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LMX2491: LMX2491/LMX2492 : No trigger activated when TRIG2 is used

Olivier Chevalerias
Prodigy 105 points

Part Number: LMX2491
Other Parts Discussed in Thread: LMX2492

Tool/software:

Hello,

I am using LMX2491 Fractional N PLL to generate a ramp as part of a radar system.

The simplified block diagram is presented below.

 

As both LMX2491 and LMX2492 are very similar except for the frequency range, I supposed that carrying tests using LMX2492 would be OK.

 

For the first step, I used LMX2492EVB and TICS Pro with an external radar connected to the board, to validate the system.

(Note that this is a bit tricky considering the frequency range, but it worked OK)

The ramp was triggered with TRIG1 pin.

This worked fine.

 

So, I have designed my own PCB board to include the different elements: PLL, radar, filters and signal conversion.

When I received the board, the initial tests failed, and I investigated what could be the problems.

I found out that I inverted TRIG1 and TRIG2 Net Labels on the schematic.

To facilitate the tests, I am still using TICS Pro to program the LMX2491 chip and to generate the trigger signal.

 

So, within TICS Pro, I changed the trigger source from TRIG1 to TRIG2.

This is not working.

 

So, I reverted to the evaluation board and got the LMX2492 replaced by an LMX2491.

I repeated the tests with TRIG1 and TRIG2 as trigger sources. (I adapted the configuration within TICS Pro each time, making sure that the trigger is applied to the pin each time.)

I could confirm that only the TRIG1 pin permits the activation of the ramp.

 

I modified my board to be able to use TRIG1 pin as the trigger source, and it is working correctly.

 

But while using TRIG1 pin is OK for a quick test, it does not permit the test of thecomplete system. I would have to cut traces and add wires. This is still possible on a prototype, but I would like to understand why the TRIG2 pin cannot be used to trigger the ramp.

 

I tried to read carefully the datasheets and the User Guide and I ran searches on the Forum, but I do not see any recommendation not to use TRIG2 pin as a trigger pin nor the requirement to program a specific configuration to activate the function.

 

Would you be able to point out what I am doing wrong and why I cannot use the TRIG2 pin as a trigger for the ramp please?

 

Looking forwards to your feedback,

Best regards,

Olivier

  • 0
    TI__Guru** 116900 points

    Hi Olivier,

    I don't think there is difference in TRIG2 between these two devices. I will see if I can verify this tomorrow. 

  • 0 in reply to Noel Fung
    Prodigy 105 points

    Thanks Noel,

    I ran some additional tests this morning on a new LMX2492EVB kit that I just received.

    I programmed a simple ramp and I probed the signal on the VCO filter output.

     

    I got the same problem:

        - when configuring TRIG1 as the TRIGGER Source B, when activating TRIG1 pin in the USER Controls window of TICS Pro, a ramp is generated

        - when configuring TRIG2 as the TRIGGER Source B, when activating TRIG2 pin in the USER Controls window of TICS Pro, no ramp is generated

    I compared the exported registers' values generated for the 2 configurations.

    The only difference is for register R59 with value 0x003B01 for TRIG1 and 0x003B02 for TRIG2 corresponding respectively to TRIG1 terminal rising edge and TRIG2 terminal rising edge.

    So the configuration seems OK, but behavior is not as expected.

    With my best regards,

    Olivier

  • 0 in reply to Olivier Chevalerias
    TI__Guru** 116900 points

    Hi Olivier,

    LMX2492: TRIG1

    LMX2492: TRIG2

    I think you forgot to set TRIG2 as input pin.

  • 0 in reply to Noel Fung
    Prodigy 105 points

    Hi Noel,

    This is exactly the problem. Thank you.

    I was about to email you about it.

    I tested lots of different things today and I finely saw this setting in the interface.

    As by default TRIG1 is defined as an input I didn't have to change the configuration og the pin.

    That resolves the issue ans solves my problem.

    Have a nice day!

    Best regards,

    Olivier