DCM与PLL

The DCM is a Digital Clock Manager - at its heart it is a Delay Locked Loop. This has the ability to deskew a clock, generate different phases of the clock, dynamically change the phase of a clock, generate related (2x) clocks, do clock division, and even generate clocks with harmonic relationships to the incoming clock. It was the only clock management block that existed in older technologies (up to Spartan-3 and Virtex-4).

In Virtex-5 and Spartan-6 the Phase Locked Loop (PLL) was introduced along with the DCM. The PLL is an analog clock management cell that can do almost everything the DCM can do with the exception of dynamic and fine phase shifting. However, it can do more precise frequency generation and can generate multiple different frequencies at the same time. It also has significantly better jitter performance than the DCM - particularly when doing frequency synthesis with large multipliers/dividers.

In Virtex-6 the MMCM - Mixed Mode Clock Manager - was introduced. This is a PLL with some small part of a DCM tacked on to do fine phase shifting (that's why its mixed mode - the PLL is analog, but the phase shift is digital). Thus the MMCM can do everything the PLL can do plus the phase shifting from the DCM. The V6 only had MMCMs.

In the 7 series, they have a combination of PLLs and MMCMs. Mostly this is so that there are more cells available for use (the PLLs are smaller, so they take less room on the FPGA die). Furthermore the PLLs are tightly bound to the I/O structures that are used for DDRx-SDRAM memory controllers (via the MIG).

As for the number of them, that is determined by the size of the device. Look at the Product Table for the device you are using - it will tell you what is in the CMT (Clock Management Tile) and how many of them are available in your device.

以上的意思就是说：

1、DCM实际上就是一个DLL，可以对输入时钟进行相位移动，补偿，产生倍频和分频时钟，但是5以及以后的产品不用了。

2、PLL相对于DCM，除了不能相移时钟，其它的都一样，但是PLL产生时钟的频率比DCM更加精准，而且时钟的jitter也更好。

3、MMCM实际上就是PLL+DCM相移功能的结合体。7系列的FPGA还会在临近I/O部分放置一些PLL，专门给MIG来产生DDR时钟。