
Revised - August 2014
LMK04800 Family
SNAU076B
25
Copyright © 2014, Texas Instruments Incorporated
Single Loop 0-Delay Mode Examples
In Single Loop 0-Delay Mode, MODE = 8, the feedback from the VCO of PLL2 to the PLL2_P/PLL2 N
divider is broken and a fed back clock output will drive the PLL2 N divider directly. This permits phase
alignment between the clock output and the OSCin input (0-Delay). As such, the PLL2_N, PLL2_R, and
PLL2_N_CAL divide values may need to be adjusted to permit the LMK04800 to lock.
Programming Steps
1.
Program the Single Loop 0-Delay mode.
2.
Enable the feedback mux. EN_FEEDBACK_MUX = 1.
3.
Select clock output for feedback with the feedback mux. FEEDBACK_MUX = User value.
4.
Program the VCO frequency of PLL2 tab to: The actual VCO frequency * PLL2_P (which is
PLL2 PreN) / CLKout Divider.
•
Entered CodeLoader 4 VCO Frequency = Actual VCO Frequency * PLL2_P / CLKout
Divider.
5.
Updated the PLL2_N_CAL register on the Bits/Pins tab to the N value when in non-0-Delay mode.
6.
Press Ctrl-L to cause all registers to be programmed.
•
The reason is to cause the programming of register R30 to start the VCO calibration
routine now that the proper PLL2_N_CAL value is programmed.
•
PLL2_N_CAL value is automatically updated when a new VCO frequency is entered and
the PLL2_N value is calculated. In this case the VCO frequency entered is wrong and the
PLL2_N_CAL value will be incorrect.
If for any reason the CLKout frequency is less than the phase detector frequency, the PLL2 R divider
must be increased so that the phase detector is at the same or lower value than the CLKout frequency.
Details
The 0-Delay mode for Single Loop mode is more complicated to program than for Dual Loop mode in
part because of the PLL2_N_CAL register. When performing the VCO calibration the device uses
PLL2_N_CAL for in non-0-Delay mode. Once the VCO is calibrated the device enters 0-Delay mode.
For more information on the PLL programming equations, refer to PLL PROGRAMMING in the
applications section of the datasheet.
In Table 7 case 1 illustrates the register programming when note using 0-Delay.
Case 2 shows 0-Delay with a clock out divider of 2. Since PLL2_P = 2, this substitution of which circuit
is performing the divide by two results in no impact o the software. All the values display correctly.
Case 3 shows 0-Delay mode with a CLKout divider not equal to the PLL2_P value. So the proper
frequency to program in the VCO to lock the VCO to 2949.12 MHz will be 491.52 MHz. This is
calculated by Actual VCO Frequency * PLL2_P / CLKoutX_Y_DIV.
Case 4 shows 0-Delay mode with CLKout divider not equal to the PLL2_P value; however the CLKout
frequency will be less than the current phase detector frequency. This requires PLL2_R to be increased
from a value of 1 to 2 to reduce the PLL2 phase detector frequency from 122.88 MHz to 61.44 MHz.
Now the adjusted VCO frequency can be programmed to allow PLL2 to lock.