Delay locked loop for use in semiconductor memory device

[0002] The present invention relates to a semiconductor memory device; and, more particularly, to a delay locked loop with finer adjustability and, thus, reduced jitter.

[0003] For achieving a high-speed operation in a semiconductor memory device, a synchronous dynamic random access memory (SDRAM) has been developed. The SDRAM operates in synchronization with an external clock signal. The SDRAM includes a single data rate (SDR) SDRAM, a double data rate (DDR) SDRAM, and the like.

[0004] Generally, when data are outputted in synchronization with the external clock signal, a skew occurs between the external clock signal and the output data. In the SDRAM, a delay locked loop (DLL) can be used to compensate for the skew that occurs between either an external clock signal and an output data, or an external clock signal and an internal clock signal.

[0005] FIG. 1 is a block diagram of a conventional DLL. Referring to FIG. 1, the illustrated conventional DLL includes a clock buffer 100, a delay monitor 110, a phase detector 120, a shift register 130 and a digital delay line 140.

[0006] The clock buffer 100 receives an external clock EXT_CLK to generate an internal clock CLK_IN. The delay monitor 110 receives a DLL clock DLL_CLK, i.e., an output of the DLL, to perform a monitoring operation and to add a predetermined amount of delay for determining a delay amount of the internal clock CLK_IN. An output of the delay monitor 110 is fed back to the phase detector 120.

[0007] The phase detector 120 compares a phase difference between the internal clock CLK_IN and the output of the delay monitor 110 to generate either a shift-left signal SHF_L or a shift-right signal SHF_R as a control signal depending on whether less or more delay is desired.

[0008] The shift register 130 decreases the delay amount in response to the shift-left signal SHF_L and increases the delay amount in response to the shift-right signal SHF_R. The digital delay line 140 delays the internal clock CLK_IN according to an output of the shift register 130 to generate the DLL clock DLL_CLK.

[0009] In FIG. 2, there is shown an exemplary diagram of the digital delay line having three delay units, generally shown as 230, 231, and 232. As shown in FIG. 2, the illustrated digital delay line 140 includes a control unit 200 for transferring the internal clock CLK_IN through a number of the delay units 230, 231, 232 in response to a first, a second, and a third shift control signals, generally shown as SL1, SL2, and SL3, respectively. The digital delay line 140 also includes a delay unit 210 for performing a time delay operation under control of the control unit 200. The digital delay line 140 further includes an output unit 220 for receiving an output of the delay unit 210 to generate the DLL clock DLL_CLK.

[0010] When only the first shift control signal SL1 is a logic high, the digital delay line 140 generates the DLL clock DLL_CLK obtained by delaying the internal clock CLK_IN through only a first delay unit 230. Then, the DLL clock DLL_CLK is transferred to the phase detector 120 through the delay monitor 110, and the phase detector 120 compares a phase of the DLL clock DLL_CLK and that of the internal clock CLK_IN.

[0011] If the internal clock CLK_IN needs further delay, the phase detector 120 activates the shift-right signal SHF_R. As a result, the first and the second shift control signals SL1 and SL2 are set to a logic low and a logic high, respectively. That is, the logic high is shifted in a right direction from SL1 to SL2.

[0012] Then, the digital delay line 140 generates the DLL clock DLL_CLK obtained by delaying the internal clock CLK_IN by two delay units 230 and 231. The DLL clock DLL_CLK is again fed back to the phase detector 120 through the delay monitor 110.

[0013] Meanwhile, if it is needed to delay the internal clock CLK_IN less, the phase detector 120 activates the shift-left signal SHF_L. As a result, the logic high signal is shifted in a left direction.

[0014] However, since each delay unit contained in the conventional digital delay line 140 is implemented with two NAND gates, the conventional DLL can make only relatively large, coarse adjustments, for example, adjustments of about several picoseconds. Therefore, as the semiconductor memory device operates at a faster speed, there is a need for a DLL with finer adjustability.

[0015] Objects and features of the disclosed device and method will become apparent from the following description of the preferred embodiments with reference to the accompanying drawings, in which:

[0016] FIG. 1 is a block diagram of a conventional delay locked loop (DLL);

[0017] FIG. 2 is a circuit diagram of a digital delay line having three delay units;

[0018] FIG. 3 is a block diagram illustrating an exemplary DLL constructed in accordance with the teachings of the disclosed invention;

[0019] FIG. 4 is a circuit diagram illustrating a first delay line and a second delay line as shown in FIG. 3;

[0020] FIGS. 5A, 5B, and 5C are circuit diagrams illustrating a shift-right operation of the DLL;

[0021] FIGS. 5D, 5E, and 5F are truth tables corresponding to the circuit diagrams illustrated in FIGS. 5A, 5B, and 5C;

[0022] FIGS. 6A, 6B, and 6C are circuit diagrams illustrating a shift-left operation of the DLL;

[0023] FIGS. 6D, 6E, and 6F are truth tables corresponding to the circuit diagrams illustrated in FIGS. 6A, 6B, and 6C; and

[0024] FIG. 7 is a timing diagram illustrating a right-shift operation of the DLL as shown in FIG. 5A.

[0025] Referring to FIG. 3, the disclosed delay locked loop (DLL) generally includes a clock buffer 300, a delay monitor 310, a phase detector 320, a first shift register 330, a first delay line 340, a second shift register 350 and a second delay line 360.

[0026] The clock buffer 300 receives an external clock EXT_CLK to generate an internal clock CLK_IN. The delay monitor 310 receives a DLL clock DLL_CLK, i.e., an output of the DLL, to perform a monitoring operation for determining a delay amount of the internal clock CLK_IN. An output of the delay monitor 310 is fed back to the phase detector 320.

[0027] The phase detector 320 compares a phase difference between the internal clock CLK_IN and the output of the delay monitor 310 to generate one of a first shift-left signal SHF_L1 and a first shift-right signal SHF_R1 depending on whether less or more delay is required.

[0028] The first shift register 330 decreases the delay amount of the internal clock CLK_IN in response to the first shift-left signal SHF_L1 and increases that delay amount in response to the first shift-right signal SHF_R1. For example, the initial state of the output of the first shift register 330 to the first delay line 340 is a logic combination of 000. If the phase detector 320 generates a first shift-left signal SHF_L1 then the output of the first shift register 330 becomes a logic combination of 111 from a logic combination of 000 and at the same time the first shift register 330 generates a second shift-left signal SHF_L2. However, if the phase detector 320 generates a first shift-right signal SHF_R1 then the output of the first shift register 330 becomes a logic combination of 100 from a logic combination of 000. In another example, if when the output of the first shift register 330 is a logic combination of 110, the phase detector 320 generates a first shift-right signal SHF_R1 then the output of the first shift register 330 becomes a logic combination of 111 from a logic combination of 110. Furthermore, if the phase detector 320 generates another first shift-right signal SHF_R1 then the output of the first shift register 330 becomes a logic combination of 000 from a logic combination of 111 and at the same time the first shift register 330 generates a second shift-right signal SHF_R2 . However, if the phase detector 320 generates a first shift-left signal SHF_L1 instead of a first shift-right SHF_R1 when the output of the first shift register 330 has a logic combination of 111 then the output of the first shift register 330 returns to a logic combination of 110 from a logic combination of 111.

[0029] The first delay line 340 delays the internal clock CLK_IN according to an output of the first shift register 330. The first delay line 340 includes a plurality of delay units having a small delay amount.

[0030] The second shift register 350 controls the delay amount of an output of the first delay line 340 in response to one of a second shift-left signal SHF_L2 and a second shift-right signal SHF_R2, which are outputted from the first shift register 330. For example, assuming the initial state of the output of the second shift register 350 to the second delay line 360 is a logic combination of 010, if the first shift register 330 generates a second shift-right signal SHF_R2 then the output of the second shift register 350 becomes a logic combination of 001 from a logic combination of 010. However, if the first shift register 330 generates a second shift-left signal SHF_L2 then the output of the second shift register 350 becomes a logic combination of 100 from a logic combination of 010. In other words, shift register 350 operates like the conventional shift register 130 described above in connection with FIGS. 1 and 2.

[0031] The second delay line 360 delays an output of the first delay line 340 by a predetermined delay amount in response to an output of the second shift register 350. Preferably, the second delay line 360 is structured and operates like the delay line 140 described above in connection with FIGS. 1 and 2. The second delay line 360 includes a plurality of delay units having a delay amount larger than that of the plurality of delay units in the first delay line 340. As a result, the first delay line 340 provides greater precision in controlling a delay to the internal clock CLK_IN because a delay amount of the first delay line 340 is a smaller increment of a delay amount of the second delay line 360.

[0032] FIG. 4 is a more detailed circuit diagram illustrating the first delay line 340 and the second delay line 360. Referring to FIG. 4, the first delay line 340 controls the delay amount of the internal clock CLK_IN in response to a first, a second, and a third shift control signals FSL1, FSL2 and FSL3, respectively. The second delay line 360 controls a delay amount of an output of the first delay line 340 in response to a fourth, a fifth, and a sixth shift control signals CSL1, CSL2, CSL3, respectively.

[0033] The first delay line 340 includes a first inverter 341 for inverting the internal clock CLK_IN. The first delay line 340 also includes a plurality of first delay units, generally shown as 342, 343, and 344, for controlling the delay amount of the internal clock CLK_IN. The first delay line 340 further includes a second inverter 348 for inverting an output of the first inverter 341.

[0034] Each of the first delay units 342, 343, 344 is implemented with an NMOS transistor 345, 346, and 347, respectively, and a capacitor C1, C2, and C3, respectively. The NMOS transistors 345, 346, 347 and the capacitors C1, C2, C3 are serially coupled to each other, respectively, between an output terminal of the first inverter 341 and a ground terminal GND as shown in FIG. 4. For example, the NMOS transistor 345 is serially coupled to the capacitor C1 in the first delay unit 342. Additionally, each gate of NMOS transistors 345, 346, 347 receives the first, the second, and the third shift control signals FSL1, FSL2, FSL3, respectively.

[0035] Each of the NMOS transistors 345, 346, 347 performs a switching operation in response to a respective one of the first, the second, and the third shift control signals FSL1, FSL2, FSL3. The capacitance of the capacitors C1, C2, C3 is selectively transferred to the second delay line 360 so that the internal clock CLK_IN is delayed by a relatively small delay amount. For example, if the first shift control signal FSL1 is a logic high and the second and third shift control signals FSL2, FSL3 are a logic low, only the NMOS transistor 345 is turned on. As a result, the delay amount of the internal clock CLK_IN is increased by the capacitance of the capacitor C1. Similarly, if the first and second shift control signals FSL1 and FSL2 are a logic high and the third shift control signal FSL3 is a logic low, the NMOS transistors 345, 346 are turned on. Accordingly, the delay amount of the internal clock CLK_IN is increased by the capacitance of capacitors C1 and C2.

[0036] The second delay line 360 generally includes a plurality of NAND gates 361, 362, 363, a plurality of second delay units 364, 365, 366, and an output unit 374. The NAND gates 361, 362, 363 are coupled to the second delay units 364, 365, 366, respectively. The output unit 374 includes a NAND gate 373, which is coupled to an output of one of the second delay units 364.

[0037] The NAND gates 361, 362, 363 receive and perform a logic NAND function with an output CLK_IN_D of the first delay line 340 and the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3, respectively. Each output of the NAND gates 361, 362, 363 is inputted to the second delay units 364, 365, 366, respectively. For example, the output of the NAND gate 361 is inputted to the second delay unit 364.

[0038] Each of the second delay units 364, 365, 366 includes a first NAND gate 368, 370, 372, respectively, and a second NAND gate 367, 369, 371, respectively. In each of the second delay units 364, 365, 366, the first NAND gates 368, 370, 372 perform a logic NAND function. In the second delay unit 364, for example, the first NAND gate 368 performs a logic NAND function with an output of a corresponding NAND gate 361 and an output of a previous second delay unit 365. Similarly, the first NAND gate 370 in the second delay unit 365 performs a logic NAND function with an output of a corresponding NAND gate 362 and an output of a previous second delay unit 366. In the second delay unit 366, however, the first NAND gate 372 performs a logic NAND function but with an output of a corresponding NAND gate 363 and a power potential VCC.

[0039] In each of the second delay units 364, 365, 366, the second NAND gates 367, 369, 371 also perform a logic NAND function with an output of a corresponding first NAND gate 368, 370, 372, respectively, and the power potential VCC. In the second delay unit 365, for example, the second NAND gate 369 performs a logic NAND function with an output of the corresponding first NAND gate 370 and the power potential VCC.

[0040] The output unit 374 includes a NAND gate 373 that generates the DLL clock DLL_CLK by performing a logic NAND function of the power potential VCC and an output of one of the second delay units 364. In other words, the output unit 374 acts as an inverter.

[0041] FIGS. 5A to 5C are circuit diagrams illustrating a shift-right operation of the disclosed DLL. FIGS. 5D to 5F are truth tables to the circuit diagrams illustrated in FIGS. 5A to 5C, respectively. A timing diagram in conjunction with FIG. 5A is shown in FIG. 7, which is further discussed below. τ_FD(fine delay) denotes the delay amount of each of the first delay units 342, 343, 344, and τ_CD(coarse delay) denotes the delay amount of each of the second delay units 345, 346, 347. τ_FDhas a value smaller than τ_CD. For example, τ_CDis preferably equal to 4τ_FD.

[0042] Referring to FIGS. 5A and 5D, the first, the second, and the third shift control signals FSL1, FSL2, FSL3 are a logic high, a logic high, and a logic low, respectively, i.e., the output of the first shift register 330 to the first delay line 340 is a logic combination of 110. Because the first and the second shift control signals are logic highs, the NMOS transistors 345, 346 in the first delay units 342, 343, respectively, are turned on. Accordingly, the capacitance of the capacitors C1, C2 in the first delay units 342, 343 increase the delay amount of the output of the first inverter 341 as one of ordinary skill in the art will readily recognize.

[0043] The second delay line 360 receives the output of the first delay line CLK_IN_D and the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3. In this example, the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3 are a logic low, a logic high, and a logic low, respectively (i.e., the second shift register 350 outputs a logic combination of 010). As noted above, each of the NAND gates 361, 362, 363 of the second delay line 360 receives the output of the first delay line CLK_IN_D and one of the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3. For example, the NAND gate 361 receives the output of the first delay line CLK_IN_D and the fourth shift control signal CSL1, which is a logic low. As a result, the output of the NAND gate 361 is a logic high. Accordingly, the output of the first delay line CLK_IN_D does not matter (i.e., a “don't care”) to the NAND gate 361 because the fourth shift control signal CSL1 is a logic low. Similarly, the output of the first delay line CLK_IN_D is also a “don't care” to the NAND gate 363 because the sixth shift control signal CSL3 is a logic low.

[0044] In contrast, the output of the NAND gate 362 depends on the output of the first delay line CLK_IN_D because the fifth shift control signal CSL2 is a logic high. In particular, when the output of the first delay line CLK_IN_D is a logic high then the output of the NAND gate 362 is a logic low. When the output of the first delay line CLK_IN_D is a logic low then the output of the NAND gate 362 is a logic high. The NAND gate 362, therefore, provides an output that is the output of the first delay line CLK_IN_D inverted.

[0045] As noted above, the output of the NAND gate 363 is a logic high and as a result, the output of the second delay unit 366, i.e., the output of the NAND gate 371, is also a logic high. Accordingly, the output of the second delay unit 365 is the same as the output of the NAND gate 362, but delayed in time by delay unit 365. Furthermore, the output of the NAND gate 361 is a logic high so the output of the second delay unit 364 is the output of the second delay unit 365 delayed through the second delay unit 364. The output unit 374 inverts the output of the second delay unit 364 to provide the DLL clock DLL_CLK. Accordingly, DLL clock DLL_CLK is the output of the first delay line CLK_IN_D delayed through two second delay units 364, 365. Therefore, the output of the output unit 374 is the DLL clock DLL_CLK, which is the internal clock CLK_IN delayed through two first delay units 342, 343 in the first delay line 340, (i.e., 2τ_FD) and two second delay units 364, 365 in the second delay line 360 (i.e., 2τ_CD) . As a result, a total delay amount is equal to (2τ_FD+2τ_CD), i.e., 10τ_FD.

[0046] Referring to FIGS. 5B and 5E, when the phase detector 320 generates the first shift-right signal SHF_R1, all of the first, the second, and the third shift control signals FSL1, FSL2, FSL3 become logic highs, i.e., the output of the first shift register 330 to the first delay line 340 is a logic combination of 111. In particular, the output of the first shift register 330 becomes a logic combination of 111 from a logic combination of 110 as shown in FIGS. 5A and 5D if the phase detector 320 generates the first shift-right signal SHF_R1. Because the first, the second, and the third shift control signals FSL1, FSL2, FSL3 are logic highs, the NMOS transistors 345, 346, 347 in the delay units 342, 343, 344, respectively, are turned on. The capacitance of the capacitors C1, C2, C3 in the delay units 342, 343, 344, respectively, increase the delay amount of the output of the first inverter 341 by a delay amount of 3τ_FD. Accordingly, the output of the first delay line CLK_IN_D is the internal clock CLK_IN delayed by 3τ_FD.

[0047] The fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3 remain the same, i.e., a logic low, a logic high, and a logic low, respectively, as in FIG. 5A. Accordingly, the output of the output unit 374 is the DLL clock DLL_CLK, which is the internal clock CLK_IN delayed through three first delay units 341, 342, 343 in the first delay line 340 (i.e., 3τ_FD) and two second delay units 364, 365 in the second delay line 360 (i.e., 2τ_CD). Therefore, a total delay amount is equal to [(2τ_FD+2τ_CD)+τ_FD], i.e., 11τ_FD.

[0048] Referring to FIGS. 5C and 5F, when the phase detector 320 again generates the first shift-right signal SHF_R1, there is no additional delay unit for further increasing the delay amount in the first delay line 340. However, because 4τ_FD=τ_CD, and delay increase corresponding to one more τ_FDcan be achieved by activating one more delay unit in the second delay line 360 (i.e., adding 1τ_CD) and turning off all three of the delay units 342, 343, 344 in the first delay line 340 (i.e., subtracting 3τ_FD), the output of the first shift register 330 is switched to a logic combination of 000 (i.e., FSL1, FSL2 and FSL3 are logic low). This change in logic state (from 111 to 000) causes the first shift register 330 to generate a second shift-right signal SHF_R2.

[0049] As described above, the second shift register 350 responds to the shift-right signal SHF_R2 by shifting the logic high signal to the right one step (e.g., from CSL2 to CSL3). Conversely, if the first shift register 330 is in its lowest state (i.e., FSL1, FSL2, FSL3 are all zero) and a shift-left signal SHF_L1 is received, the shift register 330 changes from logic combination 000 to logic combination 111 (i.e., add 3τ_FD), and generates a shift-left signal SHF_L2. The second shift register 350 responds to the shift-left signal SHF_L2 by moving the logic high signal left one step (e.g., from CSL2 to CSL1) to thereby subtract 1τ_CDso that a total of 1τ_FDof delay is reduced (3τ_FD−1τ_CD=−1τ_FD)

[0050] Returning to the example where a second shift-right signal SHF_R2 has been generated, the second shift register 350 performs a shift-right operation. Accordingly, the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3 are a logic low, a logic low, and a logic high, respectively, i.e., shifted from a logic combination of 010 to a logic combination of 001. Because the sixth shift control signal CSL3 is a logic high, the output of the NAND gate 363 is the output of the first delay line CLK_IN_D inverted. As a result, the output of the second delay unit 366 is the output of the NAND gate 363 delayed through the second delay unit 366. The fifth control signal CSL2 is a logic low so the output of the first delay line CLK_IN_D does not matter to the NAND gate 362, i.e., a “don't care.” Accordingly, the output of the NAND gate 362 is a logic high regardless of the state of CLK_IN_D. As a result, the output of the second delay unit 365 is the output of the NAND gate 363 delayed through two second delay units 365, 366. The fourth control signal CSL1 is a logic low so the output of the first delay line CLK_IN_D does not matter to the NAND gate 361, i.e., a “don't care.” Therefore, the output of the NAND gate 361 is a logic high. The output of the delay unit 364 is the output of the NAND gate 363 delayed through three second delay units 364, 365, 366. The output unit 374 performs a logic NAND function with the power potential VCC and the output of the delay unit 364 to provide the DLL clock DLL_CLK. The DLL_CLK is the output of the first delay line CLK_IN_D delayed through three second delay units 364, 365, 366. Accordingly, the output of the output unit 374 is the DLL clock DLL_CLK, which is the internal clock CLK_IN delayed through three second delay units 364, 365, 366 only in the second delay line 360, i.e., 3τ_CD. As a result, the internal clock CLK_IN is delayed by as much as 3τ_CD=12τ_FD. An increase of up to 3 more τ_FDcan be achieved for a total of 15τ_FDby turning FSL1, FSL2 and FSL3 to logic highs so that all possible delay units are activated.

[0051] FIGS. 6A to 6C are circuit diagrams illustrating a shift-left operation of the disclosed DLL. FIGS. 6D to 6F are truth tables corresponding to the circuit diagrams illustrated in FIGS. 6A to 6C, respectively. Referring to FIGS. 6A and 6D, when only the first and the fifth shift control signals FSL1 and CSL2 are a logic high (i.e., the output of the first shift register 330 to the first delay line 340 is a logic combination of 100, and the output of the second shift register 350 to the second delay line 360 is a logic combination of 010), the internal clock CLK_IN is delayed through one first delay unit 342 in the first delay line 340 and two first delay units 364, 365 in the second delay line 360. In particular, the NMOS transistor 345 is turned on, and the output of the first inverter 341 is delayed by one first delay unit 342 in the first delay line 340. As noted above, the output of the first inverter 341 is the internal clock CLK_IN inverted. Accordingly, the output of the first delay line CLK_IN_D is the internal clock CLK_IN delayed by one first delay unit 342.

[0052] The second delay line 360 receives the output of the first delay line CLK_IN_D and the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3, respectively. Similar to FIGS. 5A and 5B, the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3 in FIG. 6A are a logic low, a logic high, and a logic low, respectively (i.e., the second shift register 350 outputs a logic combination of 010). Accordingly, the output of the NAND gate 362 is delayed by two second delay units 364, 365 in the second delay line 360. The output unit 374 inverts the output of the second delay unit 364 to provide the delay locked loop clock DLL_CLK. The delay locked loop clock DLL_CLK is the internal clock CLK_IN delayed through one first delay unit 342 in the first delay line 340 (i.e., τ_FD) and two second delay units 364, 365 in the second delay line 360 (i.e., 2τ_CD). As a result, a total delay amount is equal to (τ_FD+2τ_CD) i.e., 9τ_FD.

[0053] Referring to FIGS. 6B and 6E, when the phase detector 320 generates the first shift-left signal SHF_L1, all of the first, the second, and the third shift control signals FSL1, FSL2, FSL3 become logic lows, i.e., the output of the first shift register 330 to the first delay line 340 is a logic combination of 000. None of the NMOS transistors 345, 346, 347 in the first delay units 342, 343, 344 are turned on. Accordingly, the output of the first inverter 341 is not delayed through a first delay unit in the first delay line 340. Therefore, the output of the first delay line CLK_IN_D is the internal clock CLK_IN without any first delay unit.

[0054] Similar to FIG. 6A, the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3 in FIG. 6B are a logic low, a logic high, and a logic low, respectively (i.e., a logic combination of 010). The output of the NAND gate 363 is a logic high so the output of the second delay unit 366 is also a logic high. The output of the NAND gate 362 is the output of the first delay line CLK_IN_D inverted. The output of the second delay unit 365 is the output of the NAND gate 362 delayed through the second delay unit 365. The output of the NAND gate 361 is a logic high so the output of the second delay unit 364 is the output of the NAND gate 362 delayed through two second delay units 364, 365. The output unit 374 inverts the output of the delay unit 364 to provide the delay locked loop clock DLL_CLK. The DLL_CLK is the output of the first delay line CLK_IN_D delayed through two second delay units 364, 365. As noted above, the output of first delay line CLK_IN_D in FIG. 6B is the internal clock CLK_IN without any first delay unit. Accordingly, the delay locked loop DLL_CLK is the internal clock CLK_IN delayed through two second delay units 364, 365 in the second delay line 360, i.e., a second delay amount of 2τ_CD. Therefore, a total delay amount is equal to 2τ_CD, i.e., [(τ_FD+2τ_CD)−τ_FD] or 8τ_FD.

[0055] Referring to FIGS. 6C and 6F, when the phase detector 320 again generates the first shift-left signal SHF_L1, there is no further delay unit for further decreasing the delay amount in the first delay line 340 (i.e., there is already no delay produced by the first delay line 340). Accordingly, the first, the second, and the third shift control signals FSL1, FSL2, FSL3 become logic highs, i.e., the output of the first shift register 330 to the first delay line 340 becomes a logic combination 111 from a logic combination of 000 as shown in FIGS. 6B and 6E. All of the NMOS transistors 345, 346, 347 are turned on, and the output of the first inverter 341 is delayed through three first delay units 342, 343, 344 (i.e., 3τ_FD). Therefore, the output of the first delay line CLK_IN_D is the internal clock CLK_IN delayed through three first delay units 342, 343, 344.

[0056] As a result, the first shift register 330 generates a shift-left signal SHF_L2. The second shift register 350 responds by shifting the logic high signal one step left, i.e., the fourth, the fifth, and the sixth shift control signals CSL1, CSL2, CSL3 are a logic high, a logic low, and a logic low, respectively. This has the effect of activating only delay unit 364 (i.e., delay units 365 and 366 are switched off) as explained above. The delay locked loop DLL_CLK is the first delay line CLK_IN_D delayed through the second delay unit 364. Therefore, the delay locked loop DLL_CLK is the internal clock CLK_IN delayed through three first delay units 342, 343, 344 in the first delay line 340 (i.e., a first delay amount of 3τ_FD) and one second delay unit 364 in the second delay line 360 (i.e., a second delay amount of τ_CD). As a result, the internal clock CLK_IN is delayed by (3τ_FD+τ_CD), i.e., (2τ_CD−τ_FD) or 7τ_FD.

[0057] FIG. 7 is a timing diagram of a right-shift operation of the DLL as shown in the circuit diagram of FIG. 5A. As noted above, τ_FDdenotes a delay amount of each of the first delay units 342, 343, 344, and τ_CDdenotes a delay amount of each of the second delay units 345, 346, 347. In particular, τ_CDis equal to 4τ_FD.

[0058] Referring to FIG. 7, the internal clock CLK_IN is inverted by the first inverter 341 of the first delay line 340. In this example, the first and the second shift control signals FSL1, FSL2 are logic highs. Accordingly, the NMOS transistors 345, 346 in the first delay units 342, 343, respectively, are turned on. The output of the first inverter 341 is delayed through two first delay units 342, 343 in the first delay line 340, i.e., 2τ_FD. For example, the internal clock CLK_IN goes high at 3τ_FD. The internal clock CLK_IN is inverted by the first inverter 341, delayed through two first delay units 342, 343, i.e., 2τ_FD, and inverted by the second inverter 348 to generate the output of the first delay line CLK_IN_D.

[0059] Because, in this example, the fourth and the sixth shift control signals CSL1, CSL3 are logic lows and the fifth shift control signal CSL2 is logic high, only the first and second delay units 364, 365 in the second delay line 360 are activated. Accordingly, the output of the second delay unit 365 is the output of the NAND gate 362 delayed through the second delay unit 365. As noted above, the delay amount of the second delay unit 365 is τ_CD, which is equal to 4τ_FD. Therefore, the output of the second delay unit 365 is the output of the NAND gate 362 delayed through 4τ_FD. The output of the second delay unit 364 is based on the output of the NAND gate 361 and the output of the second delay unit 365. The second delay unit 364 also has a delay amount of τ_CD, i.e., 4τ_FD. As a result, the output of the second delay unit 364 is the output of the second delay unit 365 delayed through 4τ_FD, i.e., the output of the NAND gate 362 delayed through 8τ_FD. The output unit 374 inverts the output of the second delay unit 364 to provide the delay locked loop DLL_CLK. The delay locked loop DLL_CLK is the output of the first delay line CLK_IN_D delayed through two second delay units 364, 365 in the second delay line 360, i.e., 8τ_FD. Accordingly, the delay locked loop DLL_CLK is the internal clock CLK_IN delayed through two first delay units 342, 343 in the first delay line 340, i.e., 2τ_FD, and two second delay units 364, 365 in the second delay line 360, i.e., 8τ_FD. As a result, the internal clock CLK_IN has a total delay amount of 10τ_FD.

[0060] As can be seen, a request to increase or decrease the delay amount is always responded to by a fine delay step τ_FDin the appropriate direction. As a result, the total delay amount of the internal clock CLK_IN may be controlled with greater precision than in prior art devices (i.e., τ_FDis a smaller than τ_CD). Additionally, since a minimum delay unit τ_FDdepends on the capacitance of the capacitors contained in the first delay units, it is possible to obtain a desired delay unit even at a low power voltage by properly designing the capacitors and the number of the first delay units contained in the first delay line 340.

[0061] Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.