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

A method for temperature compensating a real time clock (RTC) circuit, comprising the steps of:

• providing a real-time clock (RTC) having a crystal oscillator block comprising a crystal coupled to crystal compensation circuitry, said crystal compensation circuitry comprising a non-linear Capacitor DAC having a plurality of load capacitors having respective switches which switch respective ones of said load capacitors to change a parallel resonance frequency (fp) generated by said oscillator block, wherein said capacitor DAC is arranged so that Analog Trimming (ATR) bits received cause an arrangement of said switches to provide a non-linear change in overall load capacitance resulting in a linear relationship between said fp and said ATR bits;
• measuring a temperature of said crystal;
• generating an input code comprising said ATR bits based on said temperature to correct said fp, and
• frequency correcting by applying said input code as said input code vectors to said Capacitor DAC to provide a frequency shift to temperature correct an oscillator frequency of said crystal oscillator.

An automatic temperature compensated real-time clock (RTC) chip, comprising:

• a clock portion including a crystal oscillator block comprising crystal compensation circuitry adapted to be coupled to a crystal, said crystal compensation circuitry comprising a non-linear capacitor DAC, wherein said non-linear capacitor DAC is arranged so that Analog Trimming (ATR) bits received cause a non-linear change in overall load capacitance to result in a linear relationship between said ATR bits and a parallel resonance frequency (fp) generated by said oscillator block;
• a temperature sensor block coupled to said crystal for measuring a temperature of at least said crystal;
• an A/D converter coupled to said temperature sensor for outputting a digital temperature signal representative of said temperature of said crystal, and
• a DSP engine for receiving said digital temperature signal, said DSP engine calculating frequency correction needed to correct for frequency inaccuracy and determining a bit sequence comprising said ATR bits to achieve said frequency correction.