheet states :
"Each Device is marked with a 4–digit date code AABB.  AA designates the year of manufacture.  BB designates the work week of manufacture."    (1 to 52 weeks)
Some relics below, are  circa 1987 and 1994.    nobody I know uses a PC older than 17 years old, 2001 would be the oldest to even mess with at all. Nor runs virus magnet XP.
Super old Toshiba's used a  now leaking nasty phosphate, NiMh rechargeable (blue below) battery and only recharges for 7 hours, and PC must be on to do that, wow. (me? no love of any thing for Toshiba, buy DELL or HP. ok)
Do not buy old DS chips, 93 is way too old, many "fleabay" batteries or chips are fake, or very old. Buyer beware!
Names, used, for this battery, RTC battery, NVRAM battery, CMOS battery,clock battery, BIOS configuration battery.
All modern desktops the coin cell is clear as day present on the motherboard and snaps out easy,  some one told me I am full of beans and then pointed to 1993 relic PC, well you win ! Gee Wiz.
Do not ever install the battery backwards , as it is  in fact  POLARIZED. Plus to Plus, Negative to Negative, only. (there is backwards protection but we never tempt. fate here)
That is it for RTC batteries, most PCs use the lower coin 2032.  most not all. Only what you see there, is what you have, we look  first.

Slang used here : (some are mine others are industry standards , vast in scope)
Hack or hackable.  (hacks are possible sure, ,but I  mean: if you do you risk bricking the whole motherboard,  a $200+ huge mistake for most owners) (do not listen to basement goon web posters having you find and short out chips) OK?
RAM , random access memory that forgets with lost power.
BIOS , is firmware, BASIC Input Output system. now called UEFI windows 8 up. It is the lowest level of intelligence in the system.
Batteries:  A chemical power source (Cell) that is polarized DC voltage, and can  be chargeable or non chargeable, do not mix them here. Cell here does not mean CELL phone it means battery cell, chemical.
NVram , RAM that does not forget if the battery is still good) NV mean non-volatile. (means does not forget) The ram used is CMOS static RAM..
CMOS, (Is a transistor (MOS) configuration that uses almost no power, if static. NVram uses CMOS memory cells do to that 1 fact. This MOS device is used due to the amazing low power needs, in standby and is fast (2 reasons)
MOBO, = motherboard
PCB, = printed circuit board, like your MOBO is.
Coin, means small battery that is  coin like look and sized, some near quarter sized, or tad less. see photos, above.
RTC , real time clock , no lie the chip is just like the chip in your wrist watch, it is used in all PCs made to keep track of TIME and DATE and leap years. 
The OS and BIOS sees this DATA, and the OS will take this data (time /date) and then if turned on, in Windows, attempt to sync. it with NIST.com atomic clock data in Fort Collins Colorado. (national institute of standard's (NBS old name)
If BIOS can not hold TIME or DATE accurately  the NVRAM (RTC) battery is DEAD or NEAR DEAD. (same is true for CRC errors, or BIOS configurations get lost, or boot device is gone , wrong or changes in ramdom ways.
They key word is random, here.
If Windows sees date wrong you will get Certificate errors. 

version9.  5-9-2017