
Revised - August 2014
LMK04800 Family
SNAU076B
25
Copyright © 2014, Texas Instruments Incorporated
www.ti.com
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.
All manuals and user guides at all-guides.com