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CC2540EMK-USB: When using TI BTool app: How do we calculate dBm value from the 0-250 RSSI result?

Part Number: CC2540EMK-USB

Background information:

  • When using the pictured System #1 (TI hardware / TI firmware / TI Packet Sniffer App), the signal strength results are in dBm. These results are useful and meaningful, because they are comparable to other systems.
  • When using the pictured System #2 (TI hardware / TI firmware / TI BTool app), the signal strength results are in a 0-250 unitless scale. These results are not useful or meaningful, because we can not calculate the equivalent dBm (or mW) value, so we can not compare to other systems.
  • But we need to use System #2 for our test-rig (because it is easier to connect our test-rig API to the TI BTool app).

Question:

When using the pictured System #2 (TI hardware / TI firmware / TI BTool app), how do we calculate the dBm (or mW) value from the 0-250 result?

  • Hi Robin,

    The only information I see missing from the previous thread which Clement assisted with is that BTool provides the RSSI as a 8-bit signed integer (aka 2's complement), so the results should be interpreted as between -128 to 127.  Thus 0xB2 represents 50 (178 minus 128) and accounting for the offset (receiver sensitivity is -87 for default standard mode in the datasheet, or -93 for high gain mode) would be -37 dBm (50 minus 87) which is fairly close to your Packet Sniffer measurements.  You can refer to the TI BLE HCI Vendor Specific HCI Guide for HCI_EXT_SetRxGainCmd instructions for switching between standard/high gain modes.

    Regards,
    Ryan

  • Hi Ryan

    Thanks for your help, this sounds promising. I know from previous threads we are basically looking for an offset, based on your reply that offset total would be -215 for default standard mode (i.e.-128-87).

    I have a couple of reservations about the 2's complement aspect of this explanation (i.e. why does BTool display 0xB2 as 178 rather than converting it to 50 for us, and isn't the complement of 178 actually -78?), but then I'm not a coder! Also from a previous quick experiment we were expecting an offset closer to -239 (note the -30dBm shown on my screenshot is not meant to be equivalent to the 178 measurement, those were two random results).

    However, now we have both systems set-up properly, we will repeat that experiment and try to verify whether a total offset of -215 looks right for default standard mode. I will report back in the next few days, don't close the thread yet... 

    Regards

    Robin

  • There could be differences in radio setup between the BTool and packet sniffer firmwares which account for the remaining offset allowance.  Have you tried setting up two CC2540EMK-USB devices which are equidistant from the BLE transmitter such that you could measure the same packets on both tools?  This way you could test for a linear equivalence between the resources and produce the most accurate offset value.

    Regards,
    Ryan

  • Great minds think alike! We will be running that exact test in the next week or two. I will share the results...

    Regards

    Robin