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LMX2541 Not Starting

Calvin Pitman
Prodigy 100 points
Other Parts Discussed in Thread: LMX2541

Hello,

I am using the LMX2541 to generate a signal with a center frequency of 978 MHz. The deviation is 312.5kHz. However, I am having trouble getting it to properly start. To ensure that the device is being programmed, I have been checking the Ftest/LD pin, which I have set in the registers to go high once it is programmed. However, it is always low, and no signal seems to come from the RFout pin, leading me to believe that the device is not being programmed. I am using the SPI bus of an MSP430. I have attached my register values below, as well as a timing diagram take from a logic analyzer:

0x00,0x00,0x00,0x17,  //R7
0x00,0x00,0x00,0x1C, //R12
0x28,0x00,0x14,0x09,  //R9
0x01,0x11,0xCE,0x58, //R8
0x00,0x1F,0x33,0x26,  //R6
0xA0,0x00,0x00,0x05,  //R5
0x88,0x04,0x41,0x94,  //R4
0x80,0x24,0x2F,0x03,  //R3
0x04,0x00,0x06,0x42,  //R2
0x00,0x00,0x00,0x21,  //R1
0x50,0x30,0x09,0xC0, //R0
0x88,0x04,0x41,0x94   //R4, due to divisor value of 4

1) Does anything in the timing of the serial data appear to be incorrect? Additionally, will the gaps in between transmission of each byte over SPI cause issues (the SPI TX buffer is one byte in length)?

2) What is the default condition of the Ftest pin?

3) What is the best way to ensure that the chip is properly powered up before programming it? What voltages should I be able to measure?

Thanks,

Calvin

  • 0
    TI__Guru** 110210 points
    Hi Calvin,

    Try not toggle the uWire interface and CE pin before the first register write. Not sure if this is matter, but just in case.
    Another simple try is send a powerdown command to see if it responses.
  • 0 in reply to Noel Fung
    Prodigy 100 points

    Hi Noel,

    Here is an updated timing diagram, I fixed the CE toggle, do I need to leave CE low until after the first register write? Not sure about the uWire toggle, I think that that is a consequence of using a SPI interface to send the uWire data. 

    To send the powerdown command, do I just send the R3 register alone? Or which other registers would I need to program? And what response should I expect from the IC?

    Thanks,

    Calvin

  • 0 in reply to Calvin Pitman
    TI__Guru** 110210 points

    Hi Calvin,

    Suggest keep CE low until just before the first register write, i.e. approx. at 8us. Then the uWire toggling will be ignored. 

    Right, just write R3 with POWERDOWN bit set, check whether the current will go down.

    Also try with a slower uWire speed. Is it 20MHz ? This is the max. speed we can support. Can you try 10MHz?