Digital panel meter design
MCU-based approach
The flexibility of an MCU-based overcomes many of the disadvantages that are present in an ASIC-based solution. Figure 3 below shows the block diagram of an MCU-based digital panel meter implementation.
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As with an ASIC-based approach, the analog front-end converts / scales the input signal to within the ADC’s range. The microcontroller measures the signal using the ADC (which is integrated in many microcontrollers), applies the desired calibration coefficients and scaling factor, and then outputs the result on the display.
As the MCU is used for post-processing the ADC results and drive the display, this gives more control to developers to apply any scale factor between the input and display. For example, the input to the ADC could be 0 to 2V, and the display could be from 0 to 750 RPM.
Another advantage of an MCU-based approach is the implementation of digital calibration – two-point usually – using a highly accurate signal source and a communication interface (UART, I2C, SPI etc).
A known input at around 10% of the full scale is applied to the meter and the value of the known input is transmitted to the MCU using the communication interface. Another known input at 90% of the full scale value is applied to the instrument, and the value is transmitted to the MCU.
The MCU measures the ADC output for these two known values and calculates the offset and gain errors of the system using the actual input vs. the corresponding ADC result. The MCU then stores these coefficients in non-volatile memory and uses them when making actual measurements.
The advantage of digital calibration is that multiple instruments can be calibrated at the same time. In addition, the whole process may be automated, thus reducing manufacturing time.
Communication protocols like MODBUS can be easily implemented using on-chip UART and external RS-485 transducers. Also, using an internal PWMs, external filter, and V-I converter, a 4 to 20 mA output may be implemented.
Summarizing, the advantages of an MCU-based approach include scaling of the input vs. display may be performed in the digital domain, thus simplifying the analog front end requirement; digital calibration reduces manufacturing time; and a communication protocol like MODBUS may be implemented.
Its disadvantages include that, however simplified it may be, an analog front-end is still required, thus increasing the size and cost. In addition, experience in microcontroller programming is required.
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