Delay locked loop

Abstract: A clock latency circuit, method and system is provided which allows the synchronization of data according to the rising and falling edges of a system clock.

Description: BACKGROUND OF THE INVENTION It is important that data output transitions associated with an integrated circuit, e.g., a memory (dynamic random access memory (DRAM) or static random access memory (SRAM)), or other timing critical device occur in synchronized fashion with a system clock to the integrated circuit. Lack of data output synchronization with the system clock is often described in terms of clock to data latency. Latency problems have been addressed in the past through use of a delay locked loop (DLL) which is illustrated in the schematic drawing of figure 1. In a DLL, phase comparator 2 controls delay line 4 so that the phase difference between clock signals Ckin and Ckout is zero. The time delay provided by delay line 4 stabilizes when the delay between signals Ckin and Ckout is k*T, where T is the period of clock signal Ckin and k is a natural number. Prior art schemes to eliminate clock to data latency generally work with reference to the rising edge of a clock. Latency problems with respect to the falling edge of a clock are not addressed. Consequently, such schemes are not well suited for double data rate applications such as those concerning double data rate synchronous dynamic random access memories (DDR SDRAMs). A need exists to compensate for clock to data latency pertaining to double data rate applications. BRIEF DESCRIPTION OF THE DRAWINGS AND RELATED PRIOR ART Figure 1 is a schematic drawing of conventional delay locked loop (DLL). as e.g. known from US-A 4,899,071 which discloses an active digital line (DLL) using two delay lines. A phase comparator receives the input signal and the output signal of one of the delay lines and controls both delay lines accordingly. Figure 2 illustrates a schematic drawing of a preferred embodiment of the invention. Figure 3 illustrates a timing diagram showing the operation of the circuit in figure 2. Reference numerals and symbols have been carried forward. DETAILED DESCRIPTION OF THE INVENTION The invention solves the double data rate clock to data latency problem through the implementation of two docks: one triggered by the rising edge of the system clock and the other triggered by the falling edge of the system dock. The said two generated docks are phase adjusted through two delay lines with common control. Figure 2 illustrates a schematic drawing of preferred embodiment of a circuit according to the invention which can be fabricated on an integrated circuit. The circuit in figure 2 allows synchronization of data output from an integrated circuit with the rising and falling edges of a system clock input to the integrated circuit. Such a circuit can be ideally used in conjunction with double data rate applications. Receiver 6 receives the system clock signal CLK and produces two clock signals, CLK1 and CLK2 respectively, which are input to the respective delay lines 4 and 5, delay line 4 forms a delay-locked loop (DLL) with phase comparator 2. Delay line 4 can be constructed of active or passive variable impedance circuitry or it can be derived from microcomputer, microcontroller, or digital signal processor. In its preferred embodiment, receiver 6 comprises two conventional monostable multivibrator circuits or one-shot circuits, one of which triggers off of the rising edge of the system clock and the other of which triggers off of the falling edge of the system clock. A monostable multivibrator circuit or one-shot circuit can be implemented as a flip-flop with one stable state. Delay R is the propagation delay associated with receiver 6. Delay D is the propagation delay associated with the driver circuitry 7 comprising driver 8 and OR gate 10. Delay element 12 provides compensation via a DLL loop (which includes a path through elements 2, 4, and 12) through introduction of phase delay τ = R+D in the loop. When the synchronization is reached, the delay of the output signal with respect to the input clock is k*T. So, the delay of the signal entering the driver circuitry 7 with respect to the input clock is k*T-D, where D is the propagation delay of the driver 7. Phase comparator 2 need not output phase compensation causing data to delay lines 4 and 5 when a comparison of the phase input by phase comparator 2 results in the information at input 14 of phase comparator 2 having a delay of k*T relatively to the information at input 16 of phase comparator 2. (R delay at input 16 and tau+k*T-D = R+D+k*T-D = R+k*T at input 14). Should this relationship not exist, phase comparator 2 causes delay lines 4 to increase or decrease the delay associated with delay line 4 via a control signal from output 18 of phase comparator 2 so as to achieve the above described R delay condition at inputs 14 and 16. The delay lines 4 and 5 output a respective time-shifted version of the clock signal at delay lines 4 and 5s input (e.g., CLK1' being a time-shifted version of CLK1 and CL2' being a time-shifted version of CLK2) Logic involving the ORing of clock signals CLK1' and CLK2' to produce an input to driver 8, which also receives an input from output data 9, yields data signal DQ. A timing diagram illustrating the operation of the circuit of figure 2 is shown in figure 3. In the case of illustration 3, the integer k mentioned earlier is assumed to be equal to 1. Since the same control signal from output 18 is used to control delay lines 4 and 5 (and because each delay line 4 and 5 is constructed substantially the same) the delay between clocks signals CLK1 and CLK2 remains constant. Additionally, the pseudo clock duty cycle involving clocks CLK1 and CLK2 (defined as the time from the rising edge of clock CLK1 to the rising edge of clock CLK2 divided by the period of CLK1 or CLK2.) is maintained, by the circuit of figure 2, equal to the duty cycle of system clock CLK. The dock duty cycle is defined as the clock signal uptime divided by the clock period. As shown in figure 3, clock signal CLK1 is triggered by the rising edge of system clock CLK following a delay of time R while clock signal CLK2 is triggered by the falling edge of system clock CLK. Clock signal CLK1' is triggered in a way that the delay between the rising edge of CLK1' and the following rising edge of the system clock is exactly the delay of the output driver D. As shown by a causation arrow from the CLK1 waveform to the CLK1' waveform, signal CLK1 triggers signal CLK1' following a delay time which is equal to the time delay contributed by delay line 4. Likewise, the causation arrow from clock signal CLK2 to clock signal CLK2' demonstrates that signal CLK2 triggers signal CLK2' following a delay time which is also equal to the time delay contributed by delay line 5. Data signal DQ contains the data output of the application associated therewith, e.g. data from a DRAM. As shown by the causation arrows in figure 3, the data transitions on signal DQ are triggered by the rising edges of clock signals CLK1' and CLK2' respectively. The distance between the rising edge of clock CLK1' and clock CLK2' is the same as the distance between the rising edges of clock CLK1 and the rising edge of clock CLK2. Thus, the pseudo clock duty cycle concerning clocks CLK1' and CLK2' (defined as the time from the rising edge of clock CLK1' to the rising edge of dock CLK2' divided by the period of CLK1' or CLK2' is maintained by the circuit of figure 2, so as to equal to the clock duty cycle of system clock CLK. Thus, the foregoing invention can be used to synchronize the transmission of output data to the rising edges and the falling edges of the system clock CLK on a given integrated circuit. For instance, the transitions on DQ are synchronized with the rising and falling edges of system clock CLK. The transition timing is clearly demonstrated by data strobe signal DQS which represents the waveform timing transition on signal DQ as caused by signals CLK1' and CLK2'. The production of data strobe signal DQS corresponds to, for instance, when data becomes valid on the integrated circuit pins of DDR SDRAM. Consequently, the DQS signal (one such signal being associated with a plurality of outputs, e.g. 16 outputs) can be used by a controller to locate a data-valid window more accurately and to resynchronize data coming from a plurality of integrated circuits such as dual in-line-memory modules.

Claims: A clock to data latency compensation circuit for a system associated with at least one integrated circuit comprising: a receiver, said receiver being operable to receive a system clock signal and output a first clock signal derived from the rising edge of the system clock and a second clock signal derived from the folling edge of the system clock; delay circuitry being operable to receive said first and second clock signals and output time-shifted first and second clock signals corresponding to said first and second clock signals; and a phase comparator operable to control said delay line circuitry in connection with the detection of phase shift between at least one of said first and second clock signals and the at least one corresponding time shifted clock signals. A clock to data latency compensation circuit as recited in claim 1 which further comprises logic circuitry having an output and at least one input, said logic circuitry being operable to receive said time-shifted first one second clock signals from said delay circuitry. A clock to data latency compensation circuit as recited in claim 2 which further comprises a driver having a first input connected to the output of said logic circuitry and a second input being operable to receive data, said driver being operable to output said data such that it is synchronized with rising and falling edges of said system clock. A clock to data latency compensation circuit as recited in claim 1 wherein said receiver comprises at least one monostable multivibrator circuit. A clock to data latency compensation circuit as recited in claim 1 wherein said delay circuit comprises circuitry consisting of the following: variable passive impedance circuitry; variable active impedance circuitry; microcontroller circuitry; microcomputer circuitry; digital signal processing circuitry ; or a combination thereof. A synchronous random access memory integrated circuit chip which includes the clock latency compensation circuit as recited in claim 1. A synchronous random access memory system including a plurality of memory integrated circuit chips as recited in claim 6, each compensation circuit including on each said integrated circuit chip being operable to generate a data strobe signal, said memory system including a controller used in connection with synchronizing data coming from said plurality- of integrated circuit chips. A method of clock latency compensation for a system comprising: generating a first clock signal and a second clock signal in response to rising and falling edges of a system clock; processing and propagating said first and second clocks signals through circuitry operable to adjustably alter the propagation time of said signals therethrough; and controlling the adjustment of said propagation time in response to a comparison of the phase of at least one of said first and second clock signals prior to and after propagation through and processing by said circuitry operable to adjustably alter said propagation time. A method as recited in claim 8 which further comprises: further processing said first and second signals using Boolean logic, after propagation through said circuitry operable to adjustably alter said propagation time, and using said Boolean logic processed first and second signals to synchronize data with said rising and falling edges of said system clock signal.

Inventors: CHU ALBERT M, DORTU JEAN-MARC, CHU, ALBERT M., DORTU, JEAN-MARC
Applicants: IBM, INFINEON TECHNOLOGIES AG, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1999
Main classification: H03K5
IPC codes: G11C11, G11C7, H04L7, G06F1, H03L7