Hello all,

I'm having a slight problem getting a Dallas Semiconductor Real Time Clock (DS12887) working with an 8051 (T89C51RD2 from Atmel).

I have the clock and a HD44780U compatible display connected up to port 0, along with a keypad to port 1 as follows:

Display:



and the keypad/clock



I know that using the 74C922 isn't really necessary but I have a bucket load of them here so I might as well. Both the keypad and the display are working perfectly (also the other ports will be occupied in the final design so they are not available).
As far as I can gather from reading the datasheet for the DS12887, reading and writing to it is the same as to external RAM.

In the following code I'm trying to write a 1 into the seconds register of the clock (address 00H) when '1' is pressed on the keypad and when any other button is pressed the value of this register is read and displayed on the display. (I have a led connected to pin 3.4 just to show that the routine to write to the register is being executed).
When I try reading the the value of the register (after writing to it) I get the value "EE" written to the display. No matter what I do all I read is "EE". I've tried everything I can think of to get this working with no success so I'm not sure if there is a problem with my write code, my read code, both or just the logic behind the whole program. Any suggestions will be greatly appreciated.


Code:

DB0 EQU P0.0
DB1 EQU P0.1
DB2 EQU P0.2
DB3 EQU P0.3
DB4 EQU P0.4
DB5 EQU P0.5
DB6 EQU P0.6
DB7 EQU P0.7
EN EQU P3.7
RS EQU P3.6
RW EQU P3.5
DATA0 EQU P0
PORT_PIN_LED EQU P3.4
ENABLE_LCD EQU P1.4
FLASH EQU 07FH


CSEG

ORG 0000H
LJMP MAIN

ORG 0013H
LJMP INT1_ISR

ORG 0048H

MAIN:
SETB EX1
SETB EA
MOV A, 8EH ; Needed to access off chip external ram
ORL A, #3H
MOV 8EH, A
SETB ENABLE_LCD
LCALL INIT_LCD
CLR ENABLE_LCD
SETB FLASH
lpl1: JB FLASH, lpl1
CPL PORT_PIN_LED
LCALL delay
JMP lpl1

delay:
MOV R7, #200
MOV R5, #20
dly1: MOV R6, #229
dly2: DJNZ R6, dly2
DJNZ R7, dly1
DJNZ R5, dly1
RET



WAIT_LCD:
PUSH ACC
not_ready:
SETB EN
CLR RS
SETB RW
MOV DATA0,#0FFH
MOV A,DATA0
JB ACC.7,not_ready
CLR EN
CLR RW
POP ACC
RET


INIT_LCD:
SETB EN
CLR RS
MOV DATA0,#038H
CLR EN
LCALL WAIT_LCD
SETB EN
CLR RS
MOV DATA0,#038H
CLR EN
LCALL WAIT_LCD
SETB EN
CLR RS
MOV DATA0,#0Eh
CLR EN
LCALL WAIT_LCD
SETB EN
CLR RS
MOV DATA0,#06h
CLR EN
LCALL WAIT_LCD
RET


CLEAR_LCD:
SETB EN
CLR RS
MOV DATA0,#01H
CLR EN
LCALL WAIT_LCD
RET


WRITE_TEXT:
SETB EN
SETB RS
MOV DATA0,A
CLR EN
LCALL WAIT_LCD
RET

INT1_ISR:
PUSH PSW
PUSH ACC
PUSH 00H ;R0
PUSH 02H ;R2
PUSH 03H ;R3
MOV A, P1
ANL A, #0FH
DEC A
JZ one
MOV R0, #0
MOVX A, @R0
MOV R3, A
SETB FLASH
JMP write
one:
CLR ENABLE_LCD
MOV A, #1
MOV R0, #0
MOVX @R0, A
CLR FLASH
JMP finish
write:
SETB ENABLE_LCD
LCALL CLEAR_LCD
MOV A, R3
ANL A, #0FH
CJNE A, #0AH, next1
MOV A, #41H
JMP write1
next1: CJNE A, #0BH, next2
MOV A, #42H
JMP write1
next2: CJNE A, #0CH, next3
MOV A, #42H
JMP write1
next3: CJNE A, #0DH, next4
MOV A, #43H
JMP write1
next4: CJNE A, #0EH, next5
MOV A, #44H
JMP write1
next5: CJNE A, #0FH, digit1
MOV A, #45H
JMP write1
digit1: ADD A, #30H
write1: LCALL WRITE_TEXT
MOV A, R3
ANL A, #0F0H
SWAP A
CJNE A, #0AH, next6
MOV A, #41H
JMP write2
next6: CJNE A, #0BH, next7
MOV A, #42H
JMP write2
next7: CJNE A, #0CH, next8
MOV A, #42H
JMP write2
next8: CJNE A, #0DH, next9
MOV A, #43H
JMP write2
next9: CJNE A, #0EH, next10
MOV A, #44H
JMP write2
next10: CJNE A, #0FH, digit2
MOV A, #45H
JMP write2
digit2: ADD A, #30H
write2: LCALL WRITE_TEXT
CLR ENABLE_LCD
finish: POP 03H ; R3
POP 02H ; R2
POP 00H ; R0
POP ACC
POP PSW
RETI

END