// 24-hour clock code for SH-E 879 clock kit board (6-digit, 7-segment display with AT89C2051 microcontroller and single push button)
// ceptimus May 2022
// short-press button to increment currently selected (flashing) field or exit to normal (clock running) mode when seconds '00' is flashing
// long-press button change to next field, or enter clock-setting mode from running mode
// fields are selected in the order: units minutes, tens minutes, units hours, tens hours, seconds ('00')
// compile using SDCC: sdcc -mmcs51 --std-c99 clock24h.c
#include <mcs51/at89x051.h>

/* contrary to the circuit diagram included with the SH-E 879 kit, the seven segment display is addressed using these bits of P1:
   --0--
  |     |
  2     1
  |     |
   --3--
  |     |
  5     6
  |     |
   --4--
*/

__code unsigned char sevenSeg[10] = {0x88, 0xBD, 0xC4, 0xA4, 0xB1, 0xA2, 0x82, 0xBC, 0x80, 0xA0};
// digits are addressed in slightly quirky order by P3_x: 0, 1, 2, 4, 3, 5 left to right
__code unsigned char digitOrder[4] = {5,3,4,2};
__code unsigned char digitLimit[4] = {9,5,9,5};

volatile unsigned char running = 0; // flag for clock running
volatile unsigned char runningDigits[6] = {1, 2, 0, 0, 0, 0}; // start setting at 12:00 hours
volatile unsigned char hour = 12;
volatile unsigned char selected = 0;

void timer1_isr(void) __interrupt (3) __using (1) { // every 250us
	static unsigned char isrCountLow = 16;
	static unsigned char isrCountHigh = 250; // 250us * 16 * 250 = 1s
	static unsigned char displayDigit = 0;
	static unsigned char pressedCount = 0; // debounce and long-press detect for pushbutton
	unsigned char i;

	if (--isrCountLow == 0) { // every 16 * 250us = 4000us
		isrCountLow = 16;
		if (--isrCountHigh == 0) {
			isrCountHigh = 250;
		}
	} else if ((isrCountLow & 0x07) == 4) { // every 8 * 250us = 2000us
		// multiplex 6-digit display.  one digit per 2000us, so all six at 83.33Hz
		P1 = 0xFF; // all segments off
		P3 |= 0x3F; // no digit selected
		if (++displayDigit == 6) {
			displayDigit = 0;
		}
		P3 &= ~(1 << displayDigit); // select digit

		i = sevenSeg[runningDigits[displayDigit]];
		// flash selected digit(s) when in setting mode
		if (!running && isrCountHigh < 125) {
			if (selected == 4 && (displayDigit == 3 || displayDigit == 5)) { // seconds
				i = 0xFF;
			} else if (selected == 0 && displayDigit == 4) { // units minutes
				i = 0xFF;
			} else if (selected == 1 && displayDigit == 2) { // tens minutes
				i = 0xFF;
			} else if (selected == 2 && displayDigit == 1) { // units hours
				i = 0xFF;
			} else if (selected == 3 && displayDigit == 0) { // tens hours
				i = 0xFF;
			}
		}
		P1 = i;
	} else if (running && isrCountHigh == 1 && isrCountLow == 1) { // increment clock once per second
		for (i = 0; i < 4; i++) {
			if (++runningDigits[digitOrder[i]] <= digitLimit[i]) {
				break;
			} else {
				runningDigits[digitOrder[i]] = 0;
				if (i == 3) { // next hour
					if (++hour > 23) {
						hour = 0;
					}
					if (hour > 19) {
						runningDigits[0] = 2;
						runningDigits[1] = hour - 20;
					} else if (hour > 9) {
						runningDigits[0] = 1;
						runningDigits[1] = hour - 10;
					} else {
						runningDigits[0] = 0;
						runningDigits[1] = hour;
					}
				}
			}
		}
	} else if (isrCountLow == 8) { // debounce button every 4ms (but on a different interrupt to the display multiplex)
		if (P3_7) { // button released
			if (!running && pressedCount && pressedCount < 250) { // it was a short press
				if (selected == 4) { // seconds was selected
					isrCountLow = 16;
					isrCountHigh = 250;
					running = 1; // back to running mode
				} else if (selected == 0) { // units minutes
					if (++runningDigits[4] > 9) {
						runningDigits[4] = 0;
					}
				} else if (selected == 1) { // tens minutes
					if (++runningDigits[2] > 5) {
						runningDigits[2] = 0;
					}
				} else if (selected == 2) { // units hours
					if (++runningDigits[1] > 9) {
						runningDigits[1] = 0;
					}
					if (runningDigits[0] == 2 && runningDigits[1] > 3) {
						runningDigits[0] = 0;
					}
					hour = runningDigits[0] * 10 + runningDigits[1];
				} else if (selected == 3) { // tens hours
					if (++runningDigits[0] > 2) {
						runningDigits[0] = 0;
					}
					if (runningDigits[0] == 2 && runningDigits[1] > 3) {
						runningDigits[1] = 3;
					}
					hour = runningDigits[0] * 10 + runningDigits[1];
				}
			}
			pressedCount = 0;
		} else { // button pressed
			if (++pressedCount >= 250) {
				if (pressedCount == 250) { // long press
					if (running) {
						selected = 0;
						runningDigits[3] = runningDigits[5] = 0;
						running = 0;
					} else if (++selected > 4) {
						selected = 0;
					}
				}
				pressedCount = 251;
			}
		}
	}
}

void main(void) {
	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) continue;
}