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LMX2820: Temperature Sensor Intermittent Reading Issue

Phil Perry
Prodigy 10 points
Part Number: LMX2820
Other Parts Discussed in Thread: TMS320F2808

Hi,

I've been reading back the temperature from several LMX2820 devices and plotting them over time, but have noticed that sometimes the temp value read back from R76 drops to a value that corresponds to around -375°C (rb_TEMP_SENS = 0x2F). The majority of the time the value read is realistic - between 40°C & 80°C (0x217 to 0x246). Is this a known issue, or is there a step I'm missing or getting wrong when decoding this value?

I'm taking the following steps when reading the LMX28280 temperature

  1. Power on LMX2820
  2. Write the initial register values from the register map in descending order (R123 first)
  3. Write value 0x2138 to register R19
  4. Read the value from register R76 every 2s
  5. Convert register value to °C using the formula: T = (0.85 * R76) - 415

Other info that might help

  • Using SPI comms at 1Mbps baud with three LMX2820s in parallel - reading on the clock falling edge
    • Verified on the scope that the value of R76 does indeed drop to around 0x2F so it doesn't appear to be an SPI read issue
  • Seems to happen more often when the LMX2820 is in power-down but that's anecdotal. I've yet to find a condition that reliably causes this odd reading
  • I've tested this with approx. 12 LMX2820s and about 99% of the time the temperature reported is within the expected range.
  • Presents sometimes as a single erroneous measurement and sometimes it latches at the low measurement for several measurement cycles (up to about 10s from my observation)

Any help would be appreciated!

Cheers,

Phil

  • 0
    TI__Guru* 94320 points

    Hi Phil,

    This is the first time we've heard about temperature readback issue, we don't have any cue what went wrong. If you could find out a test condition that could replicate the problem easily, we will try it out. 

    BTW, we recommend readback on the clock rising edge, this should return robust read back.

  • 0
    Prodigy 10 points

    Hi Noel,

    Thanks for the response! I think the most likely situation is I'm sampling too close to the bit transition and, every now and then, I sample after the transition and I get a bit flip.

    One thing I forgot to mention is that I'm using a TMS320F2808 microprocessor to talk to the LMX2820 over SPI. The F2808 sets up the SPI interface and allows for clock schemes based on rising/falling edge with delay/no delay. I get no response from the synths if I use the rising edge no delay clock scheme, which should be the correct choice. But rising edge with delay and falling edge both work (albeit with this reliability issue). The delay is half a clock so rising edge with 

    I think this is probably not an issue with the LMX2820 itself but a timing issue between the LMX2820 and F2808. I'm still investigating so if anything else crops up I'll add it on

    Cheers,

    Phil

  • 0 in reply to Phil Perry
    TI__Guru* 94320 points

    Hi Phil,

    Ok, feel free to post if you have any application questions.