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The main purpose of the Clocking FMC is to test/prototype the clocking circuit proposed in the [WRS-4 main board (Hardware_Architecture)](/project/wr-switch-hw-v4/wikis/uploads/c4fa8bc97eeeb551c736146eae7b8e25/WRS-4_main_board_Hardware_Architecture_-v1.7-2020-06-09.pdf) document, section 3.4 (page 25-28). Additionally, the FMC has an SFP cage and it can be used to enable WR on a host board, such as e.g. AFCZ, or ZCU102/ZCU106 eval board.
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Figure 5 shows a detailed block diagram of the proposed solution. As can be seen in
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this figure, the main PLL is the HMC7044 that has the following features of interest for the
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new WRS-4:
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- Possibility to have a mixed scenario (part of the design working with 62.5 MHz
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recovered clock and another part with 156.25 MHz recovered clock, for 1Gbps and
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10Gbps links respectively).
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- Deterministic phase alignment between all output channels, allowing to generate the
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10 MHz output directly from the HMC7044 PLL with SYNC support.
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- Plenty of clock outputs with configurable type (LVDS or LVPECL).
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The main oscillator is the DOT050V that has a plastic cover to avoid known problems
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related to the airflow. For the helper clock, we propose to use the programmable and tunable oscillator
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Si549 that has already been tested in a WR node and is ideal to support different
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frequencies. The 1PPS and Abscal outputs will be latched with D-type FF (with 125MHz or
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250MHz). The design will have the possibility to mount an oscillator board (a type of module
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expansion) to allow better oscillator OCXO and DDS for lpGBT.
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The Grandmaster 10 MHz input clock is multiplied by deterministic LMX2594 PLL to
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create a 1-PPS aligned 62.5/125 MHz, thus the SoftPLL of the WR PTP implementation can
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align to it. As such, it is the safest solution in case of loss of the 10 MHz input reference.
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Another option for the Grandmaster can be to connect the RFU of the HMC7044 directly with
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the LMX2594 in order to clock the system directly with 10 MHz input clock (multiplied to 2.5
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GHz) and without using the main oscillator DOT050V. The main problem of this last option
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occurs when the input reference is lost, since the system might freeze. The RFU lines of
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HMC7044 can also be connected to the expansion board.
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