// 12-hour clock code for (6-digit, 7-segment display with AT89C2051 microcontroller, three push buttons, buzzer, and four LED 'colons'
// ceptimus January 2024
// compile using SDCC: sdcc clock12h.c
#include <mcs51/at89x051.h>

/* the common anode, seven segment display is addressed using these bits of P1:
   --2--
  |     |
  1     3
  |     |
   --0--
  |     |
  5     4
  |     |
   --6--
*/

// digits are addressed by P3_x: 4, 0, 5, 2, 1, 3 left to right
// the four 'colon' LEDs are lit by pulling P1.7 low
// the buzzer is sounded by pulling P3.7 low
// the three buttons pull pins to GND when pressed, and are shared with the digit select lines:
// ... left to right: S3:P3.5, S2:P3.4, S1:P3.2

__code unsigned char sevenSeg[22] = { // [10] and [21] are blank digit to replace leading '0'
  0x01, 0x67, 0x12, 0x22, 0x64, 0x28, 0x08, 0x63, 0x00, 0x20, 0x7F, // with colon lit
  0x81, 0xE7, 0x92, 0xA2, 0xE4, 0xA8, 0x88, 0xE3, 0x80, 0xA0, 0xFF  // with colon unlit
};

__code unsigned char digitSelectMask[6] = {0xEF, 0xFE, 0xDF, 0xFB, 0xFD, 0xF7};
__code unsigned char digitLimit[6] = {1, 9, 5, 9, 5, 9};

volatile unsigned char digit[6] = {1, 2, 0, 0, 0, 0}; // start at 12:00 hours
volatile unsigned char buttons = 0xFF;
volatile unsigned char isrCountHigh = 0;

unsigned char brightness = 0; // 0 is full brightness, 1 = 0.25, 2 = 0.50, 3 = 0.75
unsigned char colonMode = 2; // 0 = on, 1 = off, 2 = pulsing

void timer1_isr(void) __interrupt (3) __using (1) { // every 250us
  static unsigned char isrCountLow = 0;
  static unsigned char colonOffset = 0; // zero for lit, eleven for unlit

  static unsigned char displayDigit = 0; // 0 = left (10 hours); 5 = right (1 seconds)
  unsigned char i;

  if ((isrCountLow & 0x03) == brightness) {
    P1 = 0xFF; // all segments, and colon, off
    P3 |= 0x3F; // no digit selected: brightness gives four levels of PWM LED brightness
  }

  if (++isrCountLow & 0xF0) { // counts interrupts 0-15 and wraps
    isrCountLow = 0;
    if (++isrCountHigh == 250) { // 250us * 16 * 250 = 1s
      colonOffset = colonMode ? 0 : 11; // colon lit for first half of each second in colon modes 1 and 2
      isrCountHigh = 0;
      // clock advances one second
      for (i = 5; !(i & 0x80); i--) {
        if (++digit[i] <= digitLimit[i]) {
          break;
        } else {
          digit[i] = 0;
        }
      }
      if (i & 0x80) { // just incremented hour from _9 to 00
        digit[0] = 1; // correct 00 to 10
      } else if (i == 1 && digit[0] == 1 && digit[1] == 3) { // rollover from 12:59 to _1:00
        digit[0] = 10;
        digit[1] = 1;
      }
    } else if (isrCountHigh == 125) { // work 'colon' mask depending on colon mode
      if (colonMode == 2) {
        colonOffset = 11; // switch colon off for last half of each second, in mode 2
      }
    }
  } else if ((isrCountLow & 0x07) == 1) { // every 8 * 250us = 2000us
    // multiplex 6-digit display.  one digit per 2000us, so all six at 83.33Hz
    if (++displayDigit == 6) {
      displayDigit = 0;
      buttons = P3; // sample every 12 ms
    }
    P3 &= digitSelectMask[displayDigit]; // select digit
    P1 = sevenSeg[digit[displayDigit] + colonOffset]; // and display it
  }
}

void main(void) {
  unsigned char ticks; // counts at 250 Hz, when waiting right button up
  unsigned char prevISRcount; // detects passing of one tick when waiting right button up

  TMOD = 0x20; // timer 1 in 8-bit auto-reload mode
  TH1 = 0x06; // count from 0x06 to 0xFF then wrap.  with 12MHz crystal, will interrupt every 250us
  IE = 0x88; // enable timer 1 interrupts
  TCON |= 0x40; // run timer 1

  while (1) {
    if (!(buttons & 0x20)) { // left button
      if (++digit[1] > 9) { // advance hour
        digit[1] = 0;
        ++digit[0];
        if (digit[0] > 10) { // when changing hour from _9, will instantaneously be X0
          digit[0] = 1;  // change to 10
        }
      } else if (digit[0] == 1 && digit[1] >= 3) { // wrap from 12:XX to _1:XX
        digit[0] = 10;
        digit[1] = 1;
      }
      while (!(buttons & 0x20)) continue; // wait button up
    }
    if (!(buttons & 0x10)) { // middle button
      if (++digit[3] > 9) { // advance minute
        digit[3] = 0;
        if (++digit[2] > 5) {
          digit[2] = 0;
        }
      }
      while (!(buttons & 0x10)) { // wait button up
        digit[4] = 0;
        digit[5] = 0; // hold seconds at zero while waiting
        isrCountHigh = 0; // keep internal 'partial seconds' variable at zero (there remains a ~4ms jitter on restart due to isrCountLow)
      }
    }
    if (!(buttons & 0x04)) { // right button
      ++brightness;
      brightness &= 0x03;
      prevISRcount = isrCountHigh;
      ticks = 0;
      while (!(buttons & 0x04)) { // wait button up
        if (isrCountHigh != prevISRcount) { // button has been held for 1/250 sec
          prevISRcount = isrCountHigh;
          if (!++ticks) { // button has been held for 256/250 seconds
            P3_7 = 0; // activate buzzer
            colonMode = colonMode ? --colonMode : 2; // change colon mode
            brightness = 0; // set full brightness when long-pressing to change colon mode
            break; // only one colon mode change per long button press (no auto-repeat)
          }
        }
      }
      while (!(buttons & 0x04)) continue; // wait button up
      P3_7 = 1; // buzzer off
    }
  }
}

/* following 'modeline' configures my text editor, Xed, to use 2-space 'tabs'
   it must be either the first line of the file or in the last three lines
# vi:si:et:sw=2:sts=2:ts=2
*/