P89CV51RB2FBC datasheet(32/73 Pages) NXP

32 / 73 page

P89CV51RB2_RC2_RD2_1

Product data sheet

Rev. 01 — 5 October 2007

32 of 73

NXP Semiconductors

P89CV51RB2/RC2/RD2

80C51 with 1 kB RAM, SPI

6.5.3 Programmable clock-out

A 50 % duty cycle clock can be programmed to come out on pin T2 (P1[0], Clock-out

mode). This pin, besides being a regular I/O pin, has two additional functions. It can be

programmed:

1. To input the external clock for timer/counter 2, or

2. To output a 50 % duty cycle clock ranging from 122 Hz to 8 MHz at a 16 MHz

operating frequency.

To conﬁgure the timer/counter 2 as a clock generator, bit C/T2 (in T2CON) must be

cleared and bit T2OE in T2MOD must be set. Bit TR2 (T2CON.2) also must be set to start

the timer.

The clock-out frequency depends on the oscillator frequency and the reload value of

Timer 2 capture registers (RCAP2H, RCAP2L) as shown in Equation 2:

(2)

Where (RCAP2H, RCAP2L) = the content of RCAP2H and RCAP2L taken as a 16-bit

unsigned integer.

In the Clock-out mode, Timer 2 roll-overs will not generate an interrupt. This is similar to

when it is used as a baud rate generator.

6.5.4 Baud rate generator mode

Bits TCLK and/or RCLK in T2CON allow the UART transmit and receive baud rates to be

derived from either Timer 1 or Timer 2; see Section 6.6 for details. When TCLK = 0, Timer

1 is used as the UART transmit baud rate generator. When 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:

OscillatorFrequency

65536

RCAP2H RCAP2L

()

–

()

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

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

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