Philips Semiconductors Product data
P89C60X2/61X2
80C51 8-bit Flash microcontroller family
64KB Flash, 512B/1024B RAM
2003 Sep 11
27
OSC
÷ n
C/T2 = 0
C/T
2 = 1
TR2
Control
TL2
(8 bits)
TH2
(8 bits)
÷ 16
RCAP2L RCAP2H
EXEN2
Control
EXF2
Timer 2
Interrupt
T2EX Pin
Transition
Detector
T2 Pin
Reload
÷ 2
“0” “1”
RX Clock
÷ 16 TX Clock
“0”“1”
“0”“1”
Timer 1
Overflow
Note availability of additional external interrupt.
SMOD
RCLK
TCLK
SU01625
n = 1 in 6-clock mode
n = 2 in 12-clock mode.
Figure 13. Timer 2 in Baud Rate Generator Mode
Baud Rate Generator Mode
Bits TCLK and/or RCLK in T2CON (Table 6) allow the serial port
transmit and receive baud rates to be derived from either Timer 1 or
Timer 2. When TCLK= 0, Timer 1 is used as the serial port transmit
baud rate generator. When TCLK= 1, Timer 2 is used as the serial
port transmit baud rate generator. RCLK has the same effect for the
serial port receive baud rate. With these two bits, the serial port can
have different receive and transmit baud rates – one generated by
Timer 1, the other by Timer 2.
Figure 13 shows the Timer 2 in baud rate generation mode. The
baud rate generation mode is like the auto-reload mode, in that a
rollover 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/T
2=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 every machine cycle (i.e., 1/6
the oscillator frequency in 6-clock mode or 1/12 the oscillator
frequency in 12-clock mode). As a baud rate generator, it
increments at the oscillator frequency in 6-clock mode or at 1/2 the
oscillator frequency in 12-clock mode. Thus the baud rate formula is
as follows:
Oscillator Frequency
[n [65536 * (RCAP2H, RCAP2L)]]
Modes 1 and 3 Baud Rates =
Where:
n = 16 in 6-clock mode, 32 in 12-clock mode.
(RCAP2H, RCAP2L)= The content of RCAP2H and RCAP2L
taken as a 16-bit unsigned integer.
The Timer 2 as a baud rate generator mode shown in Figure 13 is
valid only if RCLK and/or TCLK = 1 in T2CON register. Note that a
rollover in TH2 does not set TF2, and will not generate an interrupt.
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 in use 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. As a baud rate generator, Timer 2 is
incremented every state time (osc/2) or asynchronously from pin T2;
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 7 shows commonly used baud rates and how they can be
obtained from Timer 2.
