Apparatus performing read operation

1. An apparatus comprising:

a control clock generation circuit configured to generate a first toggling reference clock from a first internal clock and generate a second toggling reference clock from a first inverted internal clock, when a read operation is performed; and

a strobe signal generation circuit configured to generate a data strobe signal whose level transitions during a toggling period in synchronization with the first and second toggling reference clocks, wherein a generation sequence of the first and second toggling reference clocks is changed depending on with which one of the first internal clock and the first inverted internal clock a read command for the read operation is synchronized.

2. The apparatus according to claim 1 , wherein the control clock generation circuit is configured to, when the read command is synchronized with the first internal clock:

output the first internal clock as the first toggling reference clock during a period in which a first toggling reference period signal is activated; and

output the first inverted internal clock as the second toggling reference clock during a period in which a second toggling reference period signal is activated, wherein the first and second toggling reference period signals are sequentially activated when the read operation is performed.

3. The apparatus according to claim 2 , wherein the control clock generation circuit is configured to, when the read command is synchronized with the first inverted internal clock:

output the first internal clock as the first toggling reference clock during a period in which the second toggling reference period signal is activated; and

output the first inverted internal clock as the second toggling reference clock during a period in which the first toggling reference period signal is activated.

4. The apparatus according to claim 2 , further comprising:

a period signal generation circuit configured to, when the read operation is performed, activate the first toggling reference period signal at an ending time point of a read latency period and activate the second toggling reference period signal later than the first toggling reference period signal.

5. The apparatus according to claim 1 , wherein the strobe signal generation circuit is configured to set the data strobe signal to a first logic level in synchronization with the first and second toggling reference clocks.

6. The apparatus according to claim 1 , wherein:

the control clock generation circuit is configured to generate a first toggling delay clock from a second internal clock and generate a second toggling delay clock from a second inverted internal clock, when the read operation is performed; and

the strobe signal generation circuit is configured to generate the data strobe signal whose level transitions during the toggling period in synchronization with the first and second toggling delay clocks, wherein a generation sequence of the first and second toggling delay clocks is changed depending on with which one of the first internal clock and the first inverted internal clock the read command is synchronized.

7. The apparatus according to claim 6 , further comprising:

an internal clock generation circuit configured to generate the first internal clock, the second internal clock, the first inverted internal clock and the second inverted internal clock having a phase difference of 90 degrees from each other, based on a clock.

8. The apparatus according to claim 6 , wherein the strobe signal generation circuit is configured to set the data strobe signal to a second logic level in synchronization with the first and second toggling delay clocks.

9. The apparatus according to claim 6 , further comprising:

a data output circuit configured to output data by serializing read data in synchronization with the first and second toggling reference clocks and the first and second toggling delay clocks.

10. The apparatus according to claim 1 , wherein:

the control clock generation circuit is configured to output one of the first internal clock and the first inverted internal clock as a preamble reference clock depending on with which one of the first internal clock and the first inverted internal clock the read command is synchronized; and

the strobe signal generation circuit is configured to generate the data strobe signal whose level transitions during a preamble period in synchronization with the preamble reference clock.

11. The apparatus according to claim 10 , wherein:

the control clock generation circuit is configured to output one of a second internal clock and a second inverted internal clock as a preamble delay clock depending on with which one of the first internal clock and the first inverted internal clock the read command is synchronized; and

the strobe signal generation circuit is configured to generate the data strobe signal whose level transitions during the preamble period in synchronization with the preamble delay clock.

12. The apparatus according to claim 10 , wherein:

the control clock generation circuit is configured to output one of the first internal clock and the first inverted internal clock as a postamble reference clock depending on with which one of the first internal clock and the first inverted internal clock the read command is synchronized; and

the strobe signal generation circuit is configured to generate the data strobe signal whose level transitions during a postamble period in synchronization with the postamble reference clock.

13. The apparatus according to claim 12 , wherein:

the control clock generation circuit is configured to output one of a second internal clock and a second inverted internal clock as a postamble delay clock depending on with which one of the first internal clock and the first inverted internal clock the read command is synchronized; and

the strobe signal generation circuit is configured to generate the data strobe signal whose level transitions during the postamble period in synchronization with the postamble delay clock.

14. An apparatus comprising:

a control clock generation circuit configured to generate a first toggling reference clock from a first internal clock and generate a second toggling reference clock from a first inverted internal clock, when a read operation is performed; and

a data output circuit configured to output read data as data in synchronization with the first and second toggling reference clocks, wherein a generation sequence of the first and second toggling reference clocks is changed depending on with which one of the first internal clock and the first inverted internal clock a read command for the read operation is synchronized.

15. The apparatus according to claim 14 , wherein the control clock generation circuit is configured to, when the read command is synchronized with the first internal clock:

output the first internal clock as the first toggling reference clock during a period in which a first toggling reference period signal is activated; and

output the first inverted internal clock as the second toggling reference clock during a period in which a second toggling reference period signal is activated, wherein the first and second toggling reference period signals are sequentially activated when the read operation is performed.

16. The apparatus according to claim 15 , wherein the control clock generation circuit is configured to, when the read command is synchronized with the first inverted internal clock:

output the first internal clock as the first toggling reference clock during a period in which the second toggling reference period signal is activated; and

output the first inverted internal clock as the second toggling reference clock during a period in which the first toggling reference period signal is activated.

17. The apparatus according to claim 15 , further comprising a period signal generation circuit configured to, when the read operation is performed:

activate the first toggling reference period signal at an ending time point of a read latency period; and

activate the second toggling reference period signal later than the first toggling reference period signal.

18. The apparatus according to claim 14 , wherein the data output circuit is configured to:

output a first bit of the read data as the data in synchronization with the first toggling reference clock; and

output a second bit of the read data as the data in synchronization with the second toggling reference clock.

19. The apparatus according to claim 14 , wherein:

the control clock generation circuit is configured to generate a first toggling delay clock from a second internal clock and generate a second toggling delay clock from a second inverted internal clock, when the read operation is performed; and

the data output circuit is configured to output the read data as the data in synchronization with the first and second toggling delay clocks, wherein a generation sequence of the first and second toggling delay clocks is changed depending on with which one of the first internal clock and the first inverted internal clock the read command is synchronized.

20. The apparatus according to claim 19 , wherein the data output circuit is configured to:

output a third bit of the read data as the data in synchronization with the first toggling delay clock; and

output a fourth bit of the read data as the data in synchronization with the second toggling delay clock.