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Saturday, January 29, 2011

Duothermo: sensor back but changed to LM335

Mystically found my DS18B20 sensor in TI LaunchPad box :) But I already started to work with LM335 analog sensor. Connected it to LaunchPad VCC and A1 channel of ADC10.
While ADC10 clock source is set to ADC10OSC read value is not constant, it's jumping around +/-3. While clock source is MCLK (with 8 divider, 16MHz/8) read value is pretty constant if temperature does not change. And with ADC10OSC value is 3 points higher. I don't really understant why it is so, but if I will need a low power mode I definitly will use ADC10OSC.
Now LM335 needs some calibration. I will use this sensor as external ranged (2-3 meters max) sensor so I want no more than 2 wires. I don't like hardware calibration, because I don't have small variable resistor pots :) I'll try to do it in firmware. I don't know if recalibration needed if I change Vref for ADC10. Now VCC as Vref used and it's 3.55V(or 3.56V) on LaunchPad. The actual project will use 3.3V. This will limit sensor temperature range to no more than 55C or so. It's ok for me. I can use 5V reference for MSP430G2231 which I use, but lose another pin, which is no go for now.

A1 read function:

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