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LMK05318: PRIREF is recognized as valid, but the DPLL doesn't lock to the 1pps reference input signal

Prodigy 120 points

Replies: 5

Views: 47

Part Number: LMK05318

I am working on a clock synchronized with a GPS signal. This clock will be the input clock of an ADC.

I am testing the LMK05318 through the evaluation board.

If I apply a voltage divider to the original PPS signal (the amplitude of the signal applied to the EVM is 1.76V), LMK05318 toggles (roughly every two seconds) between the holdover state and the PRIREF state. Even when the PRIREF is valid, both the LOPL_DPLL and the LOFL_DPLL flags are checked (the DPLL doesn't lock to the 1pps signal).

Instead, if I apply to the EVM the original 3.3V 1PPS signal, LMK05318 stays in the PRIREF state until the GPS signal is present and then with a delay (in some cases the delay is 40 seconds) returns in the holdover state. However, also in this condition, once the module has recognized the PRIREF signal as valid, the DPLL doesn't lock to the 1 pps signal (both the LOPL_DPLL and the LOFL_DPLL flags remain checked).

As output frequency I have set 4Hz in order to view better the relationship between the 1pps input signal and the LMK05318 output frequency. However, in the real application the output frequency will be 33kHz.

I attach the two configuration files one for the 4Hz output frequency and the other for the 33kHz output frequency. The behaviour observed is the same for both the frequencies generated

In the evaluation board I am using the standard 48.0048MHz XO.

I have applied to the PRIREF input the 1pps signal coming from the module NEO-M8T.

The other inputs (PRIREF_N , SECREF_P and SECREF_N ) are pulled down through a 1k resistor.

The amplitude of the PPS signal coming from the GPS module is 3.3V.

Is it correct in this case to choose "CMOS" as PRIREF interface type?

At the moment I am using only the amplitude validation option.

However, based on another post present in this forum, I am not clear if I should use the 1PPS Phase Detector validation option.

What else I am missing? Thanks in advance


  • Hi Gabriele,

    Just letting you know that I'm working on a software update for 1pps configuration. There're a few things to note for 1pps. Let me summarize the procedures (it'll become easier after the software update) and get back to you tomorrow.


  • In reply to Hao Z:

    Hi Gabriele,

    Please find attached. LMK05318 1pps 2-7-2020.zip


  • In reply to Hao Z:

    Sorry for the delay in the reply.

    I have made some tests using the configuration file you provide, I have followed the instructions present in the .word file you attached.

    I have modified the XO frequency to 48.0048MHz as I am using the XO present on the EVM and I have changed the PRI_REF interface to CMOS.

    I have manually set registers R286,R287,R288 and R289 as described in the .word document.

    However, in this configuration the Primary Reference is always considered invalid and therefore the DPLL does not lock to the reference. The flag BAW_LOCK becomes one almost immediately after having powered up the board (I have written the configuration in EEPROM).

    I have tested the configuration also with different 1PPS signal pulsewidths  (10us, 1ms,20ms,200ms and 500ms) as the GPS module (NEO-M8T) we are using is able to configure the output pulsewidth. 

    I attach a screenshot of the status window. It is the same for all the tests I have made.

    What I am missing?

    Thanks in advance

     1pps default 02-12-2020.tcs

  • In reply to Gabriele Marocco8:

    Hi Gabriele,

    I'm afraid that the on-board 48.0048MHz XO cannot be used for 1pps configuration. This is because 1pps configuration requires very narrow loop bandwidth (0.01Hz). If the XO has too much wander, then the DPLL will have trouble tracking the phase error. So the 1pps phase detector requires a stable XO because it takes in the jitter of 1pps reference in periods of XO clock. If the XO input has too much wander then the PRIREF_VALSTAT won't be cleared. 

    A regular TCXO or OCXO with 4.6ppm frequency accuracy should work. For sanity check, you can use an external signal generator which typically has an OCXO inside.

    Besides, the input type for 1pps PRIREF or SECREF needs to be SE(50Ohm) instead of CMOS.


  • In reply to Hao Z:

    Please see attached for XO/reference frequency accuracy requirements.1pps phase detection calculator.xlsx