P89LV51RB2_RC2_RD2_5 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 05 — 15 December 2009 33 of 76
NXP Semiconductors
P89LV51RB2/RC2/RD2
8-bit microcontrollers with 80C51 core
This bit can be used to generate an interrupt (by enabling the Timer 2 interrupt bit in the
IEN0 register). If EXEN2 = 1, Timer 2 operates as described above, but with the added
feature that a 1-to-0 transition at external input T2EX causes the current value in the
Timer 2 registers, TL2 and TH2, to be captured into registers RCAP2L and RCAP2H,
respectively.
In addition, the transition at T2EX causes bit EXF2 in T2CON to be set, and EXF2 like
TF2 can generate an interrupt (which vectors to the same location as Timer 2 overflow
interrupt). The Timer 2 interrupt service routine can interrogate TF2 and EXF2 to
determine which event caused the interrupt.
There is no reload value for TL2 and TH2 in this mode. Even when a capture event occurs
from T2EX, the counter keeps on counting T2 pin transitions or f
osc
/ 6 pulses. Since once
loaded contents of RCAP2L and RCAP2H registers are not protected, once Timer2
interrupt is signalled it has to be serviced before a new capture event on T2EX pin occurs.
Otherwise, the next falling edge on T2EX pin will initiate reload of the current value from
TL2 and TH2 to RCAP2L and RCAP2H and consequently corrupt their content related to
the previously reported interrupt.
6.5.2 Auto-reload mode (up or down counter)
In the 16-bit auto-reload mode, Timer 2 can be configured as either a timer or counter (via
C/T2 in T2CON), then programmed to count up or down. The counting direction is
determined by bit DCEN (Down Counter Enable) which is located in the T2MOD register
(see Table 22 and Table 23). When reset is applied, DCEN = 0 and Timer 2 will default to
counting up. If the DCEN bit is set, Timer 2 can count up or down depending on the value
of the T2EX pin.
Figure 12 shows Timer 2 counting up automatically (DCEN = 0).
Fig 11. Timer 2 in Capture mode
002aaa523
OSC ÷6
T2 pin
C/T2 = 0
C/T2 = 1
TL2
(8-bits)
TH2
(8-bits)
TF2
control
capture
TR2
timer 2
interrupt
EXF2
RCAP2L RCAP2H
control
EXEN2
transition
detector
T2EX pin
