51 Baud Rate Generator Timer Initial Value Calculation Method

In mode 0 and mode 2, the serial port baud rate is fixed, mode 0 is Fosc/12, mode 2 is Fosc/32 or Fosc/64, which is determined by the SMOD bit of the PCON register.

In mode 1 and mode 3, the baud rate is a variable value. The baud rate can be generated by timer 1 (8052 can be generated by timer 2). So what is the baud rate? The baud rate is determined by the overflow rate of timer 1:
Baud Rate = (2SMOD/32)*(Timer 1 Overflow Rate)
Note: 2SMOD Here SMOD is the exponent, SMOD is the baud rate double baud rate bit, which is in the PCON register.
When using timer 1 as a baud rate generator, it is usually necessary to configure timer 1 to the 8-bit auto-reload mode, and at the same time disable timer 1 interrupts. So how is the overflow rate of timer 1 calculated? The overflow rate is the overflow frequency. Assuming the crystal frequency is 12MHz and the TH1 value is 0xFE, it only takes two clock pulses to overflow, and the overflow period is 2us, so the overflow frequency is 500KHz. According to the timer section, the timing time is calculated as follows:
Timing time = (maximum count value - counter initial value) * machine cycle = (maximum count value - counter initial value) * (12/crystal frequency (Hz)) (s) = (256-TH1) * (12/OSC_FREQ (Hz)) (s).
Then the overflow frequency is naturally:
OSC_FREQ (Hz) / ((256-TH1) * 12).
So the final baud rate is:
(2SMOD / 32) * (OSC_FREQ (Hz) / ((256-TH1) * 12)).

Now that we have the formula for calculating the baud rate, we can naturally deduce the initial value of TH1 based on the baud rate:
BAUD_RATE=(2SMOD/32)*(OSC_FREQ(Hz)/((256-TH1)*12))
32*12*(256-TH1)=(2SMOD*OSC_FREQ(Hz))/BAUD_RATE
256-TH1=(2SMOD*OSC_FREQ(Hz))/(BAUD_RATE*32*12)
TH1=256-(2SMOD*OSC_FREQ(Hz))/(BAUD_RATE*32*12)
Assuming the crystal frequency is 11.0592MHz, the baud rate is 9600, and SMOD=0, what should the initial value of TH1 be? According to the above formula, we can calculate that TH1=256-11059200/(9600*32*12)=0xFD.