CS2100-CP

DS840F1

21

5.6

Clock Output Stability Considerations

5.6.1

Output Switching

CS2100 is designed such that re-configuration of the clock routing functions do not result in a partial clock

period on any of the active outputs (CLK_OUT and/or AUX_OUT). In particular, enabling or disabling an

output, changing the auxiliary output source between REF_CLK and CLK_OUT, and the automatic dis-

abling of the output(s) during unlock will not cause a runt or partial clock period.

The following exceptions/limitations exist:

•

Enabling/disabling AUX_OUT when AuxOutSrc[1:0] = 11 (unlock indicator).

•

Switching AuxOutSrc[1:0] to or from 01 (PLL clock input) and to or from 11 (unlock indicator)

(Transitions between AuxOutSrc[1:0] = [00,10] will not produce a glitch).

•

Changing the ClkOutUnl bit while the PLL is in operation.

When any of these exceptions occur, a partial clock period on the output may result.

5.6.2

PLL Unlock Conditions

Certain changes to the clock inputs and registers can cause the PLL to lose lock which will affect the pres-

ence the clock signal on CLK_OUT. The following outlines which conditions cause the PLL to go un-

locked:

•

Changes made to the registers which affect the Fraction-N value that is used by the Frequency Syn-

thesizer. This includes all the bits shown in Figure 17 on page 19.

•

Any discontinuities on the Timing Reference Clock, REF_CLK.

•

Discontinuities on the Frequency Reference Clock, CLK_IN, except when the Clock Skipping feature

is enabled and the requirements of Clock Skipping are satisfied (see “CLK_IN Skipping Mode” on

page 14).

•

Gradual changes in CLK_IN frequency greater than ±30% from the starting frequency.

•

Step changes in CLK_IN frequency.

5.7

Required Power Up Sequencing

•

Apply power to the device. The output pins will remain low until the device is configured with a valid ratio

via the control port.

•

Write the desired operational configurations. The EnDevCfg1 and EnDevCfg2 bits must be set to 1 dur-

ing the initialization register writes; the order does not matter.

–

The Freeze bit may be set prior to this step and cleared afterward to ensure all settings take effect

at the same time.

6. SPI / I²C CONTROL PORT

The control port is used to access the registers and allows the device to be configured for the desired operational

modes and formats. The operation of the control port may be completely asynchronous with respect to device inputs

and outputs. However, to avoid potential interference problems, the control port pins should remain static if no op-

eration is required.