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CC1200: Sniff mode with short preamble length

John Moore39
Expert 1470 points
Part Number: CC1200

 

Sirs:

      We are using sniff mode to minimize receive power consumption but we have some older systems that output minimal preamble length. One system appears to produce only 10 bit preamble bits ahead of the 11 bit sync which also contains 9 bits of preamble (that don't count as preamble bits). So the sniff must be designed to detect short preambles.

 

   Our system uses an ext 40Mhz tcxo. So I'm able to measure sniff duty cycles directly by monitoring pin 29 ( DCPL_XOSC) which is being used to power up the XOSC during sniffing.

      Used the xcel spreadsheet, CC120X_SniffMode.xlxs to calculate tevent0 in ms and the hex configuration setting. Used the config setting to set up and operate the chip in sniff mode and then measure the XOSC powering voltage with an oscilloscope.

     Following are the measurements:

 

 

Pream bytes      Event0      Width p29         Tevent0calc      Tevent0meas

 

.5                00 00      6.6 ms            0                 9.52 ms

1.0               00 85      6.37 ms          3.33              6.6 ms

2.0               01 8F      6.87 ms          9.99 ms          9.99 ms

3.0               02 9A      6.48 ms          16.65 ms          16.61 ms

4.0               03 A4      6.56      ms          23.31 ms          23.36 ms

 

 

   This data agrees with calculations for preamble bytes 2, 3, and 4 and above. But for 0.5 preamble bytes the duty cycle should be 100% but the data appears to have taken a wrong turn.

Same with 1 preamble byte which should be nearly 100%. Did notice that Sync_CFGx.pqt_gating =1 which is not recommended.

 

So I must be missing something in my config which probably an expert can spot immediately.  The config follows:

 

/***************************************************

   150Mhz   Sniff mode GPIO0 not pulsing

Rx filter=19.84khz, 2 GFSk, 2.68k dev, 2.4kbps

sync= 05,56 doesn't include type

11bits ,no seq no, fixed 3 byte 0bit

 

7 byte preamble             Sends to MCDT rec and ti rec

 

TI receiver evb: RF current=5.4ma, mcu current=2.6ma

****************************************************/

 

const registerSetting_t preferredSettings[]=

{

//{CC120X_IOCFG3,           0x3c},         //dec 60. read ext osc enable. Doesn't work

{CC120X_IOCFG2,           0x06},         //sniff 150M

//{CC120X_IOCFG1,           0x00},         //make GPIO1 output & invert to high (see 3.4)

{CC120X_IOCFG0,           0x06},

{CC120X_SYNC3,             0x55},         //was AA, 55 7:0 sets MSB sync. AA for inverted

{CC120X_SYNC2,             0x55},         //was AA, 55, 7:0 sets MSB sync.

{CC120X_SYNC1,            0x55},         //was AA, 55, 05 7:0 sets MSB sync.

{CC120X_SYNC0,             0x56},         //was A9, 56 sets lsb of sync in 7:0. Orig A6

{CC120X_SYNC_CFG0,         0x88},         //was 88. added PQT_EN & PQT_Gating_En, strict_sync_check=3

{CC120X_SYNC_CFG1,         0x28},   //28=11 bits & 8 thresh. was 48. 4x=16bits, 8x=24bits, Ax=32 bits 7-5 len sync, 4:0=sync thresh

{CC120X_DEVIATION_M,       0x8D},

{CC120X_MODCFG_DEV_E,     0x08},

{CC120X_DCFILT_CFG,       0x5D},

{CC120X_PREAMBLE_CFG1,     0x14},     //14=3bytes, 18=4b. 10=2bytes e AA. was 24. 0x31=24bytes. 0x25=7bytes

{CC120X_PREAMBLE_CFG0,     0x8A},

{CC120X_IQIC,             0xCB},

{CC120X_CHAN_BW,           0xAC},     //AC=9.46khz. AA=10khz

{CC120X_MDMCFG1,           0x60},     //40=fifo en, Man mode=20, invert data=10

{CC120X_MDMCFG0,           0x05},

{CC120X_SYMBOL_RATE2,     0x4F},

{CC120X_SYMBOL_RATE1,     0x75},

{CC120X_SYMBOL_RATE0,     0x10},

{CC120X_AGC_REF,           0x31},

{CC120X_AGC_CS_THR,       0x09},

{CC120X_AGC_CFG1,         0x40},

//{CC120X_AGC_GAIN_ADJUST,   0xAC},     //-84

{CC120X_AGC_CFG0,         0x87},         //2counts for rssi

{CC120X_FIFO_CFG,         0x00},

{CC120X_SETTLING_CFG,    0x03},

{CC120X_FS_CFG,           0x1B},

{CC120X_WOR_CFG0,         0x08},     //same

{CC120X_WOR_CFG1,         0x08},     //added

{CC120X_WOR_EVENT0_MSB,   0x00},     //02, 3B pre. was 01. was 02

{CC120X_WOR_EVENT0_LSB,   0x00},   //9a, 3b pre. was 8f for 2bytws. was e2

{CC120X_PKT_CFG2,         0x00},     //norm mode, CRc disabled, no status byte

{CC120X_PKT_CFG1,         0x00},     // 0x01=append RSSI.   Adx check & crc disabled. This line needed to rec MCDT!

{CC120X_PKT_CFG0,         0x10},     //x18 pro 6bits, x10 EMIDS 4bits. fixed length=00 in bits 6:5, bit len in bits 4:2

//{CC120X_RFEND_CFG1,       0x30},   // x30 gives return to rx after rxing

{CC120X_RFEND_CFG0,       0x0C},     //0x30 gives return to rx after txing

{CC120X_PKT_LEN,           0x08},     //pktlen, packet length 02 +4 bits for emids, 6-3/4 for pro

{CC120X_IF_MIX_CFG,       0x1C},

{CC120X_FREQOFF_CFG,       0x22},     //FOC_EN=x20, FOC_KI_Factor=02 was 20

{CC120X_MDMCFG2,           0x0C},

{CC120X_TOC_CFG,           0x40},     //TOC limit 2%

 

{CC120X_SETTLING_CFG,     0x08},     //Cal FS going from idle to rec or tx

 

{CC120X_FREQ2,             0x5A},

{CC120X_FREQ1,             0x00},

{CC120X_FREQ0,            0x00},

 

{CC120X_IF_ADC1,           0xEE},

{CC120X_IF_ADC0,           0x10},

{CC120X_FS_DIG1,           0x07},

{CC120X_FS_DIG0,           0xAF},

{CC120X_FS_CAL1,           0x40},

{CC120X_FS_CAL0,           0x0E},

{CC120X_FS_DIVTWO,         0x03},

{CC120X_FS_DSM0,           0x33},

{CC120X_FS_DVC0,           0x17},

{CC120X_FS_PFD,           0x00},

{CC120X_FS_PRE,           0x6E},

{CC120X_FS_REG_DIV_CML,   0x1C},

{CC120X_FS_SPARE,         0xAC},

{CC120X_FS_VCO0,           0xB5},

//{CC120X_FS_CFG,           0x0B},                     //136-160Mhz

{CC120X_XOSC5,             0x0E},

{CC120X_XOSC1,             0x03},

};

 

 

 

Thanks,

 

John

 

 

  

  • 0
    Expert 1470 points

    Sniff Sampling Scope wave Event0= 0.pdfAttached are scope waveforms for Event0 configured to 00 00 showing the pin29 signal that powers the xosc.

    John

  • 0 in reply to John Moore39
    TI__Guru* 79101 points
    I have a couple of questions/comments.

    First of all, how did you get to the register settings you are listing? I took Smartrf studio and input the data rate, deviation, rx bw etc. that you state you are suing, but ended up with different register settings.
    When using your settings in the CC1200 sniff mode excel sheet, I am not getting the calculated numbers you are showing.
    Have you done changes to any parameters than the once in the Dashboard? Can you please send a screenshot of your dashboard?
    What is the startup time of the TCXO?
    I would assume that you will have difficulties receiving the correct packets since your sync word is almost identical to the preamble. Have you checked that you actually can get a stable link with normal RX?

    Siri
  • 0 in reply to Siri
    Expert 1470 points

    Thanks, Siri:

    Good to have you back.

    Attached is a screen shot of the control panel with one byte preamble. It gives setting 00 85 with cs terminate as you can see. But it gives a negative number with pqt termination.


    I have been able to receive 3 byte preamble packets well. It has 118dbm +- sensitivity on 3 boards. But with the 10 bit preamble reception is sporadic.


    John

  • 0 in reply to John Moore39
    Expert 1470 points

    Attached is the xcel spreadsheet.  The tcxo is a kyocera KT2520F40 that was recommended by TI.

    JohnCC120x_RX_Sniff_ModeJohn1.xlsx

  • 0 in reply to John Moore39
    TI__Guru* 79101 points

    Hi John

    I think you need to download your excel sheet again (from the app note) and then enter the register settings over again. There has been done several edits of cells in the excel sheet that should not be edited. For example the data rate is written as 1200 instead of being calculated from cell B3, B4, and B5 (so the register values does not match) You have register settings for 38.4 kbps but have written 1200.

    By inputting the register settings from Studio you will see that when using CS you need a minimum of 1 byte preamble and if using PQT termination you need 4 bytes.

    You should not do any changes in the EVENT0 sheet or any other sheets other than in tXTAL (in the calculation sheet). There you need to input the startup time of the TCXO (the time in there by default is 300 us, which is the startup time of the crystal on our EM’s)

    BR

    Siri

  • 0 in reply to Siri
    Expert 1465 points

    Thanks, Siri:

         Will do.

         Also the startup time for this kyocera tcxo appears to be better than 40us with a scope.

    John

        

  • 0 in reply to John Moore55926
    Expert 1465 points
    Siri:
    The shortest preamble length we need to contend with is about 6 bits. Could we receive this reliably if we change from sniff mode to ordinary receive mode?

    John
  • 0 in reply to John Moore55926
    TI__Guru* 79101 points
    the radio only needs 4 bits of preamble, so in theory that should be enough. However, I think the biggest challenge you will have is the short sync word (that looks like a preamble). You might experience receiving a lot of false sync word, so you might want to play around with the sync threshold.

    Siri
  • 0 in reply to Siri
    Expert 1470 points

    Thanks, Siri:

    I'll try varying the sync threshold.

          My sync thresh is on maximum (8).  The sync detect gives me an interrupt which causes the software to read the message.  If the thresh is too loose then there will be 

    false interrupts and I'll read errors for messages.  This is not what is happening.  Instead I'm getting no response to these short preamble messages most of the time.

         Would there be any advantage in using continuous rx mode instead of sniff mode?  For some reason I can't switch back to continuous rx mode now to try it.

    John

  • 0 in reply to John Moore39
    TI__Guru* 79101 points
    For testing ordinary packet, use the Easy_link example found here:

    www.ti.com/.../swrc274

    and then use SmartRF STudio to generate settings that can be exported to the code.

    As long as sniff mode is set up correctly and you make sure that you are in RX mode long enough for the AGC to settle before your sync word comes along, there will be no difference in performance when using regular RX compared to sniff mode. However, it is not always easy to figure our if lost packets are due to poor rf performance or wrong timing, so therefore we recommend to first see that you are able to receive the packets you want without thinking about timing issues, and if you are satisfied with the performance, you can move on to the sniff implementation.

    If you can post a detailed description of how your packet looks like (preamble, sync, length info (if any) number of bytes in payload, CRC etc. I can try to help you with settings for both regular RX mode and sniff mode.

    BR

    Siri