Nodemcu-firmware: bme280 reports wrong temperature around/below 0

[dicer]

Expected behavior

Module returns actual temperature when reading it.

Actual behavior

Around 0 degrees Celsius the temperature I read has 1-2° jumps in it and generally looks wrong:

I don't know the actual temperatures but I would guess something goes wrong when going below 0°. It already starts with the jump from 0 to 1 degree. I'm pretty sure this should not happen and the temperature was already below 0, before it jumped to -2°
I think the graph should have shown the temperature sink below 0 to roughly -2 and then rise again above 0.

I've never seen these jumps before when measuring higher temperatures! So I don't think it's my backend code.

Test code

Nothing special here (I basically follow the example from the docs):

bmetype = bme280.init(PIN_BME280_SDA, PIN_BME280_SCL)
P = bme280.baro()
P = string.format("%d.%03d", P/10, P%10)
H, T = bme280.humi()
T = string.format("%d.%02d", T/100, T%100)

NodeMCU version

NodeMCU custom build by frightanic.com
branch: master
commit: 8e48483c825dea9c12b37a4db3d034fccbcba0bf
SSL: true
modules: bme280,cjson,dht,file,gpio,http,i2c,mqtt,net,node,spi,tmr,uart,wifi,ws2812,file,gpio,net,node,tmr,uart,wifi
build built on: 2016-09-16 13:13
powered by Lua 5.1.4 on SDK 1.5.4.1(39cb9a32)

I know, rather old. But looking at bme280.c nothing changed since Aug 2, 2016

Hardware

BME type: 2

[devsaurus]

There are some improvements in dev branch that aren't available yet in master: #1652 and #1662
Please cross-check your application with a build from current dev branch.
Also note the updates to the API.

[FrankX0]

You are running in unexpected (but correct) result of the modulo operation on negative numbers.
For example check Modulo operation with negative numbers

The example is correct for positive numbers, but incorrect for negative ones (and should be corrected).
Try something like:

if T<0 then
    str = "-"..(-T/100).."."..(-T%100)
else
    str = (T/100).."."..(T%100)
end

[dicer]

Oh yeah, you are right @FrankX0 !
T = -010
T = string.format("%d.%02d", T/100, T%100)
print (T)

-> 0.9
T = -110 -> -1.90

The following works:
T = -010
if T<0 then
T = string.format("-%d.%02d", -T/100, -T%100)
else
T = string.format("%d.%02d", T/100, T%100)
end
print (T)

I updated the doc in this pull request: https://github.com/nodemcu/nodemcu-firmware/pull/1735

[vsky279]

Disadvantage of programming in summer... Thanks for spotting it.

I'd prefer this syntax to fix the issue:

T = -010
local sgn = T < 0 and -1 or 1
local Ta = sgn * T
print(string.format("%s%d.%02d", sgn < 0 and "-" or "", Ta/100, Ta%100))

[marcelstoer]

Same goes for examples in module documentation: http://nodemcu.readthedocs.io/en/latest/en/modules/bme280/

[vsky279]

I will prepare a PR.