A DC +50V DVM using ADC of PIC16F887 MikroC

Input signal to ADC channel of a microcontroller is other than DC voltage source. It typically range from 0 to 5V DC, as a conventional microcontroller, and voltage reference of ADC module are around +5V. Measuring input voltage within this range is straight forward using a calculation of ADC result and its step voltage size.

A DC +50V DVM using ADC of PIC16F887 MikroC - Program sample

Embedded controller is able to measure input DC voltage exceeding this range - for example measuring an input voltage of +50V DC. From electric circuit theory, voltage divider circuit divides input voltage source to lower output using two resistors.

Reference circuit from Wikipedia

Dividing factor depends on value of these two resistor that we can not give its detail and formula here. Using this method we can design a simple digital volt meter (DVM) with a maximum range of +50V DC.

In this example we select Z1 = 100kOhm and Z2 = 10kOhm. This circuit gives an output dividing factor of 0.09 at Vout. So whenever input voltage Vin = 50V then output voltage 50V * 0.09 that is around 5V, which is the maximum input voltage to ADC input channel.

Circuit Diagram

In this program timer0 is used to schedule ADC reading before it's converted to actual DC input voltage. Using its 8MHz internal oscillator source, timer0 interrupt creates a timer tick for scheduling ADC reading. Whenever the total tick equal to 30 it's a one second timing. Hence ADC reading is activated for every 30 timer0 tick (one second).

However multiplexing display doesn't use this timer tick due total lines of C source code.

MikroC source code:

#define STEP_SIZE 0.0048
#define ONE_SECOND 30
#define RATE 3
#define DIGIT1 PORTA.RA4
#define DIGIT2 PORTA.RA5
#define DIGIT3 PORTA.RA6
#define DIGIT4 PORTA.RA7
 
unsigned ADC_Value,V_Measure;
float  Voltage;
char int_count;
char LED[10]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};
bit  ADC_RUN;
 
void interrupt() {
     if(INTCON.T0IF) {
        int_count++;
        if(int_count==ONE_SECOND)
        { int_count=0; ADC_RUN=1; }
        }
     INTCON.T0IF=0;
}
 
void PORT_SETUP() {
     PORTA=0x00;
     PORTB=0x00;
     TRISA=0x01;   // RA0 IS ANALOG IN PUT
     TRISB=0x00;   // PORTB is OUTPUT
     ANSEL=0x01;   // SELECT AN0
     ANSELH=0x00;   // OTHERS ARE DIGITAL IO
     OSCCON|=0x07;  // SELECT 8MHz INT RC
     }
 
void Timer0_Setup(){
     OPTION_REG.T0CS=0; // SELECT FOSC/4
     OPTION_REG.PSA=0;  // SELECT TIMER0 PRESCALER
     OPTION_REG|=0x07;  // SELECT 1:256 PRESCALER
     }
 
void interrupt_setup() {
     INTCON.GIE=1;
     INTCON.T0IE=1;
     INTCON.T0IF=0;
     }
 
void Read_Temp () {
     if(ADC_RUN=1) {
        ADC_Value=ADC_Get_Sample(0);   // READ AN0
        delay_ms(10);
        Voltage=ADC_Value*STEP_SIZE;  // Convert to Decimal [Volt]
      }
       ADC_RUN=0;
}
 
void SSD() {
     V_Measure=Voltage*10/0.090;  // convert Voltage to integer
 
     PORTB=LED[V_Measure/100];
     DIGIT1=1;
     delay_ms(RATE);
     PORTA=0x00;
 
     PORTB=LED[(V_Measure%100)/10]|0x80;
     DIGIT2=1;
     delay_ms(RATE);
     PORTA=0x00;
 
     PORTB=LED[V_Measure%10];
     DIGIT3=1;
     delay_ms(RATE);
     PORTA=0x00;
 
     PORTB=0x62;
     DIGIT4=1;
     delay_ms(RATE);
     PORTA=0x00;
}
 
void main() {
     ADC_Value=0;
     int_count=0;
     ADC_RUN=0;
     PORT_SETUP();
     ADC_Init();
     Timer0_Setup();
     Interrupt_Setup();
     while(1) {
     Read_Temp();
     SSD();
     }
}

 

Click here to download this example program.