Dynamic voltage scaling

With undervolting or sub voltes is defined as the deliberate lowering of the supply voltage of computer processors under the manufacturer's recommended voltage. This leads to a reduced power consumption, less power consumption and less heat. Mainly we apply it in laptops to extend battery life. However, it is also possible with stationary devices. Due to the lower heating active cooling (fans) can operate at a lower speed or switch it off completely, without causing overheating. This makes the devices more quietly or silently.

However, the voltage is lowered too far away, it comes to calculation errors, computer programs produce incorrect results (often Prime95 is used to test the system and detect such errors ) or the system crashes / freezes. If the selected voltage in the border area, problems often show only after days or weeks. It can be triggered, for example, a machine check, which results in Windows XP in a Blue Screen of Death.

Principle, one can say that for higher clock rates, a higher voltage is required to ensure that the transistors switch through fast enough. If you want to reduce very much the voltage, one must therefore also reduce the clock rate (this is done with both modern processors in low load automatically, but usually is still room for undervolting ). The procedure is therefore almost contradictory for overclocking; where the clock rate is increased to achieve better performance, which is often accompanied by an increase in voltage to obtain the stability. However, undervolting is not the exact opposite of overclocking (see: underclocking ), you might as well keep the clock rate and still reduce the voltage. In this case, no performance penalty incurred.

Technical background potential calculation errors in voltage reduction

In a microprocessor, it can be physically and manufacturing reasons give no transistors that switch exactly from the same minimum voltage, therefore the normal operating voltage is determined by the manufacturer so that all transistors can always switch in any case within a certain voltage tolerance range. Is lowered in a microprocessor but now the voltage so low that you, the boundaries within which all transistors still can safely turn, reaches or falls below, then it can happen that for some transistors the given voltage is no longer sufficient and switching not more or no longer take place in time.

To explain you can simply put in a processor to introduce an 8 -bit wide data cable with 8 parallel lines, where each line corresponds to one bit and at the end of the data line in each case a transistor (T1 -T8 ) is applied, which determines the value of the data stream. If a transistor connected, then there is voltage, which corresponds to the value 1, otherwise is no voltage, which corresponds to the value 0. The entire data line can thus display an 8 -bit value, for example 0000 0000, which corresponds to the value 0 in decimal notation. Now if by an arithmetic operation, all bits are set to 1, for example, so that the value 1111 1111 (equivalent to 255 decimal) is applied to the data line, then all transistors to scan through. If, however, the voltage for the transistors T2 and T5 is for example too low to perform a secure connection of these transistors, it is at the end of the data line is not more 1111 1111, but 1110 1101, which corresponds to the decimal to the value 237. So it's done a calculation error.

A voltage reduction can therefore cause the software provides false results or no longer runs correctly; the hardware itself will not be damaged by voltage reduction. Therefore it is necessary at a voltage drop that can still reliably switch all transistors of the microprocessor. For computers in the medical, scientific or safety related field any unnecessary risks should be addressed, the voltage should therefore always be within the specifications.