Z80 CTC Timer

;Simple 1 second timer for my Z80 SBC using the Z80 CTC
;Written using ASM80.com
;Code copyright Jacob Hahn, 2019


RAMCELL     EQU     8000h
INTTBL      EQU     1

            .ORG    $0000
            JP      START

            .ORG    $101
            DW      TMRDONE

            .ORG    $1000
TMRDONE:    PUSH    AF
            LD      A,(RAMCELL)
            CP      $06
            JR      Z,END
            INC     A
            LD      (RAMCELL),A
            EI
            POP     AF
            RETI

END:        CALL    GETCHAR1
            DI
            HALT

START:
            LD      SP,0xffff ;set the stack pointer to top of RAM

            ;.INCLUDE initdart.z80 ;initialize DART
            LD      A,00H ; initialize DART
            OUT     ($09),A ; request reg. 0
            LD      A,18H ; Channel reset
            OUT     ($09),A
            NOP
            LD      A,04H ; request reg. 4
            OUT     ($09),A
            LD      A,084H ; x32 clock, 1 stop bit, no parity
            OUT     ($09),A
            LD      A,03H ; request reg. 3
            OUT     ($09),A
            LD      A,0C1H ; 8 bit receiver, RX enable
            OUT     ($09),A
            LD      A,01H
            OUT     ($09),A
            LD      A,08H
            OUT     ($09),A
            LD      A,05H ; request reg. 5
            OUT     ($09),A
            LD      A,068H ; Transmit 8 bits, TX enable
            OUT     ($09),A ; end of DART init

            LD      a,00000011b
            OUT     $13,A ;CH2 doesn't run
            OUT     $1B,A ;CH3 doesn't run
            LD      A,7H ;init CTC ch. 0
            OUT     $03,A
            LD      A,0FAH ;start timer with time constant 0FAH
            OUT     $03,A
            LD      A,0C7H ;init CTC ch. 1
            OUT     $0B,A
            LD      A,0FAH ;load counter with time const. 0FAH
            OUT     $0B,A
            LD      A,00000000b ;int. vector
            OUT     $0B,A
            CALL    GETCHAR
            XOR     A ;0 to A
            LD      (RAMCELL),A
            LD      A,INTTBL
            LD      I,A
            IM      2 ;init interrupts
            EI       ;enable them
WAIT:       LD      A,A
            JR      WAIT

CHKOUT:     PUSH    AF
TXA:        LD      A,$00
            OUT     ($09),A
            IN      A,($09)
            AND     $04
            JR      Z,TXA
            POP     AF
            RET

GETCHAR1:   LD      HL,MSG2
            JP      WR
GETCHAR:    LD      HL,MSG1
WR:         LD      A,(HL)
            OR      A
            JR      Z,DONE
            CALL    CHKOUT
            OUT     ($01),A
            INC     HL
            JR      WR
DONE:       RET

MSG1:       DB      "Timer start",$0D,$00
MSG2:       DB      "Timer done",$0D,$00