P89CV51RB2FBC datasheet(33/73 Pages) NXP

33 / 73 page

P89CV51RB2_RC2_RD2_1

Product data sheet

Rev. 01 — 5 October 2007

33 of 73

NXP Semiconductors

P89CV51RB2/RC2/RD2

80C51 with 1 kB RAM, SPI

The Baud rate generator mode is like the Auto-reload mode, when a roll-over 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 at 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 shown in Equation 3:

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 in Baud rate generator mode is valid only if RCLK and/or TCLK = 1 in T2CON

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 not cause a

reload from (RCAP2H, RCAP2L) to (TH2, TL2). Therefore when Timer 2 is used as a

baud rate generator, T2EX can be used as an additional external interrupt, if needed.

When Timer 2 is in the Baud rate generator mode, one should not try to read or write TH2

and TL2. Under these conditions, a read or write of TH2 or TL2 may not be accurate. The

RCAP2 registers may be read, but should not be written to, because a write might overlap

a reload and cause write and/or reload errors. The timer should be turned off (clear TR2)

before accessing the Timer 2 or RCAP2 registers. Table 23 shows commonly used baud

rates and how they can be obtained from Timer 2.

6.5.5 Summary of baud rate equations

Timer 2 is in Baud rate generator mode: if Timer 2 is being clocked through pin T2 (P1[0])

the baud rate is:

Baud rate = Timer 2 overﬂow rate / 16

If Timer 2 is being clocked internally, the baud rate is:

Baud rate = fosc / (16 × (65536 − (RCAP2H, RCAP2L)))

Where fosc = oscillator frequency

To obtain the reload value for RCAP2H and RCAP2L, this equation can be rewritten as:

RCAP2H, RCAP2L = 65536

− f

osc / (16 × baud rate)

OscillatorFrequency

65536

RCAP2H RCAP2L

()

–

()

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