/* details of the program: * use AT89S51 microcontroller to produce "di, di,..." The alarm sound is output from P3.2 port, generating frequency of 1KHz. * 1KHZ square wave outputs 0.2 seconds from P3.2, followed by 0.2 seconds from P3.2 output level signal, so that it can be circulated to form the alarm sound we need. * * programming methods * (1. In life we often go to all kinds of alarm sounds, such as "di, di,..." It's the usual kind of voice call, * but for the sound of the alarm, for 0.2 seconds, then for 0.2 seconds, the cycle goes on, assuming that the frequency of the beep is 1KHz, * because of the above signal, we divide the above signal into two parts, one of which is 1KHZ square wave, * take up to 0.2 seconds; The other part is the level, which also takes 0.2 seconds. Therefore, we use the timing/counter T0 of the SCM to be the timing. * can be timed for 0.2 seconds; At the same time, it is also needed to generate 1KHZ square wave with single chip microcomputer, and 1KHZ square wave signal period is 1ms, * the high level takes up 0.5ms and the low level occupies 0.5 ms, so the timer T0 is also used to complete the time of 0.5 ms. * finally, the timing time of the time/counter T0 can be selected as 0.5 ms, while the timing 0.2 seconds is 400 times as much as 0.5 ms. * this means that a time of 0.2 seconds will be reached 400 times per half ms time. * /

didi.c

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