DCM (Digital Clock Manager)

[chipmunk22]

I am tasked to use the DCM in Xilinx's architecture wizard. Can anyone explain what is the function of a DCM and how does it actually work? What are the inputs and outputs of a typical DCM? many thanks!

 

[xez]

Check the User Guide documentation for the part you're
using. e.g. for the Spartan 3 / Spartan 3A DCMs:
http://direct.xilinx.com/bvdocs/userguides/ug331.pdf
q.v. pages 61...

Using Digital Clock Managers (DCMs)
Summary
Digital Clock Managers (DCMs) provide advanced clocking capabilities to Spartan-3
Generation FPGA applications (Spartan-3, Spartan-3E, Spartan-3A, Spartan-3AN, and
Spartan-3A DSP families). Primarily, DCMs eliminate clock skew, thereby improving
system performance. Similarly, a DCM optionally phase shifts the clock output to delay the
incoming clock by a fraction of the clock period. DCMs optionally multiply or divide the
incoming clock frequency to synthesize a new clock frequency. The DCMs integrate
directly with the FPGA’s global low-skew clock distribution network.
Introduction
DCMs integrate advanced clocking capabilities directly into the FPGA’s global clock
distribution network. Consequently, DCMs solve a variety of common clocking issues,
especially in high-performance, high-frequency applications:
• Eliminate Clock Skew, either within the device or to external components, to
improve overall system performance and to eliminate clock distribution delays.
• Phase Shift a clock signal, either by a fixed fraction of a clock period or by
incremental amounts.
• Multiply or Divide an Incoming Clock Frequency or synthesize a completely new
frequency by a mixture of clock multiplication and division.
• Condition a Clock, ensuring a clean output clock with a 50% duty cycle.
• Mirror, Forward, or Rebuffer a Clock Signal, often to deskew and convert the
incoming clock signal to a different I/O standard—for example, forwarding and
converting an incoming LVTTL clock to LVDS.
• Any or all the above functions, simultaneously.

 

[ravenprp]

The Digital Clock Manager (DCM) is a key component in Xilinx's architecture and is used to generate and manage clock signals within the FPGA. Its main function is to provide precise and stable clock signals for the various components and circuits within the FPGA.

The DCM works by taking in an input clock signal and using a phase-locked loop (PLL) to generate an output clock signal with a desired frequency and phase. The PLL constantly compares the input and output clock signals and makes adjustments to the output signal to ensure it stays in sync with the input signal.

The inputs of a typical DCM include the input clock signal, a reference clock signal, and various control signals for configuring the DCM's operations. The outputs include the generated clock signal, as well as status signals that indicate the stability and performance of the DCM.

In Xilinx's architecture wizard, the DCM can be configured to generate different types of clock signals, such as single-ended or differential, and can also be used to adjust the duty cycle and phase of the output clock. It also has features for fine-tuning the output clock's frequency and phase for precise timing requirements.

Overall, the DCM plays a crucial role in the proper functioning of an FPGA by providing reliable and precise clock signals to all components. Its flexibility and versatility make it a valuable tool for designers working with Xilinx's architecture.