STC12LE5616AD datasheet(144/377 Pages) STC

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In the above assembly language solution, five distinct sections are the interrupt vector loactions, the main

program, and the three interrupt service routines. All vector loacations contain LJMP instructions to the respective

routines. The main program, starting at code address 0030H, contains only four instructions. SETB IT0 configures

the door sensing interrupt input as negative-edge triggered. MOV TMOD, #11H configures both timers for mode 1,

16-bit timer mode. Only the external 0 interrupt is enabled initially (MOV IE,#81H), so a "door-open" condition

is needed before any interrupt is accepted. Finally, SJMP $ puts the main program in a do-nothing loop.

When a door-open condition is sensed (by a high-to-low transition of INT0), an external 0 interrupt is

generated, INT0INT begins by putting the constant 20 in R7, then sets the overflow flags for both timers to force

timer interrupts to occur.

Timer interrupt will only occur, however, if the respective bits are enabled in the IE register. The next two

instructions (SETB ET0 and SETB ET1) enable timer interrupts. Finally, INT0INT terminates with a RETI to the

main program.

Timer 0 creates the 1 second timeout, and Timer 1 creates the 400Hz tone. After INT0INT returns to the main

program, timer interrupt are immediately generated (and accepted after one excution of SJMP $). Because of the

fixed polling sequence, the Timer 0 interrupt is serviced first. A 1 second timeout is created by programming 20

repetitions of a 50,000 us timeout. R7 serves as the counter. Nineteen times out of 20, T0INT operates as follows.

First, Timer 0 is turned off and R7 is decremented. Then, TH0/TL is reload with -50,000, the timer is turned back

on, and the interrupt is terminated. On the 20th Timer 0 interrupt, R7 is decremented to 0 (1 second has elapsed).

Both timer interrupts are disabled(CLR ET0, CLR ET1)and the interrupt is terminated. No further timer interrupts

will be generated until the next "door-open" condition is sensed.

The 400Hz tone is programmed using Timer 1 interrupts, 400Hz requires a period of 1/400 = 2,500 us or

1,250 high-time and 1,250 us low-time. Each timer 1 ISR simply puts -1250 in TH1/TL1, complements the port

bit driving the loudspeaker, then terminates.

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