Generally, one calls meter a system intended to visualize a size, a quantity in a precise unit.
It is in general a question of quantifying discrete sizes (Analogique), but the everyday usage largely extended the term to sizes comprising some figures after the comma.
Example: the mileage odometer of a Véhicule posts the distances covered by the known as vehicle in Kilomètre S while posting the instantaneous speed of the vehicle.
Mechanics
Car
See also: Speedometer Odometer,
A cable contained in a sheath transfers a negligible part of the couple intended for the wheels from the vehicle for a mechanism which converts the number of revolutions of the cable into a comprehensible posting by the driver. In general the mechanism transforms this displacement (rotation) into a displacement proportional angular of a needle vis-a-vis graduation S for the posting the speed and into rotation of several rings indicating the traversed Kilomètre S.
Electromechanics
See also: Electric meter
A system combining pure mechanics and the electromagnetic effects allows the visualization of a size.
Electronics
In electronics, a meter is a numerical integrated circuit intended to count the pulse repetition frequency applied to its entry. It is composed of a certain number of rockers D, T or JK.
Binary counter
The simplest meter is obtained by putting in cascade a series of rockers T, the signal to be counted being applied to the entry of the first rocker; the exit of this rocker controls the entry of the second rocker and so on. The result of counting appears in the form of binary number, the first rocker indicating the least significant digit.Example: 0101 0111 indicates 87
It is generally envisaged a line (STRONG CURRENT, given to zero or Reset ) making it possible to give the meter to 0 after a counting.
The capacity of the meter, it is the maximum number of impulses that it can add up without error. It is worth 2N for a binary counter. If the capacity is exceeded, the meter totals 0 and recovers to count.
To avoid using an erroneous result, one can associate with the meter an indicating of going beyond ; this last is controlled by a rocker which is activated at the time of a capacity overshooting of the meter.
The maximum frequency of the signal which can be applied to the meter depends on technology used to carry out the rockers: TTL, CMOS, ECL etc
Meter DCB or decimal
The binary counter is most compact, càd. that which requires less rockers for a given capacity. As long as it is a question of memorizing the results in a memory of computer, it is very a good choice. But if one wishes a result in a more comprehensible form for the user, one will use meters DCB, often wrongly called decimal scalers. The rockers are grouped by 4, each group indicating a decimal digit. This type of meter is used in the measuring devices such as numerical voltmeter and multimeter, frequency meter…
Example: 0101 0111 indicates 57
Reversible meter
It is a meter, binary or decimal which makes it possible to count or deduct according to the logical level applied to a pin of order called Sens of counting ( Up/down ).
Meter with predetermination
It is a meter equipped with entries making it possible to introduce an unspecified number into the meter (parallel Loading or Parallel load ). Once the introduced number, the impulses applied to the entry of counting will increment or, generally, décrémenter the contents of the meter.
Example: timer, where one selects the number of minutes wished, and the meter returns gradually towards 0.
Asynchronous meter or synchronous
The binary counter describes above suffers from a defect when it is a question of memorizing the results. Indeed, as each rocker spends a certain time to change state (what is called time travel , noted tp), the various rockers react not simultaneously but gradually. In the extreme case (that where it is the rocker at the end of the chain, that which indicates the most significant digit, which must change state), it is necessary to wait a N.tp time after the impulse to have the correct indication. If the impulse intended to memorize the number in a register arrives after an impulse but before the expiry of the N.tp period, the result put in memory is perhaps erroneous.
To avoid these problems, one modifies the way of connecting the rockers. One employs rockers JK. The impulses to be added up are applied simultaneously to the entries clock of all the rockers. The first rocker, which receives the impulses, has its entries J and K permanently with 1; it thus changes state with each impulse; the entries of the other rockers are controlled by the exits of the preceding rockers so that they rock at the convenient periods: each rocker is authorized to rock when all the preceding rockers have their exit with 1.
The meters where the clock is applied simultaneously to all the rockers are called synchronous meters ( synchronous counters ), the others, asynchronous meters ( asynchronous or ripple counters ).
See too
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