// stopwatch 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 toggle between running and paused (lap timer) displays.  long-press button to stop stopwatch and reset to 00:00.00
// compile using SDCC: sdcc -mmcs51 --std-c99 stopwatch.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[6] = {5,3,4,2,1,0};
__code unsigned char digitLimit[6] = {9,9,9,5,9,9};

volatile unsigned char running = 0; // flag for stopwatch running
volatile unsigned char paused = 1; // flag for 'frozen' (lap timer) display
volatile unsigned char  pausedDigits[6] = {0, 0, 0, 0, 0, 0};
volatile unsigned char runningDigits[6] = {0, 0, 0, 0, 0, 0};

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

	if (++isrCount == 40) { // 40 * 250us = 0.01s
		isrCount = 0;
	} else if ((isrCount + 5) % 10 == 0) { // multiplex 6-digit display.  one digit per 2500us, so all six at 66.67Hz
		P1 = 0xFF; // all segments off
		P3 |= 0x3F; // no digit selected
		if (++displayDigit == 6) {
			displayDigit = 0;
		}
		P3 &= ~(1 << displayDigit); // select digit

		if (paused) {
			P1 = sevenSeg[pausedDigits[displayDigit]];
		} else { // running
			P1 = sevenSeg[runningDigits[displayDigit]];
		}
	} else if (isrCount == 10 && running) { // run clock
		for (i = 0; i < 6; i++) {
			if (++runningDigits[digitOrder[i]] <= digitLimit[i]) {
				break;
			} else {
				runningDigits[digitOrder[i]] = 0;
			}
		}
	} else if (isrCount == 20 && !paused) { // copy runningDigits to pausedDigits
		for (i = 0; i < 6; i++) {
			pausedDigits[i] = runningDigits[i];
		}
	} else if (isrCount == 30) { // debounce button
		if (P3_7) { // button released
			pressedCount = 0;
		} else { // button pressed
			if (++pressedCount == 1) { // start of press
				if (running) {
					paused = !paused; // toggle between paused display (lap timer) and running display
				} else {
					paused = 0;
					running = 1; // start running on first press after reset
				}
			} else if (pressedCount > 100) { // long press >= 1s
				pressedCount = 100;
				running = paused = 0;
				for (i = 0; i < 6; i++) {
					runningDigits[i] = 0;
				}
			}
		}
	}
}

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;
}