P89V51RD2 Datasheet, PDF (32/75 Pages) NXP Semiconductors – 8-bit 80C51 5 V low power 64 kB Flash microcontroller with 1 kB RAM

Philips Semiconductors
P89V51RD2
8-bit microcontrollers with 80C51 core
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.
(down counting reload value)
FFH
FFH
toggle
EXF2
OSC
¸6
T2 pin
C/T2 = 0
C/T2 = 1
control
TL2
TH2 ^underflow
(8-bits) (8-bits) _overflow
TF2
Timer 2
interrupt
TR2
RCAP2L RCAP2H
count direction
1 = up
0 = down
(up counting reload value)
T2EX pin
Fig 13. Timer 2 in Auto Reload mode (DCEN = 1).
002aaa525
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.
7.4.3 Programmable clock-out
A 50% duty cycle clock can be programmed to come out on pin T2 (P1.0). 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 T20E 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:
9397 750 12964
Product data
Rev. 01 — 01 March 2004
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
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