Micro Firmware Tech Support

Clearing CMOS RAM on AT Compatible PC Systems


Filename: CMOSCLR.TXT
WWW URL:  http://www.firmware.com/support/bios/cmosclr.htm
FTP URL:  ftp://ftp.firmware.com/text/cmosclr.txt
Revision: 08/16/99  TLS  Micro Firmware Technical Support
Summary:  Describes clearing CMOS RAM by using a DEBUG script or by
          shorting pins on the clock chip. 


WARNING - This document is provided by Micro Firmware, Inc. for information purposes only and is intended for qualified service personnel or experienced users. Failure to understand and follow these instructions can result in circuit damage and/or data loss.

Micro Firmware, Inc. makes no warranty of any kind with regard to this document, and assumes no responsibility for any errors that may appear in this document.

Discharging CMOS RAM will cause the hard drive type information and all other SETUP data to be lost. If the hard drive type information is not set back to the same values, data may not be able to be recovered from the drive.

Our technical support department exists to provide support to our ROM BIOS upgrade customers. Anyone who finds this document useful is welcome to it, but for further assistance you should consult a local service technician.


The BIOS code on a PC resides in one or more ROM chips, usually socketted 256KB EPROM chips. Newer motherboards often use Flash ROM chips, which can be updated without replacing the chips. The setup program is usually part of the BIOS code in the ROM chips, but on some older PCs the SETUP program is run from a file on a diskette or hard drive. This diskette would normally be provided by the manufacturer of the PC. We can provide a generic SETUP program to access CMOS Setup on most 286/386 Phoenix BIOSes. For more info on setup programs, see:

http://www.firmware.com/support/bios/setupdsk.htm

The values selected in the SETUP program are stored in CMOS RAM, which is normally in the clock chip - also called the RTC or Real Time Clock chip.

In some situations, such as when a password has been set and has been forgotten, it may be necessary to clear CMOS RAM.

Some motherboards have a CMOS clear jumper that can be used to clear CMOS RAM. Some motherboards even have a separate jumper just to clear passwords.

On many PCs CMOS RAM can be cleared by unplugging the battery and the power supply from the motherboard and waiting for anywhere from an hour or two to a day or two. It is necessary to unplug the power supply because capacitors in the power supply can hold a charge for up to several days. The purpose of the battery on the motherboard is just to hold a voltage on the CMOS RAM to keep the values stored there.

On many PCs the battery is soldered onto the motherboard, which makes it a bit more difficult to remove the battery. Normally a motherboard would need to be removed from a case before de-soldering a battery.

If the clock chip is a Dallas 1287, the CMOS RAM cannot be cleared - the chip must be replaced. The Dallas 1287A differs in that CMOS RAM can be cleared.

CMOS RAM on most clock chips can be cleared by shorting together the appropriate pins. This is done with the power turned off. A paperclip bent into a U shape is a good tool for this.

Several of the most common clock chips used in PCs are described below. CMOS RAM is sometimes located instead in proprietary chips as part of the motherboard's chipset.

When CMOS RAM loses power, a bit is set to indicate this, which should cause the BIOS to detect that the CMOS RAM is invalid and will normally result in the loading of default values.

The same results can be obtained by using a simple DEBUG script to invalidate CMOS RAM. This may be much more convenient than shorting pins on a chip in cases where it is possible to boot to a DOS prompt to run DEBUG.

Here is a DEBUG script to invalidate CMOS RAM. This should work on all AT motherboards. (XT systems do not have CMOS RAM).


A:\>DEBUG
- o 70 2E
- o 71 FF
- q (Quits to DOS)
After clearing CMOS RAM we would usually get some kind of error message on bootup such as CMOS Battery Dead, Clock Chip Lost Power, or maybe just CMOS Checksum Invalid, and default values should be loaded.

The loading of default values after invalidating CMOS RAM is usually more thorough than the loading of default values by choosing the option to load default values in CMOS setup.


P82C206 Chip (Square)

This is usually a square PLCC chip, sometimes soldered onto the motherboard, sometimes in a socket. This chip also contains 2 DMA controllers, 2 Interrupt controllers, Timer, and Real-time Clock. It is usually marked CHIPS (made by Chips and Technologies) and has the number P82C206, but a compatible chip of a different brand may be used instead.

CMOS RAM on this chip is cleared by shorting together pins 12 and 32 on bottom edge of chip or pins 74 and 75 on upper left corner of chip for a few seconds (with power off). (Pins 12 and 32 are the first and last pins on the bottom edge of the chip, pins 74 and 75 are the 2 corner pins on the upper left corner.)

Here is a picture of the 206 chip:


gnd
74
_|____________________
5v 75--| |
| |
| |
| CHIPS |
1 * | |
| P82C206 |
| |
|_____________________|
|||||||||||||||||||||
| |
| gnd | 5v
12 32

F82C206 Chip (Rectangular)

This is a small rectangular PLCC chip usually soldered onto the board. It is usually marked OPTi (the manufacturer of the chip) and has the number F82C206, but a compatible chip of a different brand may be used instead. This chip also contains 2 DMA Controllers, 2 Interrupt Controllers, Timer, and Real-time Clock.

CMOS RAM is cleared on this chip by shorting together pins 3 and 26 on bottom edge of chip for a few seconds (with power off). (Third pin in from left side and 5th pin in from right side)

Here is a picture of the 206 chip:



80 51
______________________________
81 | | 50
| |
| |
| OPTi |
| |
| F82C206 |
| |
100 |______________________________| 31
||||||||||||||||||||||||||||||
1 | | 30
| |
3 26

Dallas DS1287, DS1287A, Benchmarq bp3287MT, bq3287AMT

The Dallas DS1287 and DS1287A, and the compatible Benchmarq bp3287MT and bq3287AMT chips have a built-in battery. This battery should last up to 10 years. Any motherboard using these chips should not have an additional battery. When the battery fails, the RTC chip would be replaced.

CMOS RAM can be cleared on the 1287A and 3287AMT chips by shorting pins 12 and 21 (with power off).

The 1287 (and 3287MT) differ from the 1287A in that the CMOS RAM can not be cleared. If there is a problem such as a forgotten password, the chip must be replaced. (In this case it is recommended to replace the 1287 with a 1287A). Also the Dallas 12887 and 12887A are similar but contain twice as much CMOS RAM storage.

Here is a picture of the Dallas 1287A Real Time Clock chip:


___________
1 -| * U |- 24 5 volts DC
2 -| |- 23
3 -| |- 22
4 -| |- 21 RCL (RAM Clear)
5 -| |- 20
6 -| |- 19
7 -| |- 18
8 -| |- 17
9 -| |- 16
10 -| |- 15
11 -| |- 14
Ground 12 -|__________|- 13

NOTE: Although these are 24-pin chips, the Dallas chips may be missing 5 pins - these are unused pins. Most chips have unused pins, though usually they are still present.


Motorola MC146818AP or compatible

This is a rectangular 24-pin DIP chip, usually in a socket. Compatible chips are made by several manufacturers including Hitachi (HD146818AP) and Samsung (KS82C6818A). The number on the chip should end in 6818.

Although this chip is pin-compatible with the Dallas 1287/1287A, there is no built-in battery. This means that CMOS RAM can be cleared on this chip by just removing it from the socket for a few seconds and replacing it.

To reduce chances of damage to the chip, it is preferable instead to short pins 12 and 24 for a few seconds( with power off).

Here is a picture of the 6818 chip:


___________
1 -| * U |- 24 5 volts DC
2 -| |- 23
3 -| |- 22
4 -| |- 21
5 -| |- 20
6 -| |- 19
7 -| |- 18
8 -| |- 17
9 -| |- 16
10 -| |- 15
11 -| |- 14
Ground 12 -|__________|- 13


Dallas DS12885S or Benchmarq bq3258S

CMOS RAM is cleared on this chip by shorting pins 12 and 20 with power off. These are the pins on diagonally opposite corners - lower right and upper left. (Might also try pins 12 and 24)

Here is a picture of the Dallas 12885S chip:


(5Volts)
24 20 13
| | | | | | | | | | | |
-----------------------------------
| |
| DALLAS |
|> |
| DS12885S |
| |
-----------------------------------
| | | | | | | | | | | |
1 12
(Ground)

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While every attempt is made to ensure the accuracy of our support documents, Micro Firmware provides these documents on an AS-IS basis for information purposes only.