MMCM

MMCM stands for Mixed-Mode Clock Manager. It is a clock-management primitive used in many Xilinx field-programmable gate arrays (FPGAs) and related devices to generate, modify, and distribute clock signals. The MMCM provides a flexible way to derive multiple clocks from a single reference, aligning their phases and adjusting frequencies to meet design timing.

Core features of the MMCM include frequency synthesis, phase shifting, and duty-cycle control. Through programmable multiply/divide ratios, the MMCM can generate output clocks with different frequencies. It also supports phase shifting and fine phase interpolation to achieve precise relationships between clocks. Outputs are typically distributed via clock buffers to maintain low skew across the FPGA fabric, and a feedback path is used to lock the generated clocks to the reference clock.

Typical usage involves supplying an input clock to the MMCM, configuring the desired output frequencies and phases, and routing the resulting clocks through global clock buffers for distribution. The MMCM is commonly used to create clocks for separate timing domains, deskew signals, or to generate derived clocks required by peripherals, all while aiming to minimize jitter and meet setup/hold timing constraints. In many designs, the MMCM works alongside other clock-management resources such as PLLs and DCMs, with the choice dependent on device family and specific timing requirements.

Notes and variations: MMCM implementations are device-specific, and capabilities or ranges can vary between FPGA families. Designers often use vendor-provided tools or IP blocks to configure and validate MMCM-based clocking schemes. See also PLL, DCM, BUFG, and clock management resources in FPGA documentation.