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Extract Microcontroller P89C538 Stored Heximal need to receive and transmit the baud rates to be derived from either Timer 1 or Timer 2.

When TCLK= 0, Timer 1 is used as the serial port transmit baud rate generator. When TCLK= 1, Timer 2 is used as the serial port transmit baud rate generator. RCLK has the same effect for the serial port receive baud rate.

With these two bits, the serial port can have different receive and transmit baud rates – one generated by Timer 1, the other by Timer 2.

Below Figure shows the Timer 2 in baud rate generation mode. The baud rate generation mode is like the auto-reload mode,in that a rollover in TH2 causes the Timer 2 registers to be reloaded with the 16-bit value in registers RCAP2H and RCAP2L, which are preset by software after Extract Microcontroller P89C538 Stored Heximal.

Timer 2 in Baud Rate Generator Mode

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 if Extract Microcontroller P89C538 Stored Heximal.

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.