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When an external clock input is used for Timer0, it must meet certain requirements. The external clock requirement is due to internal phase clock (TOSC) synchronization when Crack MCU.
Also, there is a delay in the actual incrementing of Timer0 after synchronization. When no prescaler is used, the external clock input is the same as the prescaler output if Crack MCU.
The synchronization of T0CKI with the internal phase clocks is accomplished by sampling the prescaler output on the Q2 and Q4 cycles of the internal phase clocks (Figure 6-4) when Crack MCU.
Therefore, it is necessary for T0CKI to be high for at least 2 TOSC (and a small RC delay of 2 Tt0H) and low for at least 2 TOSC (and a small RC delay of 2 Tt0H) after Crack MCU.
Refer to the electrical specification of the desired device. When a prescaler is used, the external clock input is divided by the asynchronous ripple counter-type prescaler, so that the prescaler output is symmetrical before Crack MCU.
For the external clock to meet the sampling requirement, the ripple counter must be taken into account. Therefore, it is necessary for T0CKI to have a period of at least 4 TOSC (and a small RC delay of 4 Tt0H) divided by the prescaler value after Crack MCU.
The only requirement on T0CKI high and low time is that they do not violate the minimum pulse width requirement of Tt0H. Refer to parameters 40, 41 and 42 in the electrical specification of the desired device when Crack MCU.
Since the prescaler output is synchronized with the internal clocks, there is a small delay from the time the external clock edge occurs to the time the Timer0 module is actually incremented. Figure 6-4 shows the delay from the external clock edge to the timer incrementing if Crack MCU.
An 8-bit counter is available as a prescaler for the Timer0 module or as a postscaler for the Watchdog Timer (WDT), respectively (see Section 9.6 “Watch-dog Timer (WDT)”). For simplicity, this counter is being referred to as “prescaler” throughout this data sheet before Crack MCU.
The PSA and PS<2:0> bits (OPTION<3:0>) determine prescaler assignment and prescale ratio. When assigned to the Timer0 module, all instructions writing to the TMR0 register (e.g., CLRF 1, MOVWF 1, BSF 1,x, etc.) will clear the prescaler after Crack MCU.
When assigned to WDT, a CLRWDT instruction will clear the prescaler along with the WDT. The prescaler is neither readable nor writable. On a Reset, the prescaler contains all ‘0’s if Crack MCU.
The prescaler assignment is fully under software control (i.e., it can be changed “on-the-fly” during program execution). To avoid an unintended device Reset, the following instruction sequence (Example 6-1) must be executed when changing the prescaler assignment from Timer0 to the WDT when Crack MCU.
To change the prescaler from the WDT to the Timer0 module, use the sequence shown in Example 6-2. This sequence must be used even if the WDT is disabled. A CLRWDT instruction should be executed before switching the prescaler when Crack MCU.