Abstract:
An internal clock delay circuit of a semiconductor device and a method for delaying an internal clock of the semiconductor device. The semiconductor device includes a CAS latency signal generator that generates CAS latency signals comprising a first CAS latency signal, a second CAS latency signal and a third CAS latency signal, and an internal clock delay circuit that receives one of the CAS latency signals and an internal clock signal and delays the internal clock signal by a predetermined time in response to the received CAS latency signal. The internal clock delay circuit includes delay circuits that delay the internal clock signal, and the internal clock signal passes through only one among the delay circuits when the semiconductor device operates in the second CAS latency mode. The method includes: inputting an internal clock signal to an internal clock delay circuit, which includes delayers, of a semiconductor device; and inputting CAS latency signals to the internal clock delay circuit to determine CAS latency modes of the semiconductor device; and outputting the internal clock signal through the delay circuits as an output signal of the internal clock signal delay circuit. The internal clock signal passes through one of the delay circuits in a second CAS latency mode and passes through at least two delay circuits among the delay circuits in either a first CAS latency mode or a third CAS latency mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor devices and more particularly, to an internal clock signal delay circuit for a synchronous DRAM. 
     2. Description of the Related Art 
     A synchronous DRAM operates in synchronization with an internal clock signal generated in response to an external clock signal from a CPU. In reading a first data from a row of memory cells of a synchronous DRAM, a row address strobe (RAS) signal enables a reading wordline, and then column select line (CSL) is enabled. Generally, the first data from a row requires more time between enabling the wordlines and selecting the column lines for data output. However, when the clock frequency of the CPU is high, that is, when the period of a clock signal is short, the synchronous DRAM may need to delay the CSL enabling signal by more than one clock period, so that the first data is valid when read. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a synchronous DRAM in which internal clock signal is delayed in three CAS latency modes to control the timing of CSL enabling. 
     In accordance with an embodiment of the present invention, a semiconductor device includes a CAS latency signal generator that generates CAS latency signals comprising a first CAS latency signal, a second CAS latency signal and a third CAS latency signal, and an internal clock delay circuit that receives one of the CAS latency signals and an internal clock signal and delays the internal clock signal by a predetermined time in response to the received CAS latency signal. The internal clock delay circuit includes delay circuits that delays the internal clock signal, and the internal clock signal passes through only one among the delayers when the semiconductor device operates in the second CAS latency mode. 
     Another embodiment of the invention provides a semiconductor device, which includes: a CAS latency signal generator that generates CAS latency signals; and an internal clock delay circuit that receives an internal clock signal and second and third CAS latency signals and generates a delayed internal clock signal. The second and third CAS latency signals are disabled when the semiconductor device operates in a first CAS latency mode, the second CAS latency signal is enabled when the semiconductor device operates in a second CAS latency mode, and the third CAS latency signal is enabled when the semiconductor device operates in a third CAS latency mode. The internal clock delay circuit includes: a first delay circuit that receives the internal clock signal; a second delay circuit connected to the first delay circuit; a third delay circuit connected to the second delay circuit; a first controller that receives an output signal of the first delay circuit and the second CAS latency signal; a second controller that receives an output signal of the second delay circuit and the second and third CAS latency signals; and a third controller that receives an output signal of the third delay circuit and the second and third CAS latency signals. The internal clock delay circuit can further include a fourth controller that receives outputs of the first, second and third controllers. 
     Still another embodiment of the invention provides a semiconductor device, which includes: a CAS latency signal generator that generates CAS latency signals; and an internal clock delay circuit that receives an internal clock signal and first and third CAS latency signals and generates a delayed internal clock signal. The first CAS latency signal is enabled when the semiconductor device operates in a first CAS latency mode, the first and third CAS latency signals are disabled when the semiconductor device operates in a second CAS latency mode, and the third CAS latency signal is enabled when the semiconductor device operates in a third CAS latency mode. The internal clock delay circuit includes: a first delay circuit that receives the internal clock signal; a second delay circuit connected to the first delay circuit; a third delay circuit connected to the first delay circuit in parallel with the second delay circuit; a first controller that receives an output signal of the first delay circuit and the first and third CAS latency signals; a second controller that receives an output signal of the second delay circuit and the first CAS latency signal; and a third controller that receives an output signal of the third delay circuit and the third CAS latency signal. The internal clock delay circuit can further include a fourth controller that receives the outputs of the first, second and third controllers. 
     The present invention is also directed to a method for delaying the internal clock signal of a synchronous DRAM, which decreases the generation time of the delayed internal clock signal in the second CAS latency mode. The method includes: inputting an internal clock signal to an internal clock delay circuit, which includes delay circuits, of a semiconductor device; and inputting CAS latency signals to the internal clock delay circuit to determine CAS latency modes of the semiconductor device; and outputting the internal clock signal through the delayers as an output signal of the internal clock signal delay circuit. The internal clock signal passes through one of the delay circuits in a second CAS latency mode and passes through at least two delay circuits among the delay circuits in either a first CAS latency mode or a third CAS latency mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more apparent by describing in detail specific embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 is a block diagram of a synchronous DRAM according to an embodiment of the present invention; 
     FIG. 2 is a circuit diagram of an embodiment of an internal clock signal delay circuit of FIG. 1; 
     FIG. 3 is a timing diagram for signals in the internal clock signal delay circuit of FIG. 2; 
     FIG. 4 is a circuit diagram of another embodiment of the internal clock signal delay circuit of FIG. 1; and 
     FIG. 5 is a timing diagram for signals in the internal clock signal delay circuit of FIG.  4 . 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with an aspect of the present invention, an internal clock signal delay circuit delays an internal clock signal in three column address strobe (CAS) latency modes to control the timing of CSL enabling: a first CAS latency mode, a second CAS latency mode, and a third CAS latency mode. The first CAS latency mode delays the internal clock signal such that the synchronous DRAM outputs data one clock cycle of CPU clock after a row line is activated. This mode is often used in testing the reliability of the cells of the synchronous DRAM. The second CAS latency mode delays the internal clock signal such that the synchronous DRAM outputs data after the second cycle of CPU clock. The third CAS latency mode delays the internal clock signal such that the synchronous DRAM outputs data after the third cycle of CPU clock. The second and third modes are used in normal read/write operation of the synchronous DRAM depending on the CPU clock frequency. 
     FIG. 1 illustrates a synchronous DRAM  101  according to an embodiment of the present invention. The synchronous DRAM  101  includes an internal clock delay circuit  111 , a column address strobe (CAS) latency signal generator  121 , a column decoder  131 , a sense amplifying and input and output gating unit  141 , an output buffer  161 , a row decoder  171 , a wordline driver  181 , and a memory array  151 . 
     The CAS latency signal generator  121  generates first, second, and third CAS latency signals CL 1 , CL 2 , and/or CL 3 , which control the CAS latency mode of the synchronous DRAM  101 . In the first CAS latency mode, the synchronous DRAM  101  reads and outputs data DQn from the memory array  151  after the first rising edge of an internal clock signal PCLKC generated in response to an external read command. In the second CAS latency mode, the synchronous DRAM  101  reads and outputs the data DQn after the second rising edge of the internal clock signal PCLKC. In the third CAS latency mode, the synchronous DRAM  101  reads and outputs the data DQn after the third rising edge of the internal clock signal PCLKC. 
     The internal clock delay circuit  111  receives the internal clock signal PCLKC and the first, second, and third CAS latency signals CL 1 , CL 2 , and CL 3  and generates a delayed internal clock signal PCLKCD. The delayed internal clock signal PCLKCD is a delayed version of the internal clock signal PCLKC, and the first, second, and third CAS latency signals CL 1 , CL 2 , and CL 3  control and vary the amount of delay. The column decoder  131  receives a column address signal Ac from outside synchronous DRAM  101  and the delayed internal clock signal PCLKCD and generates column selection line enable signals PCSLn by decoding the column address signal Ac in synchronization with the delayed internal clock signal PCLKCD. 
     The row decoder  171  decodes a row address signal Ar received from outside synchronous DRAM  101  and generates wordline enable signals NWEn for selecting among wordlines WL 0  to WLn and activating the selected wordlines. The wordline driver  181  receives the wordline enable signals NWEn and applies a boosting voltage on the selected wordlines higher than the internal power supply voltage. The memory array  151  includes memory cells (not shown). Data is read from or written to memory cells connected to the selected wordlines. The data read from the memory cells is transmitted to the sense amplifying and input and output gating unit  141  through bitlines BL 0  to BLn. In the sense amplifying and input and output gating unit  141 , the column selection line enable signals PCSLn select among the data transmitted to the sense amplifying and input and output gating unit  141  so that the selected data are transmitted to the output buffer  161  through input and output lines IO 0  to IOn. The output buffer  161  outputs the transmitted data from the synchronous DRAM  101 . 
     FIG. 2 illustrates an internal clock signal delay circuit  401 , which is an embodiment of the internal clock signal delay circuit  111  of FIG.  1 . The internal clock signal delay circuit  401  includes three delay circuits  411 ,  412  and  413  and four controllers  421  to  424 . The internal clock delay circuit  401  receives internal clock signal PCLKC, second CAS latency signal CL 2 , and third CAS latency signal CL 3  and generates delayed internal clock signal PCLKCD, which is a version of the internal clock signal PCLKC delayed by a predetermined time. The delay time of the delayed internal clock signal PCLKCD varies according to the CAS latency mode of the synchronous DRAM. Each of the delay circuits  411  to  413  includes an even number of invertors. It is possible to control the delay time of the delay circuits  411  to  413  by varying the number of the invertors. Each of the controllers  421  to  424  includes a NAND gate. Alternatively, the controllers  421  to  424  can include transmission gates or field effect transistors. 
     When the synchronous DRAM operates in the first CAS latency mode, the second CAS latency signal CL 2  and the third CAS latency signal CL 3  are disabled to logic low. The output signal of the first controller  421  becomes logic high regardless of the output signal of the first delay circuit  411  since the second CAS latency signal CL 2  is logic low. The output signal of the second controller  422  becomes logic high regardless of the output signal of the second delay circuit  412  since the third CAS latency signal CL 3  is logic low. The output signal of the third controller  423  is determined by the output signal of the third delay circuit  413  since the output signals of the invertors  431  and  432  are logic high. Accordingly, the output signal of the fourth controller  424  is determined by the output signal of the third controller  423  since the output signals of the first controller  421  and the second controller  422  are logic high. 
     In the first CAS latency mode, the internal clock signal PCLKC passes through the delayers  411 ,  412 , and  413  and the controllers  423  and  424 . Therefore, the delayed internal clock signal PCLKCD is delayed relative to the internal clock signal PCLKC by the total delay time through the delay circuits  411  to  413 . 
     When the synchronous DRAM operates in the third CAS latency mode, the third CAS latency signal CL 3  is enabled to logic high, and the second CAS latency signal CL 2  is disabled to logic low. The output signal of the first controller  421  becomes logic high regardless of the output signal of the first delay circuit  411  since the second CAS latency signal CL 2  is logic low. The output signal of the second controller  422  is determined by the output signal of the second delay circuit  412  since the output signal of the inverter  431  and the third CAS latency signal CL 3  are logic high. The output signal of the third controller  423  becomes logic high regardless of the output signals of the third delay circuit  413  and the inverter  431  since the output signal of the inverter  432  is logic low. The output signal of the fourth controller  424  is determined by the output signal of the second controller  422  since the output signals of the first controller  421  and the third controller  423  are logic high. 
     In the third CAS latency mode, the internal clock signal PCLKC passes through the first and second delay circuits  411  and  412  and the second and fourth controllers  422  and  424 . Therefore, the delayed internal clock signal PCLKCD is delayed relative to the internal clock signal PCLKC by the total delay time through the first and second delay circuits  411  and  412 . The generation time of the delayed internal clock signal PCLKCD is shorter in the third CAS latency mode than in the first CAS latency mode because the internal clock signal PCLKC does not pass through the third delay circuit  413 . 
     When the synchronous DRAM operates in the second CAS latency mode, the second CAS latency signal CL 2  is enabled to logic high, and the third CAS latency signal CL 3  is disabled to logic low. Then, the output signal of the first controller  421  is determined by the output signal of the first delay circuit  411  since the second CAS latency signal CL 2  is logic high. The output signal of the second controller  422  becomes logic high regardless of the output signal of the second delay circuit  412  since the output signals of the third CAS latency signal CL 3  and the inverter  431  are logic low. The output signal of the third controller  423  becomes logic high regardless of the output signals of the third delay circuit  413  and the inverter  432  since the output signal of the inverter  431  is logic low. Accordingly, the output signal of the fourth controller  424  is determined by the output signal of the first controller  421  since the output signals of the second controller  422  and the third controller  423  are logic high. 
     In the second CAS latency mode, the internal clock signal PCLKC passes through the first delay circuit  411  and the first and fourth controllers  421  and  424 . Therefore, the delayed internal clock signal PCLKCD is delayed relative to the internal clock signal PCLKC by the delay time through the first delayer  411 . The generation time of the delayed internal clock signal PCLKCD is shorter in the second CAS latency mode than in the third CAS latency mode. 
     FIG. 3 illustrates the total delay time of the delayed internal clock signal PCLKCD in the first, second and third CAS latency modes of the internal clock delay circuit  401  of FIG.  2 . Symbols d 1 , d 2 , and d 3  denote the delays through the delay circuits  411 ,  412  and  413 , respectively. In the first CAS latency mode, the total delay time is the sum of d 1 , d 2 , and d 3 . In the second CAS latency mode, the total delay time is d 1 . In the third CAS latency mode, the total delay time is the sum of d 1  and d 2 . Therefore, the total delay time of the delayed internal clock signal PCLKCD is shortest in the second CAS latency mode and longest in the first CAS latency mode. 
     FIG. 4 illustrates an internal clock delay circuit  601 , which is another embodiment of the internal clock delay circuit  111 . The internal clock delay circuit  601  includes three delay circuits  611  to  613 , four controllers  621  to  624 , and invertors  631  and  632 . The internal clock delay circuit  601  receives the internal clock signal PCLKC, the first CAS latency signal CL 1 , and the third CAS latency signal CL 3  and outputs the delayed internal clock signal PCLKCD. The delayed internal clock signal PCLKCD is a version of the internal clock signal PCLKC delayed by a predetermined time. The delay time of the delayed internal clock signal PCLKCD varies according to the CAS latency mode of the internal clock delay circuit  601 . Each of the delay circuits  611  to  613  includes an even number of invertors. It is possible to control the delay time of the delay circuits  611  to  613  by varying the number of the invertors. Each of the controllers  621  to  624  includes a NAND gate. Alternatively, the controllers  621  to  624  can include transmission gates or field effect transistors. 
     In the first CAS latency mode, the first CAS latency signal CL 1  is enabled to logic high, and the third CAS latency signal CL 3  is disabled to logic low. When the first CAS latency signal CL 1  is logic high, the output signal of the inverter  631  becomes logic low. Thus, the output signal of the first controller  621  becomes logic high regardless of the output signals of the first delay circuit  611  and the inverter  632 . The output signal of the second controller  622  is determined by the output signal of the second delayer  612  since the first CAS latency signal CL 1  is logic high. The output signal of the third controller  623  becomes logic high regardless of the output signal of the third delay circuit  613  since the third CAS latency signal CL 3  is logic low. Accordingly, the output signal of the fourth controller  624  is determined by the output signal of the second controller  622  since the output signals of the first controller  621  and the third controller  623  are logic high. 
     In the first CAS latency mode, the internal clock signal PCLKC passes through the first and second delay circuits  611  and  612  and the second and fourth controllers  622  and  624 . Therefore, the delayed internal clock signal PCLKCD is delayed with respect to the internal clock signal PCLKC by the total delay time through the first and second delay circuits  611  and  612 . 
     In the third CAS latency mode, the third CAS latency signal CL 3  is enabled to logic high, and the first CAS latency signal CL 1  is disabled to logic low. Since the third CAS latency signal CL 3  is logic high, the output signal of the inverter  632  becomes logic low. Therefore, the output signal of the first controller  621  becomes logic high regardless of the output signals of the first delay circuit  611  and the inverter  631 . The output signal of the second controller  622  becomes logic high regardless of the output signal of the second delay circuit  612  since the first CAS latency signal CL 1  is logic low. The output signal of the third controller  623  is determined by the output signal of the third delay circuit  613  since the third CAS latency signal CL 3  is logic high. Accordingly, the output signal of the fourth controller  624  is determined by the output signal of the third controller  623  since the output signals of the first controller  621  and the second controller  622  are logic high. 
     In the third CAS latency mode, the internal clock signal PCLKC passes through the first and third delay circuits  611  and  613  and the third and fourth controllers  623  and  624 . Therefore, the delayed internal clock signal PCLKCD is delayed relative to the internal clock signal PCLKC by the total delay time of the first and third delay circuits  611  and  613 . In the third CAS latency mode, the number of invertors in the third delay circuit  613  affects the generation time of the delayed internal clock signal PCLKCD. 
     In the second CAS latency mode, the first CAS latency signal CL 1  and the third CAS latency signal CL 3  are disabled to logic low. When the first and third CAS latency signals CL 1  and CL 3  are logic low, the output signals of the invertors  631  and  632  become logic high. Therefore, the output signal of the first controller  621  is determined by the output signal of the first delay circuit  611 . The output signal of the second controller  622  becomes logic high regardless of the output signal of the second delay circuit  612  since the first CAS latency signal CL 1  is logic low. The output signal of the third controller  623  becomes logic high regardless of the output signal of the third delay circuit  613  since the third CAS latency signal CL 3  is logic low. Accordingly, the output signal of the fourth controller  624  is determined by the output signal of the first controller  621  since the output signals of the second controller  622  and the third controller  623  are logic high. 
     In the second CAS latency mode, the internal clock signal PCLKC passes through the first delay circuit  611  and the first and fourth controllers  621  and  624 . Therefore, the delayed internal clock signal PCLKCD is delayed relative to the internal clock signal PCLKC by the delay time of the first delay circuit  611 . The delay time of the delayed internal clock signal PCLKCD is shorter in the second CAS latency mode than in the first and third CAS latency modes. 
     FIG. 5 illustrates the total delay time of the delayed internal clock signal PCLKCD in the first, second and third CAS latency modes of the internal clock delay circuit  601  of FIG.  4 . Symbols d 1 , d 2 , and d 3  denote the delays through the delayers  411 ,  412  and  413 , respectively. In the first CAS latency mode, the total delay time is the sum of d 1  and d 2 . In the second CAS latency mode, the total delay time is d 1 . In the third CAS latency mode, the total delay time is the sum of d 1  and d 3 . As described above the delay time of the delayed internal clock signal PCLKCD is shorter in the second CAS latency mode than in the first and third CAS latency modes. The delay times of the delayed internal clock signal PCLKCD in the first and third CAS latency modes are independent from each other. 
     The internal clock signal delay circuits of the present invention, in the second CAS latency mode, reduces the delay time of the delayed internal clock signal PCLKCD and thus the output time tAA of the data output from an output buffer of a synchronous DRAM. In the first CAS latency mode, the delay time of the delayed internal clock signal PCLKCD can be controlled to be long enough to prevent the column selection line enable signals PCSLn of FIG. 1 from being invalid during the test of the memory cell. In particular, when the memory cells are to be tested in a condition that the cycle time of the internal clock signal PCLKC is very short, the invalidation of the column selection line enable signals PCSLn can be prevented by increasing the delay time of the third delayer  413  of FIG. 2 or the second delay circuit  612  of FIG.  4 . Here, the internal clock signal delay time is not affected at all in the second CAS latency mode. Also, since the internal clock signal PCLKC passes through different combinations of the delayers in the CAS latency modes, the CAS latency modes can result in different total delay times from one another. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the inventor&#39;s application and should not be taken as limiting. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.