This is the code developed (in a hurry) for an introductory electric circuits laboratory class. The goal was to use a FRDM-KL25Z board and its microcontroller, connected to a LCD screen module, creating a multimeter that should measure frequency and voltage for AC inputs and only voltage for DC inputs. It uses the [mbed library] (https://os.mbed…

ac_dc_multimeterMain.cpp

#include "mbed.h"
#include "Timer.h"
#include "math.h"
#include "TextLCD.h"


#define CONVERSAO(x) (3.3*x)

TextLCD lcd(PTA13, PTD5, PTC9, PTC8, PTA5, PTA4);
AnalogIn medida(PTB0);
AnalogIn switchMode(PTB1);
Timer timer;
Serial PC (USBTX, USBRX);

int main ()
{
    float max = 0.0, min = 0.0, max_parcial, min_parcial;            //Variavel da medida AC
    int i = 0, j = 0, l = 0, k = 0;                                         //
    float amostraInicial, referencia;                                //
    int periodoParcial;                                              //
    float frequencia = 0.0;                                          //
    int t2 = 0, t1 = 0;                                              //
    float v2, vRMS = 0.0;                                            //Variavel da medida AC
    float periodo;
    float media = 0;                                                 //Variavel da medida DC
    float tensaoMedida = 0;         //Variavel da medida DC

    while(1) {
        
        while(3.3*switchMode.read() < 1.0) {

            frequencia = periodo = vRMS = v2 = amostraInicial = referencia = max = min = max_parcial = min_parcial = 0.0;
            periodoParcial = i = j = 0;
            t2 = t1 = 0;
            
            while(i < 20 && switchMode.read()*3.3 < 1.0) {
                max_parcial = min_parcial = medida.read();
                timer.start();
                while(timer.read() < 0.03) {

                    if(medida.read() > max_parcial) {
                        max_parcial = medida.read();

                    }
                    if(medida.read() < min_parcial) {
                        min_parcial = medida.read();

                    }
                }
                timer.stop();

                timer.reset();
                max += max_parcial;
                min += min_parcial;
                i++;
            }

            max = max/20.0;
            min = min/20.0;
            
            max = max*3.3/0.3;//valor do ganho correto
            min = min*3.3/0.3;
            
            referencia = (max+min)/(2.0);
            
            referencia = referencia - 5.53;
            max = max - 5.53;//valor do offset
            min = min - 5.53;
        
            
            amostraInicial = medida.read();

            while(j < 10 && switchMode.read()*3.3 < 1.0) { //Medida da frequencia
                j++;
                timer.stop();
                timer.reset();
                
                if((amostraInicial*3.3/0.3) - 5.53 > referencia) {
                    while((medida.read()*3.3/0.3) - 5.53 > referencia) {}
                    timer.start();
                    while((medida.read()*3.3/0.3) - 5.53 < referencia) {}
                    periodoParcial = timer.read_us();
                }

                else {
                    while((medida.read()*3.3/0.3) - 5.53 < referencia) {}
                    timer.start();
                    while((medida.read()*3.3/0.3) - 5.53 > referencia) {}
                    periodoParcial = timer.read_us();
                }
                
                periodo += periodoParcial;

            }
            
            if(switchMode.read()*3.3 > 1.0) break;
            
            periodo = 2*periodo/10;
            
            frequencia = (1.0/periodo)*1000000.0;
/**************************************************************************************/
//            Analise da frequencia
            k = j = 0;
                       
            if(frequencia <= 1050)
                k = 1000;
                
            if(frequencia <= 750)     
                k = 600;             
                 
            if(frequencia <= 500)
                k = 200;
                
            if(frequencia <= 250)
                k  = 5;            
            

            while(j < k && switchMode.read()*3.3 < 1.0) { //Medida da frequencia
                j++;
                timer.stop();
                timer.reset();
                
                amostraInicial = medida.read();
                
                if((amostraInicial*3.3/0.3) - 5.53 > referencia) {
                    while((medida.read()*3.3/0.3) - 5.53 > referencia) {}
                    timer.start();
                    while((medida.read()*3.3/0.3) - 5.53 < referencia) {}
                    periodoParcial = timer.read_us();
                }

                else {
                    while((medida.read()*3.3/0.3) - 5.53 < referencia) {}
                    timer.start();
                    while((medida.read()*3.3/0.3) - 5.53 > referencia) {}
                    periodoParcial = timer.read_us();
                }
                
                periodo += periodoParcial;

            }
            
            if(switchMode.read()*3.3 > 1.0) break;
            
            periodo = 2*periodo/(k+1);
            
            frequencia = (1.0/periodo)*1000000.0;          
                        
            /*****************************************************/

            timer.stop();
            timer.reset();
            timer.start();

            while((float)timer.read_us() < 50.0 * periodo) {   //Integraçao para obter a tensao RMS
                v2 = ((medida.read()*3.3) - 1.65)/0.3;
                t2 = timer.read_us();
                vRMS += (v2)*(v2)*(t2-t1)*0.000001;
                t1 = t2;
            }

            vRMS = sqrt(frequencia*vRMS/50);

            //referencia = referencia - 5.53;
            periodo = periodo/1000000;
    
            if(switchMode.read()*3.3 > 1.0) break;
            lcd.cls();
            lcd.locate(0,0);
            lcd.printf("Freq: %.2f", frequencia);
            lcd.locate(0,1);
            lcd .printf("Vrms: %.2f", vRMS);
            wait(1);
        }


//****************************************************************************************
//******MODO DC***************************************************************************

        media = 0;

        for(l = 0; l < 1000; l++) {
            media = media + CONVERSAO(medida.read());
        }

        media = media/0.3;
        media = media /1000;
        tensaoMedida = (media - 0.0004952)/1.00246;

        if(tensaoMedida <= 0.02) {
            lcd.cls();
            lcd.locate(0,0);
            lcd.printf("Tensao baixa");
            lcd.locate(0,1);
            lcd.printf("inferior a 20mV");
        }

        else {
            lcd.cls();
            lcd.locate(0,0);
            lcd.printf("VDC: %.3f", tensaoMedida);
        }
        wait(0.05);
    }
    return 0;
}