;PROGRAM: MidKck.src  M.Whitco, CET  02-07-98
;  Routine for PIC 16C56 based midi trigger unit, to play a kick drum note on a tone
;generator or synth (selectable note value, midi channel 10, with variable velocity).
;  Accumulates a byte of information (to become 3rd byte transmitted), which is inversely
;proportional to the length of time that a SPDT switch is between contacts (ie velocity).
;  Timing begins with switch leaving initial contacts, and ends when final contacts are
;closed. Transmission then begins.
;  Transmits a 3-byte string of serial (midi) data at 31.25 Kbaud (Asynchronous 8-N-1), and
;using inverted logic levels (output current on = logic 0, per midi specs).. Baud rate
;determined by bit_K and the clock rate of the PIC.

;Constants & input/output assignments
bit_K		=	13		;For 31,250-baud operation @ 8 MHz clock rate
serial_out	=	ra.2		;Serial data output line (pin 1)
switch_up	=	ra.0		;Start of switch travel (pin 17 to NC contacts)
switch_dn	=	ra.1		;End of switch travel (pin 18 to NO contacts)
note		=	rb		;Input for note selection (via DIP switch)

;Storage space for variables above special-purpose file registers ("define space")
			org	8	;Start assigning at register 08

delay_cntr	ds	1		;Register for serial delay routine (holds bit_K)
bit_cntr	ds	1		;Number of transmitted bits
msg_cntr	ds	1		;Byte pointer, increment after each byte sent
xmt_byte	ds	1		;The transmitted byte is stored here
vel_byte	ds	1		;Register for storing velocity value accumulated
pause		ds	1		;Delay time for switch debouncing (also used to
					;slow decrementing rate of velocity value)

;Org 0 starts program at the top of ROM.
			org	0

			device	pic16c56,hs_osc,wdt_off,protect_off
			reset	start

start		mov	!ra,#00000011b		;Assign pins 17 & 18 as inputs, pin 1 output
		mov	!rb,#11111111b		;set up as input for DIP switch
		mov	vel_byte,#127		;Start with highest velocity, for 0 time
		mov	msg_cntr,#0		;Message string initialized to 1st byte of 3

;****************************************************************************
;PROCESS SENSOR SWITCH
			call	debounce
:loop			jb	switch_up,timer		;If switch is logic high, continue
			jmp	:loop

timer			mov	pause,#255		;This line sets delay time
:loop			nop
			djnz	pause,:loop		;Remain in loop until zero
			jnb	switch_dn,send		;or switch reaches bottom
			djnz	vel_byte,timer		;Decrement & re-check if not 0

;***************************************************************************
;TRANSMISSION BEGINS, now that the velocity byte is collected

send
:again			mov	bit_cntr,#8		;Eight bits in a byte
			call	datastor		;Get the next byte from storage

			setb	serial_out		;Send start_bit
			call	bit_delay		;Wait 1 bit period (32 uSec)

:xmit			rr	xmt_byte	;Rotate bits of byte being sent, into carry

			movb	serial_out,/c		;Send a bit out
			call	bit_delay		;Wait 1 bit period (32 uSec)
			djnz	bit_cntr,:xmit		;Not 8 bits yet? Send next bit

			clrb	serial_out		;Send stop bit
			call	bit_delay		;Wait 1 bit period (32 uSec)
			inc	msg_cntr		;Add 1 to byte pointer(next byte)

			cjbe	msg_cntr,#2,:again	;More bytes to send? Then repeat.

:loop			jnb	switch_up,over
			jmp	:loop			;Stay here until pedal is up

over			jmp	start			;Restart the program (next kick)
;***************************************************************************
;SUBROUTINES

;TRANSMISSION BIT DURATION
bit_delay		mov	delay_cntr,#bit_K	;Put the baud rate constant into
:loop			nop				;the register, to prepare for
			djnz	delay_cntr,:loop	;midi data transmission
			ret

;BYTE VALUES TO BE TRANSMITTED
datastor
			cje	msg_cntr,#0,byte1	;Check counter, to see which
			cje	msg_cntr,#1,byte2	;byte is needed next. Then jump
			cje	msg_cntr,#2,byte3	;below, to store that byte.

byte1			mov	xmt_byte,#99h		;Note on, midi channel 10
			ret
byte2			mov	xmt_byte,note		;Note value = DIP switch setting
			ret
byte3			mov	xmt_byte,vel_byte	;Velocity variable
			ret

;DEBOUNCE ROUTINE
debounce		mov	pause,#64		;Create delay for intermittent
:loop			nop				;contacts to become fully
			djnz	pause,:loop		;debounced
			ret