/* ATtiny85 Bargraph Voltmeter

   David Johnson-Davies - www.technoblogy.com - 20th December 2015
   ATtiny85 @ 1 MHz (internal oscillator; BOD disabled)
   
   CC BY 4.0
   Licensed under a Creative Commons Attribution 4.0 International license: 
   http://creativecommons.org/licenses/by/4.0/
*/

volatile int Display = 0;

// Display **********************************************

int Line (int n) { return (1 << n) >> 1; }

int Bar (int n) { return ((signed int) 0x8000 >> (15-n))^0xFFFF; }

// Interrupt **********************************************

volatile int Row = 0;

// Interrupt multiplexes display
ISR(TIMER0_COMPA_vect) {
  int Data;
  Row = (Row + 1) % 5;
  DDRB = 0;  // Make all pins inputs
  if (Row == 0) Data = (Display & 0xC0)>>3 | (Display & 0x20)>>4;
  else if (Row == 1) Data = Display & 0x18;
  else if (Row == 3) Data = Display>>8 & 0x03;
  else if (Row == 4) Data = (Display & 0x03) | (Display & 0x04)<<1;
  else { Display = Bar(ReadADC()/100); return; }  // When Row=2 read ADC
  PORTB = (PORTB | Data) & ~(1<<Row);    // Take row low
  DDRB = DDRB | Data | 1<<Row;           // Make row an output
}

// Analogue to Digital **********************************

unsigned int ReadADC() {
  unsigned char low,high;
  ADCSRA = ADCSRA | 1<<ADSC;             // Start
  do ; while (ADCSRA & 1<<ADSC);         // Wait while conversion in progress
  low = ADCL;
  high = ADCH;
  return (high<<8 | low);
}

// Setup **********************************************
  
void setup() {
  // Set up Timer/Counter0 to generate 625Hz interrupt
  TCCR0A = 2<<WGM00;                     // CTC mode
  TCCR0B = 3<<CS00;                      // /64 prescaler
  OCR0A = 24;                            // 625 Hz interrupt
  TIMSK = TIMSK | 1<<OCIE0A;             // Enable interrupt
  // Set up ADC
  ADMUX = 2<<REFS0 | 1<<REFS2 | 1<<MUX0; // Internal 2.56V ref, ADC1 (PB2)
  ADCSRA = 1<<ADEN | 4<<ADPS0;           // Enable, 62.5kHz ADC clock
}

void loop() {
}