Semiconductor device including a decimation filter that performs predetermined filtering

Abstract

An electric-power meter device includes a sensor which detects an electric energy and which outputs an analog signal with 50 Hz to 60 Hz, and a microcomputer which includes an analog-to-digital converter which, based on a delta-sigma modulation scheme, converts the input analog signal provided from the sensor to a digital signal based on a required sampling rate to output the digital signal, a central processing unit that performs data processing based on the digital signal converted by the analog-to-digital converter, and a timer unit that outputs a trigger signal in a cycle corresponding to the required sampling rate. The analog-to-digital converter includes a delta-sigma modulator and a decimation filter.

What is claimed is: 
     
         1 . An electric-power meter device comprising:
 a sensor which detects an electric energy and which outputs an analog signal with 50 Hz to 60 Hz; and   a microcomputer comprising:
 an analog-to-digital converter which, based on a delta-sigma modulation scheme, converts the input analog signal provided from the sensor to a digital signal based on a required sampling rate to output the digital signal; 
   a central processing unit that performs data processing based on the digital signal converted by the analog-to-digital converter; and   a timer unit that outputs a trigger signal in a cycle corresponding to the required sampling rate,   wherein the analog-to-digital converter includes:
 a delta-sigma modulator that over-samples the analog signal at a predetermined sampling rate and carries out a delta sigma modulation; and 
 a decimation filter that sequentially inputs the signal delta-sigma modulated by the delta-sigma modulator and performs predetermined filtering processing on an input signal, and furthermore converts the resulting signal to a digital signal based on a required sampling rate to output the digital signal, 
   wherein the decimation filter calculates, for the each input signal that is input within a predetermined period, a filter coefficient for performing predetermined filtering processing in response to a trigger signal continuously output from the timer unit, and furthermore sequentially multiplies the input signal by the calculated filter coefficient, accumulates multiplication value within the predetermined period, and sequentially outputs the resulting value,   wherein the predetermined period is made variable in accordance with a time interval in which the trigger signal is output,   wherein the decimation filter includes:
 a coefficient calculation unit which, in response to the trigger signal, sequentially calculates the filter coefficient every time the input signal is input; and 
 an arithmetic unit which, every time the input signal is input, multiplies the input signal by the filter coefficient calculated by the coefficient calculation unit, and accumulates this multiplication value to output the resulting value, 
   wherein the coefficient calculation unit, in a first period after the trigger signal is applied, sequentially calculates the filter coefficient so that a value increases in accordance with a number of times when the input signal is input after the trigger signal is applied, while in a second period after the trigger signal is applied, the coefficient calculation unit sequentially calculates the filter coefficient so that the value decreases in accordance with a number of times when the input signal is input after the trigger signal is applied,   wherein the arithmetic unit sequentially outputs a value obtained by accumulating the multiplication from the time when the first period is started to the time when the second period ends, and   wherein the first period and the second period are each a period that is shorter than a cycle corresponding to the required sampling rate and is a predetermined period.   
     
     
         2 . The electric-power meter device according to  claim 1 ,
 wherein the coefficient calculation unit, in a third period until the next trigger signal is output after the first period elapses, calculates the filter coefficient to serve as a fixed value.   
     
     
         3 . The electric-power meter device according to  claim 2 ,
 wherein the coefficient calculation unit, in the second period, calculates the filter coefficient by sequentially subtracting a value of the filter coefficient calculated in the previous first period from a value of the filter coefficient calculated in the third period, every time the input signal is input.   
     
     
         4 . The electric-power meter device according to  claim 2 ,
 wherein the decimation filter further includes a counter unit which, in response to the trigger signal, counts a clock signal that is generated in response to a timing at which the input signal is input,   wherein the counter unit starts a count operation for counting the clock signal when the trigger signal is output, interrupts the count operation when a count value by the count operation coincides with a first value, and resumes the count operation when the next trigger signal is input, and stops the count operation when the count value coincides with a second value larger than the first value, and   wherein the coefficient calculation unit sets a period from the time when the trigger signal is output to the time when the count value coincides with the first value, to the first period, sets a period during which the count operation is interrupted to a third period, and sets a period from the time when the next trigger signal is output to the time when the count value coincides with the second value, to the second period, and calculates the filter coefficient, with the count value as the number of times of input.   
     
     
         5 . The electric-power meter device according to  claim 1 ,
 wherein the decimation filter includes two sets of filter processing sections, each being one set of the coefficient calculation unit and the arithmetic unit, and a selection unit that inputs signals output from the calculation units in the filter processing section, respectively, and selects either one of the signals as an output signal of the decimation filter to output this signal,   wherein the respective filter processing sections complementarily operate in response to output of the trigger signal, and   wherein the selection unit, at a time interval based on the cycle in which the trigger signal is output, alternately selects the signals output from the respective arithmetic units to output the signal.   
     
     
         6 . The electric-power meter device according to  claim 1 ,
 wherein the predetermined period is made variable by the decimation filter in accordance with the time interval at which the trigger signal is applied.   
     
     
         7 . The electric-power meter device according to  claim 1 ,
 wherein the coefficient calculation unit sets the first period and the second period that are each a period that is shorter than a cycle corresponding to the required sampling rate and is a predetermined period.