Clock switch circuit and clock switch method of the same

1. A clock switch circuit comprising:

a frequency divide circuit which divides a frequency of a basic clock to generate a plurality of frequency-divided clocks having different frequency dividing ratios;

an output select signal generation circuit which outputs an output select signal according to a clock select signal having a value indicating a clock which is to be output, the output select signal specifying any one of a plurality of clocks including the plurality of frequency-divided clocks and the basic clock; and

an output select circuit which selects any one of the plurality of clocks according to the output select signal to output the selected clock, wherein

the frequency divide circuit outputs a plurality of frequency-divided count values corresponding to each of the plurality of frequency-divided clocks, the plurality of frequency-divided count values indicating the number of clocks of the basic clock from start of one cycle of each of the frequency-divided clocks, and

the output select signal generation circuit switches a value of the output select signal to a value corresponding to the clock select signal at a timing at which start timing of a cycle of a frequency-divided clock selected as a next selection clock matches start timing of a cycle of a frequency-divided clock corresponding to a current selection clock based on a frequency-divided count value corresponding to the current selection clock among the plurality of frequency-divided count values.

2. The clock switch circuit according to claim 1 , wherein the frequency divide circuit comprises:

a plurality of frequency division counters, each of which counting the basic clock and generating a frequency-divided count value of corresponding frequency-divided clock; and

a plurality of frequency dividers that generate the frequency-divided clocks based on the frequency-divided count value output from a corresponding frequency division counter among the plurality of frequency division counters.

3. The clock switch circuit according to claim 2 , wherein each of the frequency division counters includes a minimum count value and a maximum count value based on a frequency dividing ratio of a corresponding frequency-divided clock, and cyclically increases the frequency-divided count value between the minimum count value and the maximum count value.

4. The clock switch circuit according to claim 1 , wherein the output select signal generation circuit comprises:

a switch timing detection circuit that outputs a timing notification signal at a timing at which start timing of a current selection clock matches start timing of a next selection clock based on the frequency-divided count value of the current selection clock and the frequency-divided count value of the next selection clock; and

an output select signal switch circuit that switches a value of the output select signal based on the timing notification signal.

5. The clock switch circuit according to claim 1 , wherein the output select signal generation circuit comprises:

a cycle end detection circuit which detects an end timing of one cycle of a current selection clock to assert a cycle end signal;

a notice signal switch circuit which outputs a value of the clock select signal as a next cycle clock notice signal according to the cycle end signal;

a latch circuit which outputs a value of the next cycle clock notice signal as the output select signal based on the basic clock; and

a reset signal generation circuit which asserts a reset signal corresponding to the next selection clock based on the value of the clock select signal and the value of the next cycle clock notice signal, wherein

the frequency divide circuit sets a logical level of the frequency-divided clock and the frequency-divided count value corresponding to the reset signal that is asserted to initial values.

6. The clock switch circuit according to claim 5 , wherein the reset signal generation circuit comprises:

a decode circuit which decodes the value of the clock select signal to assert a decode signal corresponding to the next selection clock and negate other decode signals;

a match/mismatch judgment circuit which asserts a gating signal when the value of the clock select signal matches the value of the next cycle clock notice signal; and

a gating circuit which outputs the decode signal as the reset signal during a period at which the gating signal is asserted.

7. The clock switch circuit according to claim 5 , wherein the reset signal generation circuit comprises:

a decode circuit which decodes the value of the clock select signal to negate a decode signal corresponding to the next selection clock and assert other reset signals; and

a reset signal switch circuit that sets the value of the decode signal as the value of the reset signal based on the cycle end signal and the basic clock.

8. A clock switch method of a clock switch circuit, the clock switch circuit outputting clock signals having different cycles by switching the clock signals, the method comprising:

generating a plurality of frequency-divided clocks having different frequency dividing ratios based on a basic clock;

generating a plurality of frequency-divided count values indicating the number of clocks of the basic clock from a start point of one cycle of each of the plurality of frequency-divided clocks;

detecting one of end timing and start timing of a cycle of a current selection clock based on a frequency-divided count value corresponding to the current selection clock among the plurality of frequency-divided count values; and

switching a clock which is output to a clock having a different cycle at a timing at which start timing of a cycle of the frequency-divided clock selected as a next selection clock matches start timing of a next cycle of a frequency-divided clock corresponding to the current selection clock.

9. The clock switch method according to claim 8 , comprising:

detecting that the frequency-divided count value corresponding to the current selection clock is set to an initial value; and

switching a clock which is output to a clock having a different cycle upon detection that both of the frequency-divided count value corresponding to the current selection clock and the frequency-divided count value corresponding to the next selection clock are set to initial values.

10. The clock switch method according to claim 8 , comprising:

upon detection that the next selection clock is different from the current selection clock, initializing a logic level of a frequency-divided clock selected as the next selection clock and a frequency-divided count value corresponding to the next selection clock; and

switching a clock which is output to a clock having a different cycle according to the end timing of the current selection clock detected based on the frequency-divided count value of the current selection clock.

11. The clock switch method according to claim 10 , wherein the plurality of frequency-divided clocks are in stop states when not selected.