P89C668 Philips Semiconductors, P89C668 Datasheet - Page 14

• Manufacturer Part Number: P89C668
• Description: 80C51 8-bit Flash microcontroller family 64KB ISP FLASH with 8KB RAM
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Table 5.
Baud Rate Generator Mode
Bits TCLK and/or RCLK in T2CON (Table 5) 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 6 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:
2001 Jul 27
80C51 8-bit Flash microcontroller family
64KB ISP Flash with 8KB RAM
Ba d Rate
Baud Rate
19.2 k
750 k
5.6 k
4.8 k
2.4 k
OSC
600
220
600
220
T2EX Pin
T2 Pin
Timer 2 Generated Commonly Used
Baud Rates
Osc Freq
Osc Freq
12 MHz
12 MHz
12 MHz
12 MHz
12 MHz
12 MHz
12 MHz
6 MHz
6 MHz
Transition
Detector
C/T2 = 0
C/T2 = 1
Note availability of additional external interrupt.
RCAP2H
FF
FF
FF
FF
FE
FB
FD
F2
F9
EXEN2
Control
Figure 6. Timer 2 in Baud Rate Generator Mode
Timer 2
TR2
Control
RCAP2L
EXF2
FF
D9
B2
C8
1E
AF
8F
64
57
RCAP2L
(8-bits)
Interrupt
14
Timer 2
TL2
When Timer 2 is in the baud rate generator mode, one should not try
The timer can be configured for either “timer” or “counter” operation.
In many applications, it is configured for “timer” operation (C/T2*=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
at the oscillator frequency. 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 6, 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.
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
/
6
the oscillator frequency). As a baud rate generator, it increments
Modes 1 and 3 Baud Rates + Timer 2 Overflow Rate
Modes 1 and 3 Baud Rates =
(8-bits)
RCAP2H
TH2
Reload
[16
[65536 * (RCAP2H, RCAP2L)]]
Oscillator Frequency
“1”
“1”
2
“0”
“0”
“0”
Overflow
Timer 1
“1”
16
16
16
P89C668
SMOD
RCLK
TCLK
Preliminary data
SU01213
RX Clock
TX Clock