P89LV51RB2_RC2_RD2_5 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 05 — 15 December 2009 36 of 76
NXP Semiconductors
P89LV51RB2/RC2/RD2
8-bit microcontrollers with 80C51 core
TCLK = 1, Timer 2 is used as the UART transmit baud rate generator. RCLK has the same
effect for the UART receive baud rate. With these two bits, the serial port can have
different receive and transmit baud rates – Timer 1 or Timer 2.
Figure 14 shows Timer 2 in baud rate generator mode.
The baud rate generation mode is like the auto-reload mode, when 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 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
⁄
6
the oscillator
frequency). As a baud rate generator, it increments at the oscillator frequency. Thus the
baud rate formula is as follows:
Modes 1 and 3 baud rates =
(3)
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 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
Fig 14. Timer 2 in Baud Rate Generator mode
002aaa526
TX/RX baud rate
timer 2
interrupt
OSC ÷2
T2 pin
C/T2 = 0
C/T2 = 1
TL2
(8-bits)
TH2
(8-bits)
control
TR2
EXF2
RCAP2L RCAP2H
control
EXEN2
transition
detector
T2EX pin
reload
Oscillator Frequency
16 65536 RCAP2H RCAP2L,()–()×()
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