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The baud rates in modes 1 and 3 are determined by Timer 2’s overflow rate given below:
Modes 1 and 3 Baud Rates= (Timer 2 Overflow Rate)/16
The timer can be configured for either “timer” or “counter” operation. In many applications, it is configured for “timer” operation (C/T2*=0). Timer operation is different for Timer 2 when it is being used as a baud rate generator.
Usually, as a timer it would increment every machine cycle (i.e., 1/12 the oscillator frequency). As a baud rate generator, it increments every state time (i.e., 1/2 the oscillator frequency).
Thus the baud rate formula is as follows:
Modes 1 and 3 Baud Rates =
Oscillator Frequency
[32 [65536(RCAP2H,RCAP2L)]]
Where: (RCAP2H, RCAP2L) = The content of RCAP2H and RCAP2L taken as a 16-bit unsigned integer.
The Timer 2 as a baud rate generator mode shown in Figure 6, is valid only if RCLK and/or TCLK = 1 in T2CON register. Note that a rollover in TH2 does not set TF2, and will not generate an interrupt. Thus, the Timer 2 interrupt does not have to be disabled when Timer 2 is in the baud rate generator mode. Also if the EXEN2 (T2 external enable flag) is set, a 1-to-0 transition in T2EX
(Timer/counter 2 trigger input) will set EXF2 (T2 external flag) but will not cause a reload from (RCAP2H, RCAP2L) to (TH2,TL2). Therefore when Timer 2 is in use as a baud rate generator, T2EX can be used as an additional external interrupt, if needed.
When Timer 2 is in the baud rate generator mode, one should not try to read or write TH2 and TL2. As a baud rate generator, Timer 2 is incremented every state time (osc/2) or asynchronously from pin T2; under these conditions, a read or write of TH2 or TL2 may not be accurate. The RCAP2 registers may be read, but should not be written to, because a write might overlap a reload and cause write and/or reload errors. The timer should be turned off (clear TR2) before accessing the Timer 2 or RCAP2 registers.
Summary Of Baud Rate Equations
Timer 2 is in baud rate generating mode. If Timer 2 is being clocked through pin T2(P1.0) the baud rate is:
Baud Rate =
Timer 2 Overflow Rate
16
If Timer 2 is being clocked internally , the baud rate is:
Baud Rate =
Fosc
[32 [65536(RCAP2H,RCAP2L)]]
Where fosc= Oscillator Frequency
To obtain the reload value for RCAP2H and RCAP2L, the above equation can be rewritten as:
RCAP2H,RCAP2L = 65536—fosc/（32*Baud Rate）
Timer/Counter 2 Set-up
Except for the baud rate generator mode, the values given for T2CON do not include the setting of the TR2 bit. Therefore, bit TR2 must be set, separately, to turn the timer on. see Table 5 for set-up of Timer 2 as a timer. Also see Table 6 for set-up of Timer 2 as a counter.
Enhanced UART
When used for framing error detect the UART looks for missing stopbits in the communication. A missing bit will set the FE bit in theSCON register. The FE bit shares the SCON.7 bit with SM0 and thefunction of SCON.7 is determined by PCON.6 (SMOD0) (seeFigure 7). If SMOD0 is set then SCON.7 functions as FE. SCON.7functions as SM0 when SMOD0 is cleared. When used as FESCON.7 can only be cleared by software. Refer to Figure 8.
Automatic Address Recognition
Automatic Address Recognition is a feature which allows the UART torecognize certain addresses in the serial bit stream by using hardwareto make the comparisons. This feature saves a great deal of software overhead by eliminating the need for the software to examine every serial address which passes by the serial port. This feature is enabled by setting the SM2 bit in SCON. In the 9 bit UART modes, mode 2 and mode 3, the Receive Interrupt flag (RI) will be automatically set when the received byte contains either the “Given” address or the“Broadcast” address. The 9 bit mode requires that the 9th information bit is a 1 to indicate that the received information is an address and not data.