FS6370

3.3 Post Dividers

A post divider performs several useful functions. First, it allows the VCO to be operated in a narrower range of speeds compared to the

variety of output clock speeds that the device is required to generate. Second, it changes the basic PLL equation to:

where NP is the post divider modulus. The extra integer in the denominator permits more flexibility in the programming of the loop for

many applications where frequencies must be achieved exactly.

The modulus on two of the four post dividers (post dividers C and D in Figure 2) can be altered without reprogramming by a logic level

on the SEL_CD pin.

4.0 Device Operation

The FS6370 has two modes of operation:

• Program mode: during which either the EEPROM or the FS6370 control registers can be programmed directly with the desired PLL

settings

• Run mode: where the PLL settings stored the EEPROM are transferred to the FS6370 control registers on power-up, and the device

then operates based on those settings

Note that the EEPROM locations are not physically the same registers used to control the FS6370.

Direct access to either the EEPROM or the FS6370 control registers is achieved in program mode. The EEPROM register contents are

automatically transferred to the FS6370 control registers in normal device operation (run mode).

4.1 MODE Pin

The MODE pin controls the mode of operation. A logic-low places the FS6370 in program mode. A logic-high puts the device in run

mode. A pull-up on this pin defaults the device into run mode.

Reprogramming of either the control registers or the EEPROM is permitted at any time if the MODE pin is a logic-low.

Note, however, that a logic-high state on the MODE pin is latched so that only one transfer of EEPROM data to the FS6370 control

registers can occur. If a second transfer of EEPROM data into the FS6370 is desired, power (VDD) must be removed and reapplied to

the device.

The MODE pin also controls the function of the PD/SCL and OE/SDA pins. In run mode, these two pins function as power-down (PD)

4.2 SEL_CD Pin

The SEL_CD pin provides a way to alter the operation of PLL C, muxes C and D, and post dividers C and D without having to

reprogram the device. A logic-low on the SEL_CD pin selects the control bits with a "C1" or "D1" notation, per Table 3. A logic-high on

the SEL_CD pin selects the control bits with "C2" or "D2" notation, per Table 3.

Note that changing between two running frequencies using the SEL_CD pin may produce glitches in the output, especially if the post-

divider(s) is/are altered.

4.3 Oscillator Overdrive

For applications where an external reference clock is provided (and the crystal oscillator is not required), the reference clock should be

connected to XOUT and XIN must be left unconnected (float).

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