1. Field of the Invention
The present invention relates to a dividing method of a frequency dividing number involving a decimal.
2. Description of the Related Art
In general, a frequency divider converts a frequency of an input clock signal into a frequency of one fraction of an integer. However, depending on an application of the frequency divider, a frequency divider capable of dividing the input clock signal by a frequency dividing number involving a decimal value is required in order to produce a clock signal having a desired frequency from the input clock signal having an arbitrary frequency. As this frequency divider, in Jpn. Pat. Appln. KOKAI Publication No. 2001-251181, a fraction frequency divider and fraction frequency dividing method are disclosed.
The frequency divider by the above-described publication divides frequency in a fraction (e.g., 1/5.3) including a denominator involving a decimal point, when the dividing of 1/N (N dividing) of an arbitrary frequency signal is performed. For example, the frequency divider is applied to cellular phones in a digital mobile communication system, mobile station devices such as an information terminal device including a cellular phone function and computer function, and a circuit which produces a standard frequency signal of a base band signal processing circuit in a base station device to perform radio communication with the mobile station device.
In the publication, a decimal value is set in a frequency dividing number involving a decimal point in a decimal portion setting unit, a first adder accumulates/adds the decimal value, and a holding unit holds the accumulated/added value. When the held added value is 1 or more, the holding unit outputs 1 and also outputs the decimal value obtained by subtracting 1 from the held added value as the accumulated/added value to the first adder. Furthermore, an integer value in the frequency dividing number is set to an integer portion setting unit. When the holding unit does not output 1, the frequency divider uses the integer value as the frequency dividing number to perform the dividing. When the holding unit outputs 1, the frequency divider uses the added value of 1 and the integer value as the frequency dividing number to perform the dividing. A frequency-divided signal of the frequency divider is used as an operation timing signal of the holding unit.
The fraction frequency dividing apparatus described in the Jpn. Pat. Appln. KOKAI Publication No. 2001-251181 is capable of correctly performing the dividing involving the decimal, when a valid digit of a decimal setting section is infinite. However, since a finite digit is actually used to perform the dividing, there is a problem that precision of an accumulation/addition result drops and it is impossible to perform the correct dividing. That is, in the publication, when the precision of the decimal value to be set to the decimal portion setting unit is lowered, the precision of the dividing also drops. For example, when the frequency of the input signal is converted to a frequency of 3/10 (=1/3.3333 . . . ) times, it is necessary to set a high-precision numeric value such as 0.33333 to the decimal portion setting unit. However, when low-precision 0.3 is set instead of the numeric value with the high precision, the input signal is converted to a frequency of 6/19 (=1/3.6666 . . . ) times.