Synchronous semiconductor memory device
View Patent ↗A synchronous semiconductor memory device of the present invention has a clock generator for generating a normal and a reverse phase clocks by dividing an external clock, a command decoder for decoding an external command and outputting a command signal; latency setting means capable of selectively setting an even or odd number latency within a range of a predetermined number of clock cycles of the external clock, a latency counter which includes two counter circuits for sequentially shifting the command signal captured using the normal and reverse phase clock and being capable of switching a signal path in response to the number of clock cycles, and first and second control means which controls counting of the clock cycles equivalent to the even or odd number latency by forming an appropriate signal path.
1. A synchronous semiconductor memory device comprising:
a clock generator for generating a normal phase clock and a reverse phase clock with a phase difference of 180 degree therebetween as internal clocks by dividing an external clock having a predetermined period;
a command decoder for decoding an external command and outputting a command signal in accordance with a decoding result;
latency setting means capable of selectively setting an even number latency having a period of an even number times the period of said external clock or an odd number latency having a period of an odd number times the period of said external clock within a range of a predetermined number of clock cycles of said external clock;
a latency counter which includes a first counter circuit for sequentially shifting said command signal captured using said normal phase clock and being capable of switching a signal path in response to said number of clock cycles and a second counter circuit for sequentially shifting said command signal captured using said reverse phase clock and being capable of switching a signal path in response to said number of clock cycles;
first control means which, when said even number latency is set, controls counting of said clock cycles equivalent to said even number latency by forming a signal path such that said command signal captured using said normal phase clock passes only through said first counter circuit and said command signal captured using said reverse phase clock passes only through said second counter circuit;
second control means which, when said odd number latency is set, controls counting of said clock cycles equivalent to said odd number latency by forming a signal path such that said command signal captured using said normal phase clock is shifted from said first counter circuit to said second counter circuit and said command signal captured using said reverse phase clock is shifted from said second counter circuit to said first counter circuit.
2. A synchronous semiconductor memory device according to claim 1 , wherein said first and second counter circuits have a symmetrical circuit configuration using the same constituents.
3. A synchronous semiconductor memory device according to claim 2 , wherein each of said first and second counter circuits includes an N-stage shift register circuit for sequentially shifting said command signal.
4. A synchronous semiconductor memory device according to claim 3 , wherein each of said first and second counter circuits includes
a first selector to which said command signal and signals of respective stages of said shift register circuit are input and which passes a signal selected from the N+1 signals and
a second selector to which said command signal and signals of respective stages of said shift register circuit are input and which passes a signal selected from the N+1 signals to be shifted to the other counter circuit.
5. A synchronous semiconductor memory device according to claim 4 , wherein each of said first and second counter circuits includes an output-side circuit connected to said first selector and to said second selector of said other counter circuit,
and wherein a delay time of a signal path through said second circuit and said output-side circuit is one period of said external clock longer than that of said signal path through said first circuit and said output-side circuit.
6. A synchronous semiconductor memory device according to claim 5 , wherein 2(N+1) said latencies including N+1 even number latencies within a range from minimum M (even number) to maximum M+2N, and N+1 odd number latencies within a range from minimum M+1 to maximum M+1+2N can be selectively counted in said latency counter.
7. A synchronous semiconductor memory device according to claim 1 , wherein said latency setting means includes a mode register for storing a set latency rewritably,
and wherein switching of said signal path is controlled by the set latency stored in said mode register.
8. A synchronous semiconductor memory device according to claim 1 , wherein a plurality of latencies corresponding to s standard of DDR-SDRAM can be counted in said latency counter.