P89V51RB2BBC datasheet(33/80 Pages) NXP

33 / 80 page

P89V51RB2_RC2_RD2_5

Product data sheet

Rev. 05 — 12 November 2009

33 of 80

NXP Semiconductors

P89V51RB2/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 fosc / 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 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

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 13 shows Timer 2 counting up automatically (DCEN = 0).

Fig 12. 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

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