1) as you said:-
"almost forgot:what kind of assembler are you using that permit this:

CJNE A,#50H,NOT ? ,you can't use a reserved word as a label! "
Answer:-
Iam using X51 and ran the program many times but it did not show any error so is the assembler working ok or
there is some problem.


2) as you said:-
"by the way...look your code... I didn't do the math, but I think that you have done wrong the square wave timing and the 20s timer.

and assembly code without comments should be punished with a scourge... "

Answer:-
you are right i will use comments when ever i will post here again....next the maths for 400Hz and 20 seconds...

Time = 1/400Hz = 0.0025 seconds. x
1000000 = 2500micro-seconds. = 2500/2

1250 times ON and 1250 Times OFF should create 400Hz Square Wave.

using timer 0 in 16 bit Mode.

65536 (Max. value 16 bit timer can over flow) - 1250 = 64286

converting 64286 into hex = FB1E

so FB is the value to be loaded in TH0 and 1E is the value for TL0.

each time timer over flows P2.0 is complemented.

Now the above process will generate 400Hz Sqaure Wave..rite ?

Next is generating the same sound for 20 seconds :-

For that i have used DPTR increment command using the following maths:-

timer over flow at = 1250
DPH (inner Loop) = 200 times
DPL (outer loop) = 80 times.

20 seconds time = {1250*(200*80) }/ 1000000 = 20 seconds.

please indicate if there is any problem in above maths ?

Now i want to make the program without using the second timer i mean that 400Hz tone upto 20 seconds using only one timer.


program on simulator AVS51 still show that only 7.89 seconds has passed
i think there is something i have to do with using DPH and DPL in a proper way which at this moment iam not understanding..

ORG 00H
LJMP MAIN

ORG 000BH ;timer 0 vector address
JMP TOISR


MAIN:
MOV TMOD,#01B ;timer 0 in 16 bit mode
MOV DPH,#0 ;inner loop
MOV DPL,#0 ;outer loop
JNB P1.0,$ ;program waiting to detect telephone off hook
SETB TF0 ;forcing timer 0 interrupt
SETB EA ;enable global interrupt
SETB ET0 ;enable timer 0 interrupt


TOISR:
CLR TR0
MOV TH0,#FBH ;count 400hz
MOV TL0,#1EH
CPL P2.0 ;DIAL tone Sqaure Wave
SETB TR0
MOV A,DPH
CJNE A,#C8H,DO_NOTHING ;inner loop will run 200 times
MOV A,DPL
CJNE A,#50H,DO_NOTHING ;out loop will run 80 times

DO_NOTHING:
SJMP $
END