Figure 1. Dual MMCM Using DESKEW
- Each MMCM fits in its own I/O bank / clock area. Both MMCMs get the same input clock (2) from a differential clock input (1).
- Without the DESKEW circuit the output clocks of each MMCM, 5, 7 and 5', 7', will be phase-aligned to the input clock of each MMCM (5, 7 are phase-aligned to 2 and 5', 7' are phase aligned to 2’). Because of the difference in delay routing between the output of the clock input buffer (IBUFDS), and the clock input of each MMCM, the output clocks of both MMCMs will not be phase-aligned to each other.
- When DESKEW unit(s) is/are used, it can help to phase align selected clock outputs of one MMCM to clock outputs of the other MMCM.
- Take one of the outputs of an MMCM and use it as CLKIN_DESKEW input on the second MMCM.
- Take an output clock of the MMCM where the DESKEW unit is used and route that to the CLKFB_DESKEW input.
- The output clock (CLKOUT4 (7) of the MMCM will be phase-aligned to the output clock (CLKOUT4 - 7’) of the other MMCM.