keytable: subroutine 82-89
	input = scancode of key ( = row * 4 + column + 1) in keycode
	output = ASCII code of key

eep_read: subroutine 91-110
	read the code from EEPROM to RAM
	the actual code is read into the RAM @ address cod

eep_write: subroutine 112-147
	write the code into EEPROM from RAM
	the actual code is written into the EEPROM from RAM address cod

udelay: subroutine 149-161
	input = delay length in 100 usecs

	delay main program execution for the given time
	delay constant is calculated from the defined mhz variable
	for the actual frequency

beep: subroutine 163-172
	input = length of the beep
	generate a beep on the PB3 output pin

keyscan: subroutine 174-206
	output = ASCII code of a pressed key or 0 if no key is found

	scan the keyboard for a pressed key

	keycode = 0, rowcnt = 4
	select row 0 (pull PA0 pin low, float PA1, PA2, PA3)
rowscan: scan a selected keyboard row for a pressed key
	delay 100 ms (debounce delay & charge row)
	read PB4-PB7 pins as column inputs from the row into colstatus
	colcnt = 4
colscan: keycode = keycode + 1 (keycode is the scancode of the actual key checked)
	rotate right colstatus bits
	if the lowest bit was 0, a pressed key is found: exit with sub keytable
	otherwise colcnt = colcnt - 1
	if colcnt > 0, goto label colscan
	otherwise select next row (pull the according PA# pin low, float others)
	rowcnt = rowcnt - 1
	if rowcnt > 0, goto label rowscan
	return with 0: no key found pressed

main: 208-217
	program execution starts here
	setup PORTA, PORTB pin states

	PORTA pins are outputs, PORTB0-3 are outputs
	PORTB4-7 are inputs with internal pullup resistors

warm: 219-221
	generate a beep, indicating that the program is running
	call subroutine eep_read to read the actual code from EEPROM

loop: 223-
	program main loop
	clear all PORTB pins
	call subroutine read to read a code from the keyboard
	call subroutine compbuf to compare the code read in with the actual code
	if combuf returned with * as last character entered, goto codechange
	otherwise
pulseout: send out a pulse on PB2 pin. Use the defined pulsewidth variable as
	delay in 20 ms steps. Go back to loop when finished

codechange: setup PB1 pin to indicate code change on the LED
	call subroutine read to read the new code from the keyboard
	copy read in code into the RAM storage area of the actual code
	call subroutine read to read the new code again from the keyboard
	call subroutine compuf to compare the code read in with the previous one
	if the new code was not correctly entered twice, goto label warm
	(this will reset the actual code in RAM from the EEPROM and beep to indicate
	the code change was not successful)
	otherwise call subroutine eep_write to write the new code into EEPROM
	and go back to loop

read:	subroutine 266-351
	read a sequence of keys with length clen into the RAM buffer @ readbuf
	only the last clen characters are stored, previous characters are lost.
	go sleep when the keyboard is inactive. The * or # key is used to terminate
	the sequence. If pressed and at least clen characters were entered, the
	subroutine returns. Otherwise, all entered characters are cleared and a new
	read starts.

compbuf: subroutine 353-378
	input = RAM address of buffer to compare with
	output = Z, status of match

	compare the read buffer with the given buffer, set flag Z

copybuf: subroutine 380-401
	input = RAM address of buffer to copy to

	copy the read buffer to the given buffer