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hi can somebody please advise me what a dallas chip is. i heard this term earlier as its something to do with the cmos on a laptop but what exactly is it and is it easy to reset a forgotten cmos password on a laptop with this installed on it.

please can anyone shed a little light

thanks

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The Dallas Basics and the Basic Failures

The Dallas Chip is the real-time clock (RTC) — system time and date — as well as storage for console-level settings.

There's arguably a basic Alpha system design fault — or at least a “it could have been done better” case — in the Alpha system console itself, as the console code zero should have been reserved for an uninitialized or bad battery status or or such, and not used as the console code for ARC/AlphaBIOS. This is one of the usual hallmarks of a failed Dallas chip; your SRM console reverts from SRM to AlphaBIOS, or the system time or the console environment variables are lost.

As it stands now and into the future, when the Alpha BB_WATCH battery fails, the box inexplicably reverts to ARC/AlphaBIOS console, in addition to the other settings losses that ensue.

Lesson for those designing embedded systems, remember to take power loss or Dallas component or battery replacement or failure into account as part of your design. Here, you'd want to have the system flag the likely error. Not revert.
The Dallas Chip

This chip is manufactured by Dallas Semiconductor (now a division of Maxim Integrated Products), and is the typical BB_WATCH clock chip used on later-vintage VAX systems, on many of the Alpha systems, and various other computers including IBM PS/2 and IBM AT computers. The chip provides a lithium battery, a quartz crystal time source, 114 bytes of battery-backed storage, and a mechanism for providing system interrupts for functions such as the interval timer.

Older VAX systems used a NiCd battery pack, or various other mechanisms, to store the system time.

The lithium battery used to power the Dallas chip tends to last five to ten years. The battery is not a rechargeable battery. When the battery fails, the chip — a 24-pin DIP package — must be removed from its mounting socket and replaced. (HoffmanLabs recommends proper electro-static precautions; if you are not experienced working inside a system enclosure, don't. Static-sensitive devices and dangerous voltages can be present within the system enclosure.)

The Dallas part number is DS12887, and sometimes DS1287, DS1287A, or potentially other variants; check your specific box for details. The HP (Compaq, DIGITAL) part for this DS12887 component is/was the 21-39125-01, usually listed as “Battery, Real Time Clock”; replacement Dallas chips are available from various sources including HP Services. Prices for this component are typically around US$20.

This info came from here - http://labs.hoffmanlabs.com/node/441

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