P89CV51RB2_09 datasheet(33/76 Pages) NXP

33 / 76 page

P89CV51RB2_RC2_RD2_3

Product data sheet

Rev. 03 — 25 August 2009

33 of 76

NXP Semiconductors

P89CV51RB2/RC2/RD2

80C51 with 1 kB RAM, SPI

In this mode, there are two options selected by bit EXEN2 in T2CON register. If

EXEN2 = 0, then Timer 2 counts up to 0FFFFH and sets the TF2 (overﬂow ﬂag) bit upon

overﬂow. This causes the Timer 2 registers to be reloaded with the 16-bit value in

RCAP2L and RCAP2H. The values in RCAP2L and RCAP2H are preset by software

means.

Auto reload frequency when Timer 2 is counting up can be determined from Equation 1:

(1)

Where SupplyFrequency is either fosc (C/T2 = 0) or frequency of signal on T2 pin

(C/T2 = 1).

If EXEN2 = 1, a 16-bit reload can be triggered either by an overﬂow or by a 1-to-0

transition at input T2EX. This transition also sets the EXF2 bit. The Timer 2 interrupt, if

enabled, can be generated when either TF2 or EXF2 is 1.

The microcontroller’s hardware will need three consecutive machine cycles in order to

recognize falling edge on T2EX and set EXF2 = 1: in the ﬁrst machine cycle pin T2EX has

to be sampled as 1; in the second machine cycle it has to be sampled as 0, and in the

third machine cycle EXF2 will be set to 1.

In Figure 13, DCEN = 1 and Timer 2 is enabled to count up or down. This mode allows pin

T2EX to control the direction of count. 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 timer overﬂow also causes the 16-bit value in

RCAP2L and RCAP2H to be reloaded into the timer registers TL2 and TH2.

When a logic 0 is applied at pin T2EX this causes Timer 2 to count down. The timer will

underﬂow when TL2 and TH2 become equal to the value stored in RCAP2L and

RCAP2H. Timer 2 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.

SupplyFrequency

65536

RCAP2H RCAP2L

()

–

----------------------------------------------------------------------------

Fig 13. Timer 2 in Auto reload mode (DCEN = 1)

002aaa525

TL2

(8-bits)

TH2

(8-bits)

TF2

EXF2

underflow

timer 2

interrupt

RCAP2L RCAP2H

FFH

overflow

(down-counting reload value)

(up-counting reload value)

count direction

1 = up

0 = down

T2EX pin

toggle

OSC

÷6

T2 pin

C/T2 = 0

C/T2 = 1

control

TR2

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