Implementation of a MSP430-based digital thermometer using the slope ADC of the timer port module

Abstract

This report describes the slope A/D measurement of a resistance and the ease with which it can be applied to MSP430 microcontrollers. It describes a digital thermometer design that uses the slope ADC capabilities of the Timer Port module on the MSP430x3xx microcontrollers. It is used more generally as a reference on how to connect resistive sensors and reference resistors to the Timer Port module. All MSP430x3xx devices include the Timer Port module. The module allows several resistive sensors and reference resistors to be connected in an application. Unused module pins can be used as independent outputs. Slope A/D conversion is an analog-to-digital conversion technique that can be implemented with a comparator rather than a standalone ADC module or device. The technique is based on the charging/discharging of a capacitor with a known value. The number of clock cycles necessary to discharge the capacitor is then counted. Longer discharge times indicate larger voltages. The voltage is derived from the discharge time using the standard equation for capacitor discharge. In addition to digitizing voltages, a variation of the technique can be used to measure resistance. This is valuable in measuring any component that can have varying resistance, such as potentiometers and various types of transducers. Unlike voltage measurement, where the key relationship is between voltage and time while the resistance is constant, the key relationship in resistance measurement is between resistance and time, while the initial voltage remains constant. The R-relationship is linear, which means the calculation is easier and less- costly to implement in a microcontroller than for the exponential V-t relationship. The thermometer has been simulated by using a variable resistance instead of a thyristor. In addition care has been taken to optimize the power consumption by forcing the microcontroller to several low-power modes during the operation. The combination of the Timer Port module, the 16-bit CPU, and the ultra low power design provide unmatched MIPS per watt performance. The set up can be extended to provide a low power thermostat.