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Clocking 
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HackRF clock signals are generated by the Si5351. The plan so far:

    * crystal frequency: 25 MHz (supports 25 or 27 MHz)
    * optional clock input frequency: 10 MHz recommended (supports 10 to 40 MHz, or higher with division)
    * VCO frequency: 800 MHz (supports 600 to 900 MHz)
    * MAX2837 clock: 40 MHz
    * preferred MAX5864 clocks: 8, 10, 12.5, 16, 20 MHz
    * A clock at double the MAX5864 rate will be delivered to the CPLD and SGPIO.
    * LPC43xx clock: 12 MHz (from separate crystal so the ROM-based USB DFU will work)

Lemondrop+Jellybean Si5351 output mapping:

    * CLK0 -> MAX2837
    * CLK1 -> MAX5864/CPLD
    * CLK2 -> CPLD
    * CLK3 -> CPLD
    * CLK4 -> LPC4330
    * CLK5 -> RFFC5072
    * CLK6 -> extra
    * CLK7 -> extra

Jawbreaker output mapping:

    * CLK0 -> MAX5864/CPLD
    * CLK1 -> CPLD
    * CLK2 -> SGPIO
    * CLK3 -> external clock output
    * CLK4 -> RFFC5072
    * CLK5 -> MAX2837
    * CLK6 -> none
    * CLK7 -> LPC4330 (but LPC4330 will start up on its own crystal)

