Teacup, Thermistor, wrong temperature

Hi everybody,

I'm currently building a 3d printer from scratch. Yesterday I tested the temperature sensing with the following setup:

Microcontroller: Arduino Uno R3
Firmware: Teacup
Thermistor: B57560G104F (100k)

I used the double thermistor table, keeping the default values as they apply to my thermistor.
I wired the Thermistor like this: [playground.arduino.cc] (but with R2 = 4.7k)

This is the relevant config:
config.h
DEFINE_TEMP_SENSOR(extruder, TT_THERMISTOR, AIO1, THERMISTOR_EXTRUDER)
DEFINE_HEATER(extruder, PB7, 1)
#define HEATER_EXTRUDER HEATER_extruder

Now the problem: When I measure the temperature with pronterface (M105) I always get crazy values above 200 (celsius?) at roomtemperature. Any ideas what could be wrong? If I use a multimeter I measure approx. 100k resistance at room temperature, so I guess the Thermistor is ok.

Thanks for any hints!
Nik

It was a hard learning lesson, but I found out what was wrong.

For future reference:

1) I somehow inversed the measuring circuit, so instead of 0.5Volts there were 4.5Volts flowing into the analog input. After restudying the temperature sensor circuit in the reprap wiki I noticed this
2) Therefore the thermistor tables had to be modified: all first values needed to be inversed (1023 - value) and the order in the ThermistorTable.h needed to be rearranged after that (smaller values first, then larger)
3) I also had jitter in the signal which I killed with a condensator acting as a low pass filter (as proposed in the reprap temperature sensor circuit)

I hope these insights will help other lost souls

cheers,
Nik