Temperature Measurement Challenges
In many applications, there arises a need to measure the temperature of a circuit or the external environment. Achieving this task is feasible through additional components like ICs or employing an RC network and a software routine. However, the challenge arises when all I/O port pins are already in use, leaving few options. A clever circuit trick can offer a solution to this dilemma.
Leveraging Microcontroller Oscillators
Modern microcontrollers typically feature RC oscillators with relatively large temperature coefficients. These oscillators, used for instruction processing, cause the execution time of a software loop to fluctuate with the chip’s temperature. By incorporating a loop that increments a counter in your program, you can obtain unique counter values corresponding to different chip temperatures. This approach allows specific temperature values to be assigned to each counter value, enabling precise temperature measurement without requiring additional I/O port pins.
Stable Time Reference for Oscillation
To ensure reliable oscillation, a highly stable time reference is essential. This can be provided by the 50-Hz mains frequency or a secondary (crystal) oscillator network linked to the microcontroller. In the case of low-power microcontrollers, a secondary oscillator circuit operates them at very low clock rates, like 32 kHz, conserving power. The RC oscillator is then activated only as necessary, meeting the software’s demands effectively.
Versatility of RC Oscillators
RC oscillators offer stability and generate a well-shaped sine wave output. The frequency is proportional to 1/RC, allowing for a broader frequency range, especially when using a variable capacitor for tuning purposes.
Principle of RC Phase Shift Oscillator
The fundamental concept behind the RC phase shift oscillator involves passing a portion of the amplifier output through a phase shift network before feeding it back to the input. For stable oscillation, the total phase shift around the loop must be 360 degrees. Three types of RC oscillators—Wien Bridge, Twin-T, and Phase-Shift—exist. These oscillators employ a negative-gain operational amplifier and a three-section RC network to produce the required 180-degree phase shift, ensuring consistent oscillation.