ESP8266 - Async DallasTemperature - Exception (28)

Question

I am starting a temperature measurement (sensors.requestTemperatures) in a timer interrupt and I need a way to know whether it is ready. I tried using the sensors.isConversionAvailable method, but it throws an exception. I looked at the source of the isConversionAvailable method and tried to call the readScratchPad method myself, and an exception was thrown there.

Full code:

#include <DallasTemperature.h>
#include <OneWire.h>

#define TIMER_SECONDS 1
#define SENSOR_PIN 5
OneWire sensorWire(SENSOR_PIN);
DallasTemperature sensors(&sensorWire);

void setup() {
    Serial.begin(115200);
    Serial.println("Initializing...");

    sensors.begin();
    sensors.setResolution(10);
    sensors.setWaitForConversion(false);

    timer1_isr_init();
    timer1_attachInterrupt([] {
        sensors.requestTemperatures();
    });
    timer1_enable(TIM_DIV265, TIM_EDGE, TIM_LOOP);
    timer1_write(F_CPU / 256 * TIMER_SECONDS);

    Serial.println("Initialization complete");
}

void loop() {
    Serial.println("Started the loop method");

    //When this block is uncommented, the second exception is thrown
    /*uint8_t number;
    sensors.readScratchPad(0, &number);
    Serial.print("Scratch pad number: ");
    Serial.println(number);*/

    if (sensors.isConversionAvailable(0)) {
        Serial.print("Temperature: ");
        Serial.println(sensors.getTempCByIndex(0));
    } else {
        Serial.println("The conversion is unavailable");
    }

    Serial.println("Finished the loop method");
}

Exception (isConversionAvailable):

Initializing...
Initialization complete
Started the loop method

Exception (28):
epc1=0x402025af epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000

ctx: cont 
sp: 3ffef230 end: 3ffef460 offset: 01a0

>>>stack>>>
3ffef3d0:  00000000 3ffee354 00000000 402025ad  
3ffef3e0:  3ffef420 00000000 3ffee338 40201e57  
3ffef3f0:  00000000 00000017 3ffef4a0 40202bcd  
3ffef400:  3ffe8588 00000000 3ffef4a0 3ffee42c  
3ffef410:  3fffdad0 3ffee338 3ffef4a0 40202144  
3ffef420:  40201052 88011627 3ffef4a0 40202c1c  
3ffef430:  3fffdad0 00000000 3ffef4a0 40201d0e  
3ffef440:  feefeffe 00000000 3ffee424 40202888  
3ffef450:  feefeffe feefeffe 3ffee440 40100114  
<<<stack<<<

ets Jan  8 2013,rst cause:2, boot mode:(1,6)

Exception (readScratchPad):

Initializing...
Initialization complete
Started the loop method

Exception (28):
epc1=0x402025cf epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000

ctx: cont 
sp: 3ffef250 end: 3ffef470 offset: 01a0

>>>stack>>>
3ffef3f0:  00000000 3ffee364 00000000 402025cd  
3ffef400:  3ffef440 00000000 3ffee348 40201e77  
3ffef410:  00000000 3ffee348 3ffef4b0 40202c18  
3ffef420:  3ffe8438 00000000 3ffef4b0 3ffee43c  
3ffef430:  3fffdad0 3ffee348 3ffef4b0 40201d14  
3ffef440:  3fffdad0 3ffee348 3ffef4b0 40201cd0  
3ffef450:  feefeffe 00000000 3ffee434 402028a8  
3ffef460:  feefeffe feefeffe 3ffee450 40100114  
<<<stack<<<

ets Jan  8 2013,rst cause:2, boot mode:(1,6)

You should only set a flag in the interrupt handler and put sensors.requestTemperatures() in your main loop. Your reset cause 28 is probably caused by a pointer pointing to an uninitialized object. Are you sure the temperature-IC is ready? — BMelis
@BMelis I have created a volatile bool variable and now I'm only setting that in the interrupt handler. Why is this the correct/preferred way? Is it because the interrupts should finish as soon as possible and setting a boolean is faster than calling that method? I have put a 10 second delay in the setup method after the configuration of the sensors variable and I have also (successfully) printed out the temperature (which was requested synchronously) there, in the setup method. The exception was still thrown when calling the isConversionAvailable method. — Trigary

Maximilian Gerhardt Maximilian Gerhardt · Accepted Answer · 2017-08-25T19:33:14

Looking at the function's code (a few comments omitted):

bool DallasTemperature::readScratchPad(const uint8_t* deviceAddress, uint8_t* scratchPad){

    // send the reset command and fail fast
    int b = _wire->reset();
    if (b == 0) return false;

    _wire->select(deviceAddress);
    _wire->write(READSCRATCH);

    for(uint8_t i = 0; i < 9; i++){
        scratchPad[i] = _wire->read();
    }

    b = _wire->reset();
    return (b == 1);
}

You call the function with

sensors.readScratchPad(0, &number);

Then deviceAddress = NULL. So then there is the call _wire->select(deviceAddress). The _wire object is of type OneWire*. The function select is implemented as follows:

void OneWire::select(const uint8_t rom[8])
{
    uint8_t i;

    write(0x55);           // Choose ROM

    for (i = 0; i < 8; i++) write(rom[i]);
}

And in rom[i] you have a dereference on the NULL pointer you gave it initially. I.e.: Crash.

Looking at another function in the lib shows how to obtain the a valid deviceAddress pointer.

float DallasTemperature::getTempCByIndex(uint8_t deviceIndex){

    DeviceAddress deviceAddress;
    if (!getAddress(deviceAddress, deviceIndex)){
        return DEVICE_DISCONNECTED_C;
    }
    return getTempC((uint8_t*)deviceAddress);
}

So, if you want the address of your first sensor, you can get it by doing:

 DeviceAddress deviceAddress;
 if (!sensors.getAddress(deviceAddress, 0)){
     Serial.println("No device address found for index 0!!!");
     return;
 }
 //use deviceAddr in the next call
 ScratchPad readScratchPad; //this is a typedef to uint8_t[9]
 sensors.readScratchPad( (uint8_t*)deviceAddress, readScratchPad);
 //do whatever you like with the values in the readScratchPad array..

For further references look at the library files at https://github.com/milesburton/Arduino-Temperature-Control-Library and https://github.com/PaulStoffregen/OneWire .

ESP8266 - Async DallasTemperature - Exception (28)

1 Answers