BLE Beacon Scanner example might not handle negative temperatures?

[Humancell]

Board

Olimex ESP32 PoE ISO

Device Description

PoE PCB using the ESP32 WROOM 32UE

Hardware Configuration

Nothing special

Version

latest master (checkout manually)

IDE Name

Arduino IDE

Operating System

All

Flash frequency

80Mhz

PSRAM enabled

no

Upload speed

115200

Description

https://github.com/espressif/arduino-esp32/blob/master/libraries/BLE/examples/BLE_Beacon_Scanner/BLE_Beacon_Scanner.ino#L118

When low temperatures occur, and go below 0 C, the calculations do not seem to be correct. It appears that using these 16-bit values in a 32-bit INT doesn't detect/use twos complement to handle the negative numbers properly.

In my final code I moved to a "signed short" which seems to fix the issue.

Sketch

signed short value;

          // extract the temperature
          value = (cServiceData[TEMP_MSB] << 8) + cServiceData[TEMP_LSB];

Debug Message

No debug ... just wrong calculated values.

Other Steps to Reproduce

No response

I have checked existing issues, online documentation and the Troubleshooting Guide

• I confirm I have checked existing issues, online documentation and Troubleshooting guide.

[mrengineer7777]

Your sketch doesn't match the example. Note the (int) cast on each payload byte. Also we don't have your hardware. It would be very helpful to know the actual bytes received in ServiceData[] as well as the actual temperature.

[SuGlider]

This seems to be just a casting matter.
uint32_t with just 16 bits in the LSB will be always positive.

Casting it to whatever struct or type is an application subject, not related to the BLE Beacon Scanner example.

[SuGlider]

I'll add this fix. Thanks.

[SuGlider]

@PilnyTomas

The temperature is represented in signed 8.8 fixed-point notation and measured in Celsius.
https://github.com/google/eddystone/blob/master/eddystone-tlm/tlm-plain.md 

When Temperature is not supported BLE will return 0x8000, which means -128 °C.
In that case, it could be nice to check this value and send out the correct message, instead of reporting the temperature.

Note: All multi-byte values are big-endian --> MSB:LSB

[SuGlider]

As we talked, the change shall be in
https://github.com/espressif/arduino-esp32/blob/master/libraries/BLE/examples/BLE_Beacon_Scanner/BLE_Beacon_Scanner.ino#L118-L120

I think that it just has to use signed short, like

          // BIG ENDIAN payload in signed 8.8 fixed-ppint notation. Unit is Celsius. 
          int16_t temp_payload = payLoad[16] + (payLoad[15] << 8);
          if (payLoad[15] == 0x80 && payLoad[16] == 0) {
            Serial.printf("This device does not support measuring temperature.\n");
          } else {
            float calcTemp = temp_payload / 256.0f;
            Serial.printf("Reported temperature from data: %.2fC\n", calcTemp);
          }

But it shall be verified by testing some cases, like below:

For instance:

Signed Short Value -32768 = 0x8000 shall be calculated as -128 °C
Signed Short Value -9024 = 0xDCC0 shall be calculated as -35.25 °C
Signed Short Value 9024 = 0x2340 shall be calculated as +35.25 °C
Signed Short Value 384 = 0x0180 shall be calculated as +1.50 °C
Signed Short Value -1024 = 0xFC00 shall be calculated as -4.00 °C