The DMM using the MSP432 utilizes the ADC, Comparator and TimerA peripherals. The ADC is configured to return a 32 bit value. TimerA is configured to Capture Mode, which is used to capture the period. Pin 5.4 is configured to act as the ADC, while Pin 2.5 is configured to act as the TimerA.
Pin 5.2 behaves as a voltmeter, measuring voltages between 0-3.3V. From the measurements, the program can determine the maximum and minimum values. The maximum and minimum values can also be used to calculate that RMS voltage of a sine wave. It is important to note that this method of calculating RMS only works for sine waves. Pin 2.5 is able to capture the period by determining the time difference between peak values of an oscillating wave. The program cannot measure voltage and frequency/period at the same time due to the fact that the ADC and TimerA are on the same pin.
Sometimes, the amplitude of the signal is not high enough to trigger the Timer on P2.5. TimerA requires a signal of 3.3V to trigger the counting process for calculating frequency and period. To solve this, a Comparator is implemented to amplify the signal into a square wave that has a 3.3V Peak to Peak voltage. The signal goes to the Comparator input on P8.0 and the square wave is created on output P7.1. This signal is now able to trigger the Timer. A jumper is used to connect P7.1 to P2.5.
Upon determining the frequency and voltage measurements using the MSP432 peripherals and interrupts, the program sends the information via UART protocol RS232 with baud rate of 115.3kilo-bits per second. The data sent to the transfer buffer is echoed on the receive buffer which is what we see. Voltage measurements are displayed in mV and frequency is displayed in Hz.