;****************************** CTC ******************************
; Program functions:
; 	- initialize the CTC
; 	- implement an interrupt mechanism
; 	- write an interrupt service routine that handles the TASK.
;****************************************************************

.ORG 0000h					



; defineD the hardware addresses of the control port of your four ctc channels.

CH0: 	.equ	0h			; Zero Count / TimeOut 0 output, Clock / Trigger 0 input.
CH1: 	.equ	1h			; ZC/TO1, C/TRG1.
CH2: 	.equ 	2h			; ZC/TO2, C/TRG2.
CH3: 	.equ 	3h			; Clock/Trigger 3 input only (no output).

InitCTC: ; Configure the CTC.
; Init CH 0 and CH1.
		LD  A, 00000011b 	; int off, timer on, prescaler=16, don't care ext. TRG edge,
							; start timer on loading constant, no time constant follows
							; sw­rst active, this is a ctrl cmd
		OUT	(CH0),	A 		; CH0 is on hold now
		OUT	(CH1),	A 		; CH1 is on hold now
		
; Init CH2.
;CH2 divides CPU CLK by (256*256) providing a clock signal at TO2. TO2 is connected to TRG3.
		LD  A,00100111b 	; int off, timer on, prescaler=256, no ext. start,
							; start upon loading time constant, time constant follows
							; sw reset, this is a ctrl cmd
		OUT	(CH2),	A
		LD A,	0FFh 		; time constant 255d defined and loaded into channel 2.
		OUT	(CH2),	A 		; T02 outputs f= CPU_CLK/(256*256)

; Init CH3.
; Input TRG of CH3 is supplied by clock signal from TO2.
; CH3 divides TO2 clock by AFh.
; CH3 interupts CPU appr. every 2sec to service int routine CT3_ZERO (flashes LED)
		LD	A,	11000111b 	; int on, counter on, prescaler don't care, edge don't care,
							; time trigger don't care, time constant follows
							; sw reset, this is a ctrl cmd
		OUT	(CH3),	A
		LD	A,	0AFh 		; time constant AFh defined and loaded into channel 3
		OUT	(CH3),	A 		
		LD	A,	10h 		; it vector defined in bit 7­3,bit 2­1 don't care, bit 0 = 0
		OUT	(CH0),	A 		; and loaded into channel 0


INT_INI:  ; Set up the CPU interrupt mode 2.
		LD	A,	0
		LD	I,	A 			; load I reg with zero
		IM	2 				; set int mode 2
		EI 					; enable interupt

; CT3 zero count routine.
CT3_ZERO:	;flashes LED
		PUSH	AF 			; backup registers A and F
							; now address your periphery that turns the LED on/off e.g. a D­Flip­Flop
		POP	AF 				; restore registers A and F
		EI ;re­enable interrups
		RETI

.ORG	0016h				; CTC interrupt vector table.
		.DW CT3_ZERO		; Every time CTC channel 3 (CH3) count equals zero, an interrupt is triggered 
							; causing the CPU to jump to the memory address specified by the term CT3_ZERO.

.END