p89c668hbbd NXP Semiconductors, p89c668hbbd Datasheet - Page 38

• Manufacturer Part Number: p89c668hbbd
• Description: 80c51 8-bit Flash Microcontroller Family
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Table 10. Timer 2 Commonly Used Generated
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.
2002 Oct 28
12 clock
80C51 8-bit Flash microcontroller family
16KB/32KB/64KB ISP/IAP Flash with 512B/1KB/2KB/8KB RAM
mode
375 k
9.6 k
2.8 k
2.4 k
1.2 k
300
110
300
110
OSC
T2EX Pin
Baud Rate
T2 Pin
Baud Rates
6 clock
mode
19.2 k
750 k
5.6 k
4.8 k
2.4 k
600
220
600
220
Transition
Detector
Osc Freq
C/T2 = 0
C/T2 = 1
12 MHz
12 MHz
12 MHz
12 MHz
12 MHz
12 MHz
12 MHz
6 MHz
6 MHz
Note availability of additional external interrupt.
EXEN2
RCAP2H
Control
Figure 25. Timer 2 in Baud Rate Generator Mode
FF
FF
FF
FF
FE
FB
FD
F2
F9
TR2
Timer 2
Control
RCAP2L
EXF2
D9
C8
AF
FF
B2
64
1E
8F
57
RCAP2L
(8-bits)
Interrupt
38
Timer 2
TL2
In many applications, it is configured for “timer” operation (C/T2 = 0).
frequency in 12 clock mode). As a baud rate generator, it increments
The baud rates in modes 1 and 3 are determined by Timer 2’s
overflow rate given below:
The timer can be configured for either “timer” or “counter” operation.
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
at the oscillator frequency in 6 clock mode (f
mode). Thus the baud rate formula is as follows:
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.
/
6
the oscillator frequency in 6 clock mode,
Modes 1 and 3 Baud Rates + Timer 2 Overflow Rate
Modes 1 and 3 Baud Rates =
(8-bits)
RCAP2H
TH2
P89C660/P89C662/P89C664/
Reload
[ n *
* n =
[65536
Oscillator Frequency
“1”
16 in 6 clock mode
32 in 12 clock mode
“1”
2
“0”
“0”
“0”
(RCAP2H, RCAP2L)]]
Overflow
Timer 1
“1”
1
OSC
/
12
16
/2 in 12 clock
16
16
the oscillator
P89C668
SMOD
RCLK
TCLK
SU01213
Product data
RX Clock
TX Clock