Intel 8XC251SB Timer 1 Generated Baud Rates Modes 1, Selecting Timer 1 as the Baud Rate Generator

SERIAL I/O PORT

10.6.3.1Timer 1 Generated Baud Rates (Modes 1 and 3)

Timer 1 is the default baud rate generator for the transmitter and the receiver in modes 1 and 3. The baud rate is determined by the timer 1 overflow rate and the value of SMOD, as shown in the following formula:

SMOD1 × Timer 1 Overflow Rate

Serial I/O Modes 1 and 3 Baud Rate = 2 -----------------------------------------------------------

32

10.6.3.2Selecting Timer 1 as the Baud Rate Generator

To select timer 1 as the baud rate generator:

•Disable the timer interrupt by clearing the ETI bit in the IE0 register (Figure 6-2 on page 6-6).

•Configure timer 1 as a timer or an event counter (set or clear the C/T# bit in the TMOD register, Figure 8-5 on page 8-7).

•Select timer mode 0–3 by programming the M1, M0 bits in the TMOD register.

In most applications, timer 1 is configured as a timer in auto-reload mode (high nibble of TMOD = 0010B). The resulting baud rate is defined by the following expression:

Serial I/O Modes 1 and 3 Baud Rate = 2	SMOD1	×		FOSC
			32----------	×-----12--------×-----[---256-----------–----(---TH1-------------)---]

Timer 1 can generate very low baud rates with the following setup:

•Enable the timer 1 interrupt by setting the ET1 bit in the IE register.

•Configure timer 1 to run as a 16-bit timer (high nibble of TMOD = 0001B).

•Use the timer 1 interrupt to initiate a 16-bit software reload.

Table 10-4 lists commonly used baud rates and shows how they are generated by timer 1.

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