How to use DCM(PLL) as frequency multiplier on FPGA

     Phase Locked Loop (PLL) can be used as frequency multiplier. Figure3.1 shows how PLL works as frequency multiplier.

                                                Figure 3.1 PLL as frequency multiplier

Phase Locked Loop (PLL) circuits are used for frequency control. They can be configured as frequency multipliers, demodulators, tracking generators or clock recovery circuits. Each of these applications demands different characteristics but they all use the same basic circuit concept. Figure 3.1 contains a block diagram of a basic PLL frequency multiplier. The operation of this circuit is typical of all phase locked loops. It is basically a feedback control system that controls the phase of a voltage controlled oscillator (VCO). The input signal is applied to one input of a phase detector. The other input is connected to the output of a divide by N counter. Normally the frequencies of both signals will be nearly the same. The output of the phase detector is a voltage proportional to the phase difference between the two inputs. This signal is applied to the loop filter. It is the loop filter that determines the dynamic characteristics of the PLL. The filtered signal controls the VCO. Note that the output of the VCO is at a frequency that is N times the input supplied to the frequency reference input. This output signal is sent back to the phase detector via the divide by N counter. Normally the loop filter is designed to match the characteristics required by the application of the PLL. If the PLL is to acquire and track a signal the bandwidth of the loop filter will be greater than if it expects a fixed input frequency. The frequency range which the PLL will accept and lock on is called the capture range. Once the PLL is locked and tracking a signal the range of frequencies that the PLL will follow is called the tracking range. Generally the tracking range is larger than the capture range. 

            The Xilinx Spartan-3 and VirtexIIPro series FPGA are all equipped with DCM (Digital Clock Manager) modules which can precisely control the system clock [8]. There are four DCMs available in spartan-3 kit which is considered as target device for this project. There are three ways to use the DCM.  First, calling DCM as a component is through HDL. Second is through schematic i.e. taking as a symbol and assign ports for required pins, then create user symbol to use in schematic. Third is through XILINX clocking wizard from IP (Coregen & Architecture wizard).   Third one is the easiest way which is explained in subsection 

Procedure to Use DCM

Step1: Double-click Create New Source

                 – Select IP (COREGen & Architecture Wizard), and click Next

The Architecture Wizard Contains Several Wizards as shown in figure 3.2.

Figure 3.2 Selecting a DCM from clocking wizard

Step2: The Clocking Wizard helps you define the DCM. In main window

            – Select pins

            – Specify

                        • Reference source

                        • Clock frequency

                        • Phase shift

Figure 3.3 General setup wizard for DCM as a frequency multiplier

Step3: Specify the Types of Clock Buffers

              Connect clock buffers (BUFG, BUFGMUX) to the selected output pins of the DCM

                                 Figure 3.4 Wizard shows specifying the clock buffers to be used

The DCM will appear in source window and ready to use as a component. It can also be used in schematic by making it as a user symbol through ‘design utilities’ option as shown in Figure3.5. Figure 3.6 shows behavioral simulation of DCM as frequency multiplier.

Figure 3.5 DCM used as a symbol in schematic design entry

Figure 3.6 Behavioral simulation of frequency multiplier