Programming and Application of Timer/Counter Mode 3

　　T0 and Tl are very different in mode 3. When T0 works in mode 3, TL0 and TH0 are divided into two independent 8-bit timer/counters. Among them, TL0 can be used as an 8-bit timer/counter, while TH0 can only be used as an 8-bit timer. At this time, T1 can only work in mode 0, 1 or 2.
　　
　　Generally, when timer Tl is used as a serial port baud rate generator, T0 is set to mode 3. At this time, timer Tl is often set to mode 2 and used as a baud rate generator.
　　
　　Assume that two external interrupt sources of an AT89S51 microcontroller application system are occupied, and timer Tl is set to work in mode 2 and used as a baud rate generator. Now it is required to add an external interrupt source and control the P1.0 pin to output a 5 kHz (period is 200μs) square wave. Assume that the system clock is 12 MHz.
　　
　　Basic idea: Set TL0 to work in mode 3 counting mode, the initial value of TL0 is set to 0FFH, when a negative transition is detected on the T0 pin signal, TL0 overflows and requests an interrupt to the CPU at the same time, where the T0 pin is used as an external interrupt request input terminal triggered by a negative transition. In the interrupt handling subroutine, start TH0, which is set to 100 μs timing of mode 3 in advance, thereby controlling P1.0 to output a square wave signal with a period of 200 μs, as shown in the figure below.

      (1) Calculation of initial value X. The initial value of TL0 is set to 0FFH.
　　
　　The period of the 5 kHz square wave is 200 μs, so the timing time of TH0 is 100 μs. The initial value X of TH0 is calculated as follows:
　　

        
　　(2) Program design. The reference program is as follows: