An improved delay-locked loop circuit

Abstract: A delay-locked loop circuit ("DLL") comprises a delay line (1) with a delay (Δt) which can be varied in a controlled manner in order to delay a periodic input signal (CKin) of period T, and circuit means (2, 7) for controlling the delay line (1) in order to lock the delay (Δt) to the period T. The delay line (1) supplies to the control circuit means (2, 7) a plurality of periodic signals (CK1-CKN) each delayed relative to the periodic input signal by a respective fraction of the delay (Δt), and the control circuit means (2, 7) comprise sequence-detector circuit means (2) which can periodically detect, in the delayed signals, characteristic sequences of digital values indicative of the delay (Δt) and, in dependence on the type of characteristic sequence, can bring about a reduction or an increase in the delay (Δt) for locking to the period T.

Description: The present invention relates to an improved delay-locked loop circuit (or briefly DLL). A DLL is a circuit with feedback which, on the basis of a periodic input signal of period T, can theoretically output a periodic signal with the same period T, with a delay At relative to the input signal equal to and locked to the period T of the input signal. Known DLLs comprise a delay line the delay of which can be varied in a controlled manner and which receives the periodic signal of period T as an input and produces the periodic signal delayed by Δt as an output. The periodic input signal and the delayed periodic signal are supplied to a phase comparator circuit. On the basis of the phase difference detected between the delayed periodic signal and the periodic input signal, the phase comparator controls delay-line control logic circuitry which acts on the delay line to bring about an increase or a decrease in the delay At to keep the delay equal to the period T. The delay line is normally constituted by a plurality of N elemental delay units which are normally identical to one another, at least theoretically, and each of which can introduce an elemental delay equal to At/N. In the locked condition (that is Δt = T), the DLL can thus supply, on the basis of the periodic input signal, N periodic signals all of equal period and each delayed relative to the preceding signal by a time T/N. DLL circuits have various applications. In particular, these circuits may be used in the synchronous digital communications field, in which it is essential to have synchronization circuits in order correctly to decode data flows travelling on the communication channels. In European patent application No. 99830518.9 filed on 06.08.1999 in the name of STMicroelectronics S.r.1., it is proposed to use a DLL in an "SDH" (synchronous digital hierarchy) communication interface, as an alternative to the usual phase-locked loop (briefly "PLL") circuits, for synchronization with a received data flow. However, a disadvantage of known DLLs is that the locked condition may not occur, as would be desirable, with a delay Δt = T, but may occur instead with delays which are multiples or submultiples of the period T of the input signal. This is due basically to the fact that conventional phase comparators used in known DLLs cannot detect the phase difference correctly outside the range (-π rad; π rad). For a better understanding of the reason for the above statement, reference is made to appended Figure 1, in which the periodic input signal is indicated CKin and the output signal of the DLL, that is, the output signal of the delay line, the delay of which relative to the input signal is to be locked to the period of the input signal, is indicated CKout. The phase comparator operates on the leading edge RE of the input signal CKin and of the output signal CKout. In case a) of Figure 1, the output signal CKout has a delay Δt1 where t/2<Δt1<T, relative to the input signal CKin; the phase comparator detects the phase advance of the signal CKout relative to the signal CKin and communicates it to the control logic circuitry which modifies the delay At of the delay line so as to increase the delay. In case b), the output signal CKout has a delay Δt2, where T<Δt2<(3/2)T, relative to the input signal CKin; the phase comparator detects the phase delay of the output signal CKout relative to the signal CKin and communicates it to the control logic circuitry which modifies the delay Δt of the delay line so as to reduce the delay. In cases a) and b), the DLL can lock onto the period T of the input signal. Case c) will now be examined: the output signal CKout has a delay Δt3, where Δt3>(3/2)T, relative to the input signal CKin and, in order for the DLL to be locked, the delay introduced by the delay line would have to be decreased. However, the phase comparator interprets this delay as an advance and informs the control logic circuitry of the need to increase the delay of the line, thus going in the opposite direction to that required to achieve locking. Similarly, in case d), the output signal CKout has a delay Δt4, where Δt4<T/2, relative to the input signal CKin and, for the DLL to lock, it would therefore be necessary to increase the delay introduced by the line. However, the phase comparator interprets this advance as an excessive delay and informs the control logic circuitry to reduce the delay of the line, again going in the opposite direction to that required to achieve locking. In the two cases c) and d), the delay locked loop does not tend towards the locking condition, but tends away from this condition until, in case c), the maximum value of the delay which can be introduced by the line is reached and, in case d), the minimum value of the delay is reached, that is, until the delay locked loop reaches the limit of its own dynamic range and the feedback loop can no longer operate. Documents US-5663665 and EP-A-460274 describe a DLL circuit employing a delay line, sampling flip-flops and a charge pump circuit. The pre-characterising portion of claim 1 reflects the DLL circuit of document EP-A-460274. In view of the prior art described above, an obj ect of the present invention is to provide an improved DLL which is not affected by the disadvantages of conventional DLL circuits. According to the present invention, this object is achieved by means of a delay-locked loop (DLL) circuit as described in the appended claim 1. The characteristics and advantages of the present invention will become clearer from the following detailed description of a possible practical embodiment thereof, illustrated purely by way of non-limiting example in the appended drawings, in which: Figure 1 is a time graph showing the operation of a conventional DLL circuit, Figure 2 is a schematic block diagram of a DLL circuit according to the present invention, Figures 3 to 6 are time graphs showing the operation of the DLL circuit according to the present invention, Figures 7 and 8 are schematic views of characteristic sequences of digital values which can be detected by the DLL circuit according to the invention for locking the loop, and Figure 9 is a circuit diagram of a possible practical embodiment of a functional block of the circuit of Figure 2. With reference to Figure 2, a DLL circuit according to the present invention comprises a delay line 1 with variable delay, comprising a chain of N substantially identical elemental delay units 11 - 1N. Each delay unit 11 - 1N introduces a delay equal to Δt/N, where At is the total delay introduced by the delay line 1. The delay line 1 receives as an input a periodic signal CKin of period T and outputs N delayed periodic signals CK1 - CKN, each of the delayed periodic signals CK1 - CKN being taken at the output of a respective elemental delay unit 11 - 1N. The periodic signal CK1 is thus delayed by Δt/N relative to the periodic input signal CKin; each of the periodic signals CKi, where i = 2...N, is delayed by Δt/N relative to the preceding periodic signal CKi-1 and by i(Δt/N) relative to the periodic input signal CKin. The delayed periodic signals CK1 - CKN and the periodic input signal CKin are supplied to a circuit 2 which can detect the magnitude of the delay At introduced by the delay line 1 relative to the period T of the periodic input signal CKin. The circuit 2 comprises a sampling circuit 3 which operates in synchronism with the periodic input signal CKin, for example, on the leading edge of the signal CKin, and which receives as inputs the N delayed periodic signals CK1 - CKN. The sampling circuit 3 outputs a word 4 composed of N bits, each bit corresponding to a respective signal of the N signals CK1 - CKN. The word 4 constitutes a photograph of the levels of the signals CK1 - CKN at the moment of sampling. The N-bit word 4 is supplied to a sequence-interpreter or detector circuit 5, for example, a decoder, which interprets the N-bit word 4 and consequently activates or does not activate a signal 6. The signal 6 is supplied to control logic circuitry 7 of the DLL which controls the elemental delay units 11 - 1N of the delay line with variable delay 1, so as to increase or reduce the elemental delay Δt/N introduced by each of them and hence to increase or reduce the total delay At of the delay line 1. According to the invention, when there are changes in the elemental delay Δt/N, and hence in the total delay Δt of the delay line 1, the levels of the delayed periodic signals CK1 - CKN at a predetermined generic moment in time change; the logic states of the individual bits of the N-bit word 4 which, as stated, constitutes a photograph of the levels of the signals CK1 - CKN taken at a preselected moment in time, consequently change. A certain value of the delay Δt corresponds to each configuration of the N bits of the word 4, that is, to each sequence of "0"s and "1"s in the N-bit word 4. By interpreting the configuration of the N bits of the word 4, that is, by detecting the particular sequence of "0"s and "1"s, it is therefore possible to deduce the value of the delay Δt and to determine whether the delay Δt is greater than or less than the value T which corresponds to the locked condition of the loop. By repeating the operation periodically with a period of repetition equal to T, it is possible to keep the DLL loop in the locked condition. The time graphs shown in Figures 3 to 6 which, for simplicity, relate to the case of a delay line 1 comprising eight elemental delay units which supply eight delayed periodic signals CK1 - CK8, will assist an understanding of the foregoing explanation. Naturally, the selection to show an example in which there are eight delayed periodic signals is purely for the purposes of greater simplicity of the drawings, since it may clearly be extended to examples in which delay lines supply a different number of periodic delayed signals, for example, sixteen signals. Figure 3 shows the case of a delay Δt which is too small, in particular less than T/2. At the moment tc at which the periodic signal CKin has a leading edge and the sampling of the signals CK1 - CK8 therefore takes place, the configuration of the eight bits of the word 4 is as follows: bit 1 (CK1) bit 2 (CK2) bit 3 (CK3) bit 4 (CK4) bit 5 (CK5) bit 6 (CK6) bit 7 (CK7) bit 8 (CK8) 0 0 0 0 0 0 0 0 If the word 4 is regarded as a sequence of eight bits, it will be noted that, in the sequence, moving from bit 1 corresponding to the signal CK1 towards bit 8 corresponding to the signal CK8, there is no transition from "1" to "0" between one bit and the next bit. This configuration is interpreted by the circuit 5 as corresponding to a delay Δt which is too small; the circuit 5 informs the control logic circuitry 7 by means of the signal 6, of the need to increase the elemental delay At/N of the delay units 1N - 18. Figure 4 shows the case of a delay Δt which is still too small but is closer to the desired value T (in particular, a delay Δt of between T/2 and T). At the moment in time tc, the configuration of the eight bits of the word 4 is as follows: bit 1 (CK1) bit 2 (CK2) bit 3 (CK3) bit 4 (CK4) bit 5 (CK5) bit 6 (CK6) bit 7 (CK7) bit 8 (CK8) 0 0 0 0 1 1 1 1 If the word 4 is regarded as a sequence of eight bits, it will be noted that, in the sequence, moving from bit 1 corresponding to the signal CK1 towards bit 8 corresponding to the signal CK8, there is a "0" → "1" transition (between bit 4 corresponding to the signal CK4 and bit 5 corresponding to the signal CK5) but no "1" "0" transition; this sequence is indicative of the fact that the delay Δt is still too small but is tending towards the locked condition Δt = T. The circuit 5 informs the control logic circuitry 7, by means of the signal 6, of the need to increase the elemental delay Δt/N of the elemental delay units 11 - 18. Figure 5 shows the case of a delay At which is slightly too great relative to the locking value (in particular a delay Δt of between T and (3/2)T). At the moment tc, the configuration of the eight bits of the word 4 is as follows: bit 1 (CK1) bit 2 (CK2) bit 3 (CK3) bit 4 (CK4) bit 5 (CK5) bit 6 (CK6) bit 7 (CK7) bit 8 (CK8) 0 0 0 1 1 1 1 0 If the word 4 is regarded as a sequence of eight bits, it will be noted that, in the sequence, moving from bit 1 corresponding to the signal CK1 towards bit 8 corresponding to the signal CK8, there is a "1" → "0" transition, in particular, between bit 7 corresponding to the signal CK7 and bit 8 corresponding to the signal CK8; this sequence is indicative of the fact that the delay Δt is too great, but is tending towards the locked situation Δt = T. The circuit 5 informs the control logic circuitry 7, by means of the signal 6, of the need to reduce the elemental delay Δt/N of the elemental delay units 11 - 18. Figure 6 shows the case of a very long delay Δt, in particular, greater than (3/2)T. At the moment tc, the configuration of the eight bits of the word 4 is as follows: bit 1 (CK1) bit 2 (CK2) bit 3 (CK3) bit 4 (CK4) bit 5 (CK5) bit 6 (CK6) bit 7 (CK7) bit 8 (CK8) 0 0 1 1 1 0 0 1 Once again, if the word 4 is regarded as a sequence of eight bits, it will be noted that, in this sequence, moving from bit 1 corresponding to the signal CK1 towards bit 8 corresponding to the signal CK8, there is an alternation of "0"s and "1" s and hence several "1" → "0" transitions between consecutive pairs of bits; this situation is indicative of the fact that the delay At is very great. The circuit 5 informs the control logic circuitry 7, by means of the signal 6, of the need to reduce the elemental delay Δt/N of the elemental delay units 11 - 18. Thus, according to the type of sequence of logic values present in the N-bit word 4 and, in particular, according to whether or not there are one or more "1" "0" transitions in the sequence, moving from the bit corresponding to the signal CK1 towards the bit corresponding to the signal CKN, the circuit 5 can establish the magnitude of the delay Δt correctly and can communicate to the control logic circuitry 7 the type of correction to be performed. A first type of sequence composed entirely of "0"s means that the delay of the line 1 is very short and should be increased. A second type of sequence, composed of "0"s followed by "1"s (no "1" → "0" transition) means that the delay of the line is still too small but is approaching the locking value T. A third type of sequence composed of "0"s followed by "1"s followed by "0"s means that the delay of the line is too great but is approaching the locked value T. A fourth type of sequence, composed of "0"s alternating with "1"s means that the delay of the line is very large and should be reduced. This is true in general, irrespective of the number of delayed periodic signals CK1 - CKN (that is, irrespective of the number of elemental delay units 11 - 1N) and hence not only for the example of eight signals described above, but also for a larger number of signals, for example, for sixteen signals. Figures 7 and 8 show schematically the variation of the sequences of logic values within the word 4 in the generic case of a delay line with N elemental delay units in order to reach the locked condition, starting from a condition of very great delay of the line (Figure 7), or very slight delay (Figure 8). In both figures, the hatched areas within the words 4 represent logic "1"s, and the blank areas represent logic "0"s. With reference to Figure 7, it will be noted that, as long as, within the word 4, there is a "1" → "0" transition between the i-th bit and the (i+1)-th bit, the delay At of the delay line is decreased; over time, the 1" → "0" transition moves towards the bit N corresponding to the signal CKN until it disappears. A situation of dynamic equilibrium is reached, in which bit N continues to change from "1" to "0" at successive sampling moments. With reference to Figure 8, as long as the word 4 does not show a "1" to "0" transition between two consecutive bits, the delay At of the delay line is increased. As the delay Δt increases, the last bits of the word 4 (bit N and the immediately preceding bits) change state, changing to "1". In this case also, a situation of dynamic equilibrium in which bit N continues to change from "1" to "0" at successive sampling moments is reached. It is interesting to observe the similarity between the sampling circuit 3 and the sequence-detector circuit 5 and a "bucket brigade" circuit. It will be noted that, by virtue of the invention, the information concerning the correction which the control logic circuitry of the delay line should make is always correct, even for delays At less than T/2 or greater than (3/2)T. Figure 9 shows the circuit diagram of a possible practical embodiment of the circuit 2 of Figure 2 in an example of a delay line with sixteen elemental delay units which therefore supplies sixteen delayed periodic signals CK1 - CK16. The circuit comprises fourteen D-type flip-flops FF2 - FF15, each of which receives at a data input a respective delayed periodic signal CK2 - CK15; the clock inputs of all of the flip-flops FF2-FF15 receive the periodic input signal CKin. Each flip-flop FF2 - FF15 has two outputs Q and QN; the output Q adopts the state present in the respective data input signal at the moment when the signal CKin has a leading edge, the output QN adopts the complementary state. The output Q of each of the flip-flops FF2 - FF14 is supplied, together with the complementary output QN of the following flip-flop FF3 - FF15, to a respective AND gate A2 - A14 with two inputs; the complementary output QN of the flip-flop FF2 is supplied to an AND gate A1 with two inputs the other input of which is connected to earth; the output Q of the flip-flop FF15 is supplied to an input of an AND gate A15 with two inputs the other input of which is supplied by the output of a phase comparator PC (for example, a NAND gate or another phase comparator of known type) which receives the periodic signals CKin and CK16 as inputs. The outputs of the AND gates A1 - A7 are supplied to seven inputs of an OR gate 01 with eight inputs; the eighth input of the OR gate 01 is connected to earth. The outputs of the AND gates A8 - A15 are supplied to the eight inputs of an OR gate 02 with eight inputs. The outputs of the OR gates 01 and 02 are supplied to the two inputs of an OR gate 03 with two inputs, the output of which constitutes the signal 6 for the control logic circuitry 7 (Figure 2). The flip-flips FF2 - FF16 sample the delayed periodic signals CK2 - CK15 upon each leading edge of the periodic signal CKin. The AND gates A1 - A15 detect the presence of "1" → "0" transitions between consecutive bits within the word 4, moving from the bit corresponding to the signal CK1 to the bit corresponding to the signal CK16. The presence of one or more "1" → "0" transitions within the word 4 means that one or more inputs of the OR gates 01, 02 has been brought to level "1" and the output 6 of the gate 03 has consequently been brought to "1". This level informs the control logic circuitry 7 of the need to reduce the value of the delay Δt. If there is no "1" → "0" transition between consecutive bits in the word 4, the output 6 remains at "0". This level informs the control logic circuitry 7 of the need to increase the delay value Δt. It will be noted that the fact that no flip-flop is provided for sampling the signal CK1 and that an input of the AND gate A1 and an input of the OR gate 01 are connected to earth prevents the occurrence of errors due to sampling of the signal CK1 with a lesser delay relative to the input signal CKin. In all cases, the first bit in the word 4 (corresponding precisely to the signal CK1) is always equal to "0"; to have the first bit of the word 4 equal to "1" would in fact presuppose that the elemental delay Δt/N were greater than T/2, that is, that the total delay Δt introduced by the line were greater than N(T/2), for example, greater than 4T with eight elemental delay units or 8T with sixteen units; in practice, such delay values are never reached. It can be noted that, for sequences of the first and fourth types, the information supplied by the phase comparator PC is inhibited, since this information is combined in an AND gate with the output Q of the flip-flop FF15 which is at "0". This is because the sequences of the first and fourth types correspond to very short and very long delays Δt, respectively, for which, as described with reference to Figure 1, the phase comparator PC supplies incorrect information on the type of correction to be performed on the delay Δt. The information supplied by the phase comparator PC is enabled, however, for sequences of the second and third types, in which the output Q of the flip-flop FF15 is "1", which correspond to values of Δt close to the locking value T. Clearly, variations and/or additions may be applied to the embodiment described and illustrated above. For example, the provision of a circuit for sampling the delayed periodic signals is not strictly essential since it suffices, more generally, to provide a circuit which can interpret the sequences of "0"s and "1"s which are present periodically in an N-bit word, each bit corresponding to a respective signal of the N signals CK1 - CKN, for example, corresponding to the leading or trailing edges of the periodic signal input to the DLL circuit. It is also clear that the particular circuit embodiment shown in Figure 7 is purely an example and is absolutely non-limiting. Moreover, the N delayed periodic signals supplied by the delay line are not necessarily delayed equally and may, more generally, be periodic signals each delayed by a fraction of the total delay Δt relative to the periodic input signal. In this case, the sequences of "0"s and "1"s in the N-bit word would clearly be different but it would nevertheless still be possible to establish a relationship between the current value of the delay Δt and the current sequence. If a fast speed is required because the circuit 2 has to be able to operate more quickly than the minimum delay Δt which can be introduced by the delay line 1, it may be necessary to use structures with analog reading amplifiers ("sense amplifiers") instead of the flip-flops FF2 - FF15.

Claims: A delay-locked loop circuit ("DLL") comprising: a delay line (1) with a delay (At) which can be varied in a controlled manner in order to delay a periodic input signal (CKin) of period T and provide a plurality of delayed periodic signals (CK1-CKN) each delayed relative to the periodic input signal by a respective fraction of the delay (Δt), sampling circuit means (3) for periodic sampling said plurality of signals (CK1-CKN) providing a plurality of sampling digital values, a phase comparator (PC) for phase comparison between the periodic input signal (CKin) and that signal of said plurality which is delayed by the delay (Δt), said comparator providing a comparison digital value forming, together with said digital values, a digital word, circuit means (5, 7) for periodically detecting, in said word, characteristic sequences of digital values indicative of the delay (Δt) and, in dependence on the type of characteristic sequence, controlling said line, characterized in that said circuit means comprises: sequence-detector circuit means (5) for generating from said word a single two-level digital signal (6) representative of the need of increasing or reducing the delay (At) for locking to the period T, logic circuit means (7) for controlling the delay line (1) and carrying out a reduction or an increase in the delay (Δt) on the bases of said two level signal (6). A circuit according to Claim 1, characterized in that the sequence-detector circuit means (5) includes a plurality of logic gates (A1-A15, O1, 02, 03) for generating said two-level signal on the bases of said word. A circuit according to Claim 2, characterized in that the delay line (1) comprises a plurality of elemental delay units (11-1N) in cascade, each of which introduces a respective elemental delay (Δt/N) which can be varied in a controlled manner by the logic circuit means (7), each delayed periodic signal (CK1-CKN) being picked up at the output of a respective elemental delay unit. A circuit according to Claim 3, characterized in that the elemental delays (Δt/N) introduced by the elemental delay units (11-1N) are equal to one another, the delayed periodic signals (CK1-CKN) being delayed equally. A circuit according to Claim 4, characterized in that the sampling circuit means (3) receives said input signal (CKin) for periodic sampling of the plurality of delayed periodic signals (CK1-CKN) in synchronism with said periodic input signal (CKin) A circuit according to Claim 5, characterized in that the sampling circuit means (3) comprises flip-flops (FF2-FF15) with synchronization inputs supplied by the periodic input signal (CKin). A circuit according to Claim 5 or Claim 6, characterized in that the sequence-detector circuit means (2) can detect transitions from a first logic level ("1") to a second logic level ("0") between consecutive bits of a word (4) in which each bit corresponds to a respective delayed periodic signal (CK1-CKN), the word (4) constituting a photograph of the digital values of the delayed periodic signals (CK1-CKN) at the moment of sampling. A circuit according to any one of Claims 5, 6 and 7, characterized in that the sequence-detector means (2) comprises means (A15) for inhibiting the phase comparator in the presence of characteristic sequences indicative of values of the delay (Δt) outside a predetermined range of values.

Inventors: GUINEA JESUS, MAGGIO SANTO, TOMASINI LUCIANO, GUINEA, JESUS, MAGGIO, SANTO, TOMASINI, LUCIANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: H03L7
IPC codes: H03L7, H04L7, H03K5