Automatic circuit and method for temperature compensation of oscillator frequency variation over temperature for a real time clock chip

摘要

An automatic temperature compensated real-time clock (RTC) chip includes a clock portion having a crystal oscillator block including crystal compensation circuitry adapted to be coupled to a crystal. The crystal compensation circuitry includes a non-linear capacitor DAC including a plurality of load capacitors, wherein the load capacitors have respective switches which switch respective ones of the load capacitors to change a parallel resonance frequency (fp) generated by the oscillator block. The capacitor DAC is arranged so that Analog Trimming (ATR) bits received cause an arrangement of the switches to provide a non-linear change in overall load capacitance to result in a linear relationship between fp and the ATR bits. A temperature sensor block is coupled to the crystal for measuring a temperature of at least the crystal. An A/D converter is coupled to the temperature sensor for outputting a digital temperature signal representative of the temperature of the crystal. A DSP engine receives the digital temperature signal and calculates frequency correction needed to correct for frequency inaccuracy and determines a bit sequence including the ATR bits appropriate to achieve the frequency correction.