CMOS

CMOS

Real Time Clock (RTC), Non-Volatile RAM (NVRAM) or CMOS RAM, CMOS is short for Complementary Metal-Oxide Semiconductor. CMOS is an on-board semiconductor chip powered by a CMOS battery inside computers that stores information such as the system time and system settings for your computer. A CMOS is similar to the Apple Macintosh computer's PRAM. To the right is an image of a CMOS battery on a computers motherboard and the most common CMOS battery you're likely to encounter with your computer.To the right is some examples of other types of batteries that may be used in a computers to power the CMOS memory. As mentioned above the most common type of battery is the coin cell battery (lithium battery), usually about the size of a U.S. Nickel.

The standard lifetime of a CMOS battery is around 10 Years. However, this amount of time can change depending on the use and environment that the computer resides.Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Significant power is only drawn while the transistors  in theCMOS device are switching between on and off states. Consequently, CMOS devices do not produce as much waste heat as other forms of logic, for example transistor-transistor logic (TTL) or NMOS logic, which uses all n-channel devices without p-channel devices. CMOS also allows a high density of logic functions on a chip. It was primarily this reason why CMOS won the race in the eighties and became the most used technology to be implemented in VLSI chips.

The phrase "metal–oxide–semiconductor" is a reference to the physical structure of certain field-effect transistors, having a metal gate electrode placed on top of an oxide , which in turn is on top of a semiconductor material. Aluminum was once used but now the material is polysilicon. Other metal gates have made a comeback with the advent of high-k dielectric materials in the CMOS process, as announced by IBM and Intel for the 45 nanometer node and beyond.

The main principle behind CMOS circuits that allows them to implement logic gates is the use of p-type and n-type metal–oxide–semiconductor field-effect transistors to create paths to the output from either the voltage source or ground. When a path to output is created from the voltage source, the circuit is said to be pulled up. The other circuit state occurs when a path to output is created from ground and the output pulled down to the ground potential.

Output is inversion of input


CMOS circuits are constructed so that all PMOS transistors must have either an input from the voltage source or from another PMOS transistor. Similarly, all NMOS
transistors must have either an input from ground or from another NMOS transistor. The composition of a PMOS transistor creates low resistance between its source and drain contacts when a low gate voltage is applied and high resistance when a high gate voltage is applied. On the other hand, the composition of an NMOS transistor creates high resistance between source and drain when a low gate voltage is applied and low resistance when a high gate voltage is applied.

PMOS transistor (top of diagram) and an NMOS transistor. When the voltage of input A is low, the NMOS transistor's channel is in a high resistance state. This limits the current that can flow from Q to ground. The PMOS transistor's channel is in a low resistance state and much more current can flow from the supply to the output. Because the resistance between the supply voltage and Q is low, the voltage drop between the supply voltage and Q due to a current drawn from Q is small. The output therefore registers a high voltage.On the other hand, when the voltage of input A is high, the PMOS transistor is in an off (high resistance) state so it would limit the current flowing from the positive supply to the output, while the NMOS transistor is in an on (low resistance) state, allowing the output to drain to ground. Because the resistance between Q and ground is low, the voltage drop due to a current drawn into Q placing Q above ground is small. This low drop results in the output registering a low voltage.In short, the outputs of the PMOS and NMOS transistors are complementary such that when the input is low, the output is high, and when the input is high, the output is low. Because of this opposite behavior of input and output, the CMOS circuits' output is the inversion of the input.

Duality

An important characteristic of a CMOS circuit is the duality that exists between its PMOS transistors and NMOS transistors. A CMOS circuit is created to allow a path always to exist from the output to either the power source or ground. To accomplish this, the set of all paths to the voltage source must be the complement of the set of all paths to ground. This can be easily accomplished by defining one in terms of the NOT of the other. Due to the De Morgan's laws based logic, the PMOS transistors in parallel have corresponding NMOS transistors in series while the PMOS transistors in series have corresponding NMOS transistors in parallel.

Logic

More complex logic functions such as those involving AND and OR gates require manipulating the paths between gates to represent the logic. When a path consists of two transistors in series, then both transistors must have low resistance to the corresponding supply voltage, modeling an AND. When a path consists of two transistors in parallel, then either one or both of the transistors must have low resistance to connect the supply voltage to the output, modeling an OR.
An advantage of CMOS over NMOS is that both low-to-high and high-to-low output transitions are fast since the pull-up transistors have low resistance when switched on, unlike the load resistors in NMOS logic. In addition, the output signal swings the full voltage between the low and high rails. This strong, more nearly symmetric response also makes CMOS more resistant to noise.

How to reset CMOS?

Every motherboard has a battery built in which provides the trickle of power the CMOS memory needs to store its information. If you remove the battery for a brief period, the CMOS should lose its stored data.In all modern motherboards, the battery is a standard CR 2032 Lithium Cell, which looks like this:

To clear the CMOS memory, power down you system and remove this battery (you will probably need a flathead screwdriver to do it) for 5-10 minutes, then pop it back in and turn your system on. You will be prompted to set the BIOS defaults. From the BIOS screen, save and exit, and the password should no longer be set.
While this works well with desktop PCs, notebooks and laptops employ slightly different methods of storing the BIOS password. In most cases, like 99% of the time, you will not be able to reset a forgotten BIOS password by this, or any other user accessible method. Since notebooks are easily stolen, manufacturers have evolved their security measures significantly to help you protect your data, and investment. Look for PCstats Guide to preventing data theft from a stolen laptop soon… In the mean time, when it comes to resetting a BIOS-level password on a notebook, usually the only option available is to send the entire notebook back to the manufacturer for support.

• < Prev
• Next >