AP160

Version 0.0

10

AMIC Technology, Inc.

Baud Rate Generator

Timer 2 is selected as the baud rate generator by setting TCLK and/or RCLK in T2CON (Table 2). Note that the baud rates

for transmit and receive can be different if Timer 2 is used for the receiver or transmitter and Timer 1 is used for the other

function. Setting RCLK and/or TCLK puts Timer 2 into its baud rate generator mode, as shown in Figure 4. The baud rate

generator mode is similar to 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. The baud rates in modes 1 and 3 are

determined by Timer 2’s overflow rate according to the following equation.

Mode 1 and 3 Baud Rates =

16

2

Rate

Overflow

Timer

The Timer can be configured for either timer or counter operation. In most applications, it is configured for timer operation

(CP/T2=0). The Timer operation is different for Timer 2 when it is used as a baud rate generator. Normally, as a timer, it

increments every machine cycle (at 1/12 the oscillator frequency). As a baud rate generator, however, it increments every

state time (at 1/2 the oscillator frequency). The baud rate formula is given below.

)]

2

,

2

(

65536

[

32

3

1

L

RCAP

H

RCAP

Frequency

Oscillator

Rate

Baud

and

Modes

−

×

=

where (RCAP2H,RCAP2L) is the content of RCAP2H and RCAP2L taken as a 16-bit unsigned integer.

Timer 2 as a baud rate generator is shown in Figure 4. This figure is valid only if RCLK or TCLK=1 in T2CON. Note that a

rollover in TH2 does not ser TF2 and will not generate an interrupt. Note too, that if EXEN2 is set, a 1-to-0 transition in T2EX

will set EXF2 but will not cause a reload from (RCAP2H, RCAP2L) to (TH2,TL2). Thus when timer 2 is in use as a baud rate

generator, T2EX can be used as an extra external interrupt. Note that when Timer 2 is running (TR2=1) as a timer in the

baud rate generator mode. TH2 or TL2 should not be read from or written to. Under there conditions, the Timer is

incremented every state time, and the results of a read or write 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 register.

TIMER 2

INTERRUPT

OSC

T2 PIN

CONTROL

TH2

TL2

RCAP2H

RCAP2L

EXF2

T2EX PIN

EXEN2

TR2

CONTROL

TRANSITION

DETECTOR

RX

CLOCK

TX

CLOCK

"1"

"1"

NOTE:OSC FREQ. IS DIVIDED BY 2, NOT 12

"1"

"0"

TIMER 1 OVERFLOW

"0"

SMOD1

RCLK

TCLK

"0"

C/T2=0

C/T2=1

2

÷

16

÷

16

÷

2

÷

Figure 4. Timer 2 in Baud Rate Generator Mode