STC89LE52RC datasheet(102/165 Pages) STC

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The following figure shows DCEN=1, which enables Timer 2 to count up or down. This mode allows pin T2EX

to control the counting direction. When a logic 1 is applied at pin T2EX, Timer 2 will count up. Timer 2 will

overflow at 0FFFFH and set the TF2 flag, which can then generate an interrupt if the interrupt is enabled. This

overflow also causes the 16-bit value in RCAP2L and RCAP2H to be reloaded into the timer registers TL2 and

TH2. A logic 0 applied to pin T2EX causes Timer 2 to count down. The timer will underflow when TL2 and TH2

become equal to the value stored in RCAP2L and RCAP2H. This underflow sets the TF2 flag and causes 0FFFFH

to be reloaded into the timer registers TL2 and TH2.

The external flag EXF2 toggles when Timer 2 underflows or overflows. This EXF2 bit can be used as a 17th bit

of resolution if needed. The EXF2 flag does not generate an interrupt in this mode.

Timer 2

Interrupt

SYSclk

TF2

control

C/T2=0

C/T2=1

T2 Pin

T2EX Pin

MCU in 12T mode

MCU in 6T mode

TL2

(8 Bits)

RCAP2L

Timer 2 in Auto-Reload Mode (DCEN=1)

÷12

÷6

TR2

TH2

(8 Bits)

RCAP2H

EXF2

FFH

Toggle

Count Direction

1=UP

0=DOWN

Overflow

Down Counting Reload Value

Up Counting Reload Value

Buad-Rate Generator Mode

Timer2 can be configured to generate various baud-rate. Bit TCLK and/or RCLK in T2CON allow the serial port

transmit and receive baud rates to be derived from either Timer1 or Timer2. When TCLK=0, Timer1 is used as

the serial port transmit baud rate generator. When TCLK=1, Timer2 is used as the serial port transmit baud rate

generator. RCLK has the same effect for the serial port baud rate. With these two bits, the serial port can have dif-

ferent receive and transmit baud rates – one generated from Timer 1 and the other from Timer 2.

In BRG mode, Timers is operated very like auto-reload up-only mode except that the T2EX pin cannot control re-

load. An overflow on Timer 2 will load RCAP2H, RCAP2L contents onto Timer2, but TF2 will not be set. A 1-to-0

transition on P2EX pin can set EXF2 to request interrupt service if EXEN2=1.

The following figure shows the Timer 2 in baud rate generation mode to generate RX Clock and TX Clock into

UART engine. 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.

The baud rate in UART Mode 1 and Mode 3 are determined by Timer2’s overflow rate given below:

Baud Rate=

Timer 2 overflow rate

(counting T2EX)

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

configured for "timer" operation(C/T2=0)."Timer" operation is a little different for Timer 2 when it's being used

as a baud rate generator. Normally, as timer it would increment every machine clcye(thus at 1/6 or 1/12 the

system clock).In that case the baud rate is given bu the formalu:

Baud Rate=

SYSclk

n×[65536 - (RCAP2H, RCAP2L)]

(as timer)

when MCU in 12T mode, n=32; When MCU in 6T mode, n=16.

102

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