p89c538nbbb NXP Semiconductors, p89c538nbbb Datasheet - Page 8

• Manufacturer Part Number: p89c538nbbb
• Description: Cmos Single-chip 8-bit Microcontrollers With Flash Program Memory
• Manufacturer: NXP Semiconductors

Philips Semiconductors
TIMER 2 OPERATION
Timer 2
Timer 2 is a 16-bit Timer/Counter which can operate as either an
event timer or an event counter, as selected by C/T2* in the special
function register T2CON (see Figure 1). Timer 2 has three operating
modes:Capture, Auto-reload, and Baud Rate Generator, which are
selected by bits in the T2CON as shown in Table 3.
Capture Mode
In the capture mode there are two options which are selected by bit
EXEN2 in T2CON. If EXEN2=0, then timer 2 is a 16-bit timer or
counter (as selected by C/T2* in T2CON) which, upon overflowing
sets bit TF2, the timer 2 overflow bit. This bit can be used to
generate an interrupt (by enabling the Timer 2 interrupt bit in the
IE register/SFR table). 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). The capture mode is
Table 3. Timer 2 Operating Modes
1997 Jun 05
CMOS single-chip 8-bit microcontrollers
with FLASH program memory
RCLK + TCLK
EXF2
TCLK
Symbol
TF2
RCLK
EXEN2
TR2
C/T2
CP/RL2
X
0
0
1
Position
T2CON.7
T2CON.6
T2CON.5
T2CON.4
T2CON.3
T2CON.2
T2CON.1
T2CON.0
EXEN2 = 1. When either RCLK = 1 or TCLK = 1, this bit is ignored and the timer is forced to auto-reload
(MSB)
TF2
Name and Significance
Timer 2 overflow flag set by a Timer 2 overflow and must be cleared by software. TF2 will not be set
when either RCLK or TCLK = 1.
Timer 2 external flag set when either a capture or reload is caused by a negative transition on T2EX and
EXEN2 = 1. When Timer 2 interrupt is enabled, EXF2 = 1 will cause the CPU to vector to the Timer 2
interrupt routine. EXF2 must be cleared by software.
Receive clock flag. When set, causes the serial port to use Timer 2 overflow pulses for its receive clock
in modes 1 and 3. RCLK = 0 causes Timer 1 overflow to be used for the receive clock.
Transmit clock flag. When set, causes the serial port to use Timer 2 overflow pulses for its transmit clock
in modes 1 and 3. TCLK = 0 causes Timer 1 overflows to be used for the transmit clock.
Timer 2 external enable flag. When set, allows a capture or reload to occur as a result of a negative
transition on T2EX if Timer 2 is not being used to clock the serial port. EXEN2 = 0 causes Timer 2 to
ignore events at T2EX.
Start/stop control for Timer 2. A logic 1 starts the timer.
Timer or counter select. (Timer 2)
Capture/Reload flag. When set, captures will occur on negative transitions at T2EX if EXEN2 = 1. When
cleared, auto-reloads will occur either with Timer 2 overflows or negative transitions at T2EX when
on Timer 2 overflow.
CP/RL2
X
X
0
1
EXF2
Figure 1.
0 = Internal timer (OSC/12)
1 = External event counter (falling edge triggered).
RCLK
Timer/Counter 2 (T2CON) Control Register
TCLK
TR2
1
1
1
0
8
EXEN2
illustrated in Figure 2 (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 T2EX pin transitions or osc/12 pulses.).
Auto-Reload Mode
In the 16-bit auto-reload mode, Timer 2 can be configured as either
a timer or counter (C/T2* in T2CON).
Figure 3 shows the auto–reload mode of Timer 2. 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. If
EXEN2=1, then 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 are 1.
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. The EXF2 flag does not generate an interrupt in this mode
of operation.
16-bit Auto-reload
16-bit Capture
Baud rate generator
(off)
TR2
89C535/89C536/89C538
C/T2
MODE
CP/RL2
(LSB)
Preliminary specification
SU00866