Measuring an RC Time Constant with an ATmega8

By Prabeesh Keezhathra

- 3 minutes read - 500 words

The time constant of an RC circuit equals R * C (in seconds). It is the time for the capacitor to charge through the resistor to 63.2% of the supply voltage, or to discharge to 36.8% of its starting level.

Approach

The ATmega8’s ADC samples the voltage across the capacitor continuously. A digital output pin (PB0) supplies the input voltage to the RC network. The moment PB0 goes high, Timer1 starts counting. When the ADC reading hits 63.2% of the rail (roughly 161 out of 255 for a 5 V supply), we capture the timer value.

Timer1 is a 16-bit counter, clocked at 8 MHz with a 1024 prescaler. To convert timer ticks to seconds, multiply by 1024 / 8000000. An LCD shows both the raw tick count and the ADC reading.

Test results

RCCalculated RCTimer ticksMeasured RC
1 kOhm100 uF0.100 s8460.108 s
2 kOhm100 uF0.200 s18640.239 s

Both within ~20% of the ideal value, which is typical for low-precision components.

Code

  1
  2#include<avr/io.h>
  3#define F_CPU 8000000UL
  4#include <util/delay.h>
  5
  6#define RS 6 //PD6
  7#define EN 7 //PD7
  8#define databits PORTC //PC0 to PC3
  9#define row1 cmd(0x80)
 10#define row2 cmd(0xc0)
 11
 12void adc_init()
 13{
 14   //select AVCC reference voltage , left alignment of data and ADC4
 15   ADMUX=((1<<REFS0)|(1<<ADLAR)|(1<<MUX2));
 16
 17   //enable ADC, set prescaler to divide the clock by 64 and auto triggering mode
 18   ADCSRA=((1<<ADEN)|(1<<ADFR)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0));
 19
 20}
 21
 22void conversion()
 23{
 24   //start conversion
 25   ADCSRA|=(1<<ADSC);
 26}
 27
 28void port_init()
 29{
 30   DDRC = 0xcf;
 31   DDRD = (1 << RS)|(1 << EN);
 32}
 33
 34void LCD_STROBE(void)
 35{
 36   PORTD |= (1 << EN);
 37   _delay_us(1);
 38   PORTD &= ~(1 << EN);
 39}
 40
 41void data(unsigned char c)
 42{
 43   PORTD |= (1 << RS);
 44   _delay_us(50);
 45   databits = (c >> 4);
 46   LCD_STROBE();
 47   databits = (c);
 48   LCD_STROBE();
 49}
 50
 51void cmd(unsigned char c)
 52{
 53   PORTD &= ~(1 << RS);
 54   _delay_us(50);
 55   databits = (c >> 4);
 56   LCD_STROBE();
 57   databits = (c);
 58   LCD_STROBE();
 59}
 60
 61void clear(void)
 62{
 63   cmd(0x01);
 64   _delay_ms(5);
 65}
 66
 67void lcd_init()
 68{
 69   _delay_ms(15);
 70   cmd(0x30);
 71   _delay_ms(1);
 72   cmd(0x30);
 73   _delay_us(100);
 74   cmd(0x30);
 75   cmd(0x28); // Function set (4-bit interface, 2 lines, 5*7Pixels)
 76   cmd(0x28); // Function set (4-bit interface, 2 lines, 5*7Pixels)
 77   cmd(0x0c); // Make cursorinvisible
 78   clear(); // Clear screen
 79   cmd(0x6); // Set entry Mode(auto increment of cursor)
 80}
 81
 82void print(char *p)
 83{
 84   while(*p) data(*p++);
 85}
 86
 87void main()
 88{
 89   char a[5],b[5];
 90   int c,d; 
 91   DDRB=0x01;
 92   _delay_ms(50);
 93   TCCR1B|=(1<<CS10)|(1<<CS12);//prescale 1024
 94   port_init();
 95   adc_init();
 96   lcd_init();
 97   PORTB=0x01;//applying vcc to RC circuit
 98   TCNT1=0x00;//start the timer
 99
100   while(1)
101   { 
102      conversion();
103
104      while(!( ADIF));
105
106      if(ADCH==161)//63% of the input voltage
107      { 
108         c=TCNT1; 
109         d=ADCH;
110         itoa(c,a,10);//integer to ASCII
111         itoa(d,b,10);//integer to ASCII
112         row1; 
113         print(a);//showing timer value
114         row2;
115         print(b);//showing adc value
116      }
117   } 
118}

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