Delay-locked loop
Encyclopedia
In electronics, a delay-locked loop (DLL) is a digital circuit similar to a phase-locked loop
(PLL), with the main difference being the absence of an internal voltage-controlled oscillator
. A DLL can be used to change the phase of a clock signal (a signal with a periodic
waveform
), usually to enhance the clock rise-to-data output valid timing characteristics of integrated circuit
s (such as DRAM
devices). DLLs can also be used for clock recovery
(CDR). From the outside, a DLL can be seen as a negative-delay gate placed in the clock path of a digital circuit.
Another way to view the difference between a DLL and a PLL is that a DLL is a first order loop and a PLL is a second order loop. A DLL compares the phase of one of its outputs to the input clock to generate an error signal which is then integrated and fed back as the control to all of the delay elements. The integration allows the error to go to zero while keeping the control signal, and thus the delays, where they need to be for phase lock. Since the control signal directly impacts the phase this is all that is required. A PLL compares the phase of its oscillator with the incoming signal to generate an error signal which is then integrated to create a control signal for the voltage-controlled oscillator
. The control signal impacts the frequency of the oscillator, and phase is the integral of frequency, so a second integration is unavoidably performed by the oscillator itself. A first order feedback system is significantly easier to stabilize than a second order feedback system, which is a major advantage of DLLs.
The main component of a DLL is a delay chain composed of many delay gates connected front-to-back. The input of the chain (and thus of the DLL) is connected to the clock that is to be negatively delayed. A multiplexer is connected to each stage of the delay chain; the selector of this multiplexer is automatically updated by a control circuit to produce the negative delay effect. The output of the DLL is the resulting, negatively delayed clock signal.
The phase shift can be specified either in absolute terms (in delay chain gate units), or as a proportion of the clock period, or both.
Compared to phase-locked loops, delay-locked loops are a relatively recent innovation, first found in Dr. Combes' work in the early 1990s, then popularized by Xilinx
in their Virtex
family of FPGA
products.
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
(PLL), with the main difference being the absence of an internal voltage-controlled oscillator
Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
. A DLL can be used to change the phase of a clock signal (a signal with a periodic
Periodic function
In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...
waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
), usually to enhance the clock rise-to-data output valid timing characteristics of integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s (such as DRAM
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
devices). DLLs can also be used for clock recovery
Clock recovery
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...
(CDR). From the outside, a DLL can be seen as a negative-delay gate placed in the clock path of a digital circuit.
Another way to view the difference between a DLL and a PLL is that a DLL is a first order loop and a PLL is a second order loop. A DLL compares the phase of one of its outputs to the input clock to generate an error signal which is then integrated and fed back as the control to all of the delay elements. The integration allows the error to go to zero while keeping the control signal, and thus the delays, where they need to be for phase lock. Since the control signal directly impacts the phase this is all that is required. A PLL compares the phase of its oscillator with the incoming signal to generate an error signal which is then integrated to create a control signal for the voltage-controlled oscillator
Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
. The control signal impacts the frequency of the oscillator, and phase is the integral of frequency, so a second integration is unavoidably performed by the oscillator itself. A first order feedback system is significantly easier to stabilize than a second order feedback system, which is a major advantage of DLLs.
The main component of a DLL is a delay chain composed of many delay gates connected front-to-back. The input of the chain (and thus of the DLL) is connected to the clock that is to be negatively delayed. A multiplexer is connected to each stage of the delay chain; the selector of this multiplexer is automatically updated by a control circuit to produce the negative delay effect. The output of the DLL is the resulting, negatively delayed clock signal.
The phase shift can be specified either in absolute terms (in delay chain gate units), or as a proportion of the clock period, or both.
Compared to phase-locked loops, delay-locked loops are a relatively recent innovation, first found in Dr. Combes' work in the early 1990s, then popularized by Xilinx
Xilinx
Xilinx, Inc. is a supplier of programmable logic devices. It is known for inventing the field programmable gate array and as the first semiconductor company with a fabless manufacturing model....
in their Virtex
Virtex
* Virtex , a series of FPGAs produced by Xilinx* Virtex, a series of comic books published by Oktomica Comics* Virtex L, otherwise known as sodium dithionite...
family of FPGA
Field-programmable gate array
A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...
products.