Philips Semiconductors Product data
P89C660/P89C662/P89C664/
P89C668
80C51 8-bit Flash microcontroller family
16KB/32KB/64KB ISP/IAP Flash with 512B/1KB/2KB/8KB RAM
2002 Oct 28
38
OSC
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
SU01213
Figure 25. Timer 2 in Baud Rate Generator Mode
Table 10. Timer 2 Commonly Used Generated
Baud Rates
Baud Rate Timer 2
12 clock
mode
6 clock
mode
Osc Freq
RCAP2H RCAP2L
375 k 750 k 12 MHz FF FF
9.6 k 19.2 k 12 MHz FF D9
2.8 k 5.6 k 12 MHz FF B2
2.4 k 4.8 k 12 MHz FF 64
1.2 k 2.4 k 12 MHz FE C8
300 600 12 MHz FB 1E
110 220 12 MHz F2 AF
300 600 6 MHz FD 8F
110 220 6 MHz F9 57
Baud Rate Generator Mode
Bits TCLK and/or RCLK in T2CON (see Figure 20) 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.
Table 10 shows commonly used baud rates and how they can be
obtained from Timer 2.
Figure 25 shows Timer 2 in baud rate generation mode. The baud
rate generation mode is similar to 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,
1
/
12
the oscillator
frequency in 12 clock mode). As a baud rate generator, it increments
at the oscillator frequency in 6 clock mode (f
OSC
/2 in 12 clock
mode). Thus the baud rate formula is as follows:
Oscillator Frequency
[n* [65536 ǒ (RCAP2H, RCAP2L)]]
Modes 1 and 3 Baud Rates =
* n = 16 in 6 clock mode
32 in 12 clock mode
Where: (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 25, 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, 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.
