# Problem in sending negative values in the payload

**URL:** <https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604>\
**Category:** LMIC\
**Created:** [March 24, 2020, 3:06am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604 "2020-03-24T03:06:47Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![samdanisms](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/samdanisms/32/25844_2.png) [@samdanisms](https://www.thethingsnetwork.org/forum/u/samdanisms)\
**Post date:** [March 24, 2020, 3:06am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/1 "2020-03-24T03:06:47Z")

</div>

I’ve a LoRaWAN end-node (Feather M0 LoRa) sending data to the gateway (IMST+Pi3). I’m using MCCI LoRaWAN LMiC library to send solar battery current data. The code I’m using is an adapted version of [https://github.com/mcci-catena/arduino-lmic/blob/master/examples/ttn-otaa-feather-us915-dht22/ttn-otaa-feather-us915-dht22.ino](https://github.com/mcci-catena/arduino-lmic/blob/master/examples/ttn-otaa-feather-us915-dht22/ttn-otaa-feather-us915-dht22.ino) to suit to my application.

The battery current is positive during charging time and negative during discharging time. I could see battery charging current properly in my application server (i.e., positive values), however when it comes to discharging current, I’m seeing the values at +645 in the application server instead of negative values.

I carefully looked at my code and I suspect if the following line could be the problem.  
LMIC\_setTxData2(1, (uint8\_t\*)payload, sizeof(payload), 0);

As such uint8\_t has a range of 0 to 255, and it cannot take negative values. Am I right here? How do I overcome this error?

---

<div class="post-metadata">

**Author:** ![bialy](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/bialy/32/25844_2.png) [@bialy](https://www.thethingsnetwork.org/forum/u/bialy)\
**Post date:** [March 24, 2020, 7:00am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/2 "2020-03-24T07:00:03Z")

</div>

Probably you are sending more than one byte, in that case just send an as positive value, and in dedicated byte send a sign - this is the easiest way.  
More complex solution is present in your example:

int16\_t payloadTemp = …;  
byte tempLow = lowByte(payloadTemp);  
byte tempHigh = highByte(payloadTemp);  
// place the bytes into the payload  
payload[0] = tempLow;  
payload[1] = tempHigh;

---

<div class="post-metadata">

**Author:** ![kersing](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/kersing/32/44777_2.png) [@kersing](https://www.thethingsnetwork.org/forum/u/kersing)\
**Post date:** [March 24, 2020, 8:54am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/3 "2020-03-24T08:54:58Z")

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> [@samdanisms](#):
>
> I carefully looked at my code and I suspect if the following line could be the problem.  
> LMIC\_setTxData2(1, (uint8\_t\*)payload, sizeof(payload), 0);

Please provide a larger sample of your code including the calculation of the payload values. (Or post a link to the entire code)  
Also post the code you are using to decode the received payload in your application,

The code you are quoting is probably _not_ the cause of your issues.

> [@bialy](#):
>
> in that case just send an as positive value, and in dedicated byte send a sign - this is the easiest way.

Please don’t suggest wasting a byte just to send a minus sign. Sending negative values works without overhead if both the encoding and decoding side are crafted correctly.

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<div class="post-metadata">

**Author:** ![bialy](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/bialy/32/25844_2.png) [@bialy](https://www.thethingsnetwork.org/forum/u/bialy)\
**Post date:** [March 24, 2020, 9:34am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/4 "2020-03-24T09:34:57Z")

</div>

> [@kersing](#):
>
> Please don’t suggest wasting a byte just to send a minus sign. Sending negative values works without overhead if both the encoding and decoding side are crafted correctly.

For non-commercial projects, it really does not matter - make it work as soon as possible. You will have time for optimization later on.

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<div class="post-metadata">

**Author:** ![kersing](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/kersing/32/44777_2.png) [@kersing](https://www.thethingsnetwork.org/forum/u/kersing)\
**Post date:** [March 24, 2020, 9:42am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/5 "2020-03-24T09:42:13Z")

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> [@bialy](#):
>
> For non-commercial projects, it really does not matter - make it work as soon as possible. You will have time for optimization later on.

For both commercial and non commercial projects it makes sense to do it right the first time. After it works there is no incentive to make improvements, as it is probably being used so changes will interrupt operation.

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<div class="post-metadata">

**Author:** ![arjanvanb](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/arjanvanb/32/46626_2.png) [@arjanvanb](https://www.thethingsnetwork.org/forum/u/arjanvanb)\
**Post date:** [March 24, 2020, 9:49am UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/6 "2020-03-24T09:49:23Z")

</div>

Also, the following still does not guarantee that the value will be decoded correctly. In fact, negatives values will erroneously decode into very large positive values if the decoder does not [support negative values](https://www.thethingsnetwork.org/forum/t/decrypting-messages-for-dummies/4894/4).

> [@bialy](#):
>
> More complex solution is present in your example:
> 
> ```cpp
> int16_t payloadTemp = …;
> byte tempLow = lowByte(payloadTemp);
> byte tempHigh = highByte(payloadTemp);
> // place the bytes into the payload
> payload[0] = tempLow;
> payload[1] = tempHigh;
> 
> ```

---

<div class="post-metadata">

**Author:** ![samdanisms](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/samdanisms/32/25844_2.png) [@samdanisms](https://www.thethingsnetwork.org/forum/u/samdanisms)\
**Post date:** [March 24, 2020, 2:17pm UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/7 "2020-03-24T14:17:55Z")

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> [@kersing](#):
>
> Please provide a larger sample of your code including the calculation of the payload values. (Or post a link to the entire code)  
> Also post the code you are using to decode the received payload in your application,

I’m posting my full code here: [Final\_circuit\_final\_code.ino - Google&nbsp;Drive](https://drive.google.com/file/d/1gku6AN52f6bMvK-BJjrBdCc1JhO04AWx/view?usp=sharing) and decoder function here: [Decoder function.txt - Google&nbsp;Drive](https://drive.google.com/file/d/1Mln4lido2gN2El1_jeygyg1Nnu-jKhsr/view?usp=sharing)

I’m actually measuring the voltage and current of load, battery and solar of my standalone solar PV system. In addition to that I’m measuring ambient temperature and relative humidity, thus making the total number of measurements to eight. That’s why I’ve declared an array of int type of size 8.

```cpp
value[3]= (val[3]/2.0)+1000.0;
value[4]= (val[4]/2.0)+1000.0;
value[5]= (val[5]/2.0)+1000.0;

```

val[3], val[4] and val[5] are the load, battery and solar currents respectively. The reason I’ve added 1000 is to account for the negative values; I’ve made sure that this 1000 is taken care of in the decoder function.

```cpp
LMIC_setTxData2(1, (uint8_t*)payload, sizeof(payload), 0);

```

Again, could this be the issue?

> [@arjanvanb](#):
>
> Also, the following still does not guarantee that the value will be decoded correctly. In fact, negatives values will erroneously decode into very large positive values if the decoder does not [support negative values](https://www.thethingsnetwork.org/forum/t/decrypting-messages-for-dummies/4894/4)

You’re right @arjanvanb, the negative values of val[4] i.e., battery current are erroneously being decoded to very large positive values, while there’s no issue with positive values. How do we address it?

---

<div class="post-metadata">

**Author:** ![arjanvanb](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/arjanvanb/32/46626_2.png) [@arjanvanb](https://www.thethingsnetwork.org/forum/u/arjanvanb)\
**Post date:** [March 24, 2020, 6:07pm UTC](https://www.thethingsnetwork.org/forum/t/problem-in-sending-negative-values-in-the-payload/35604/8 "2020-03-24T18:07:57Z")

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> [@samdanisms](#):
>
> ```cpp
> LMIC_setTxData2(1, (uint8_t*)payload, sizeof(payload), 0);
> 
> ```
> 
> Again, could this be the issue?

Like @kersing already wrote: no. The above “cast” only tells the compiler that you know what you’re doing, and that whatever is in the variable `payload` can be handled as if it were of type `uint8_t`. That happens to be what this LMIC function expects, but your code has declared `payload` to be of a different type:

```cpp
byte payload[16];

```

Even though that is really the same (both `uint8_t` and `byte` define 8 bits unsigned integer values, from 0 thru 255), you need to tell the compiler that you know it’s okay. You could also use:

```cpp
uint8_t payload[16];
...
// No need to cast now:
LMIC_setTxData2(1, payload, sizeof(payload), 0);

```

So what’s your code doing? I’ve added some comments:

```cpp
// Each val holds the sum of 200 samples.
float val[6] = {0};
...
// Arduino int is 16 or 32 bits, depending on the board.
int value[8];
...
// Take the average of the 200 samples and multiply by 100, to
// send 12.35 as 1235. Add +1000 to always send positive values,
// hence assuming the average is larger than -1000. For a 16 bits
// unsigned integer, which supports 0 thru 65,535, this allows
// for actual averages between -10.00 and +645.35. To allow for,
// e.g., -300.00 thru +355.35, one would add 30,000 instead.
value[3]= (val[3] / 2.0) + 1000.0;
...
// Send MSB first
payload[6] = highByte(value[3]);
// ...and LSB next
payload[7] = lowByte(value[3]);

```

Like I linked to earlier, for the last two lines, [you could also use](https://www.thethingsnetwork.org/forum/t/decrypting-messages-for-dummies/4894/4):

```cpp
// MSB
payload[6] = value[3]>>8;
payload[7] = value[3];

```

All looks good so far. You’re using two bytes to send each value, [MSB first](https://en.wikipedia.org/wiki/Most_significant_byte). And you’re not sending negative values, so no magic needed in the decoder.

However, while decoding, you’re using:

```javascript
var LC = (bytes[6]<<15) | bytes[7];
...
return {
  ...
  LC: (LC-1000)/100,
  ...
}

```

That’s wrong: why shift the MSB 15 bits to the left when concatening the two bytes into a single value? I’d say this should read:

```javascript
var LC = bytes[6]<<8 | bytes[7];

```

(No parentheses needed either. It’s okay to add them, of course.)

If in the Arduino code you would _not_ add the 1,000, so if you would actually be sending _signed_ 16 bits values, then in a JavaScript decoder you would need:

```javascript
var LC = bytes[6]<<24>>16 | bytes[7];

```

See [Decrypting messages for dummies - #4 by arjanvanb](https://www.thethingsnetwork.org/forum/t/decrypting-messages-for-dummies/4894/4) that I linked to before.