Try these :

Option 1

; CNVASCI converts the lower nibble in accumulator to ASCI equivalent
; and returns Acc with converted value. ( Not my idea but got it from the net- dunno where... )

CNVASCI: ANL A, #00FH ; Keep Only Low bits
ADD A, #090H ; Add 144
DA A
ADDC A, #040H ; Add 64
DA A
RET

Option 2

; CNVASCI converts the lower nibble in accumulator to ASCI equivalent
; and returns Acc with converted value.

CNVASCI: MOV B, A ; Save Acc
CLR C
SUBB A, #0AH ; Find out if its 0-9 or A-F
JNC ADD37
MOV A, B
ADD A, #30H
RET
ADD37: MOV A, B
ADD A, #37H
RET

Raghu.