The Design Of Temperature Compensating Circuit For High Accuracy Real Time Clock

High-accuracy real-time clock(RTC)modules have an important applications in the Internet of things,wireless sensors,smart meters and other fields.A 32.768 kHz crystal oscillator is used in RTC as frequency source.The temperature dependence of the 32 kHz quartz crystal in RTC varies by ±100-±200 ppm from-40? to 85?.Aiming at this problem,this thesis designs a kind of temperature compensation circuit.The temperature compensation circuit includes temperature sensor and digital compensation circuit in two parts.The core of the temperature sensor is a RC oscillator based on a thermistor resistor which senses the temperature.A counter is used to convert the frequency of the RC oscillator into a digital output.In order to improve the accuracy of the temperature sensor,a low-temperature-drift linear regulator is used to power the RC oscillator,moreover,a high DC gain self-bias op is used to reduce the impact of offset voltage in the RC oscillator.The digital compensation circuit calculates the frequency offset of the crystal oscillator and calibrates it according to the output of the temperature sensor.There are two way to calibrate the cystal oscillator:absolute deviation compensation and relative deviation compensation.In this thesis,the compensation accuracy,resolution,jitter and other aspects of the two methods are compared.The accuracy and resolution of absolute deviation compensation are relatively high,while the jitter is relatively high.In this thesis,the jitter of absolute deviation compensation is improved by using the high-frequency RC oscillator in the temperature sensor to calibrate the second signal.Because of the low frequency of digital compensation circuit,the DFF,INV,NOR2,NAND2 and so on used in the compensation circuit are customized to reduce the leakage power by using the PMOS and NMOS with high threshold voltage.In this thesis,the design and simulation of the temperature compensation circuit are completed under the GF180nm process.The total area of the circuit is 0.28mm2.Simulation results show that the sensor draws 19?A from a 1.5V to3,6V supply,and achieves a resolution of 28mK in a conversation time of 1ms.After a 3-point calibration,the temperature sensor achieves an inaccuracy of less than ±0.6?(min-max)from-40? to 85 ?.This translates into a frequency stability of better than ±2 ppm when the sensor is used to compensate the crystal oscillator.