Semiconductor device including a decimation filter that performs predetermined filtering

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.