83C754 Philips Semiconductors, 83C754 Datasheet - Page 6

• Manufacturer Part Number: 83C754
• Description: 80C51 8-bit microcontroller family 4K/256 OTP/ROM/ DAC/ comparator/ UART/ reference
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Serial Port Control Register
The serial port control and status register is the Special Function
Register SCON, shown in Figure 1. This register contains not only
the mode selection bits, but also the 9th data bit for transmit and
receive (TB8 and RB8), and the serial port interrupt bits (TI and RI).
Baud Rates
The baud rate in Mode 0 is fixed: Mode 0 Baud Rate = Oscillator
Frequency / 12. The baud rate in Mode 2 depends on the value of
bit SMOD in Special function Register PCON. If SMOD = 0 (which is
the value on reset), the baud rate is 1/64 the oscillator frequency.
If SMOD = 1, the baud rate is 1/32 the oscillator frequency.
In the 8XC754, the baud rates in Modes 1 and 3 are determined by
the Timer 1 overflow rate.
1998 Apr 23
80C51 8-bit microcontroller family
4K/256 OTP/ROM, DAC, comparator, UART, reference
SM2
REN
TB8
RB8
TI
RI
Mode 2 Baud Rate
Enables the multiprocessor communication feature in Modes 2 and 3. In Mode 2 or 3, if SM2 is set to 1, then Rl will not be
activated if the received 9th data bit (RB8) is 0. In Mode 1, if SM2=1 then RI will not be activated if a valid stop bit was not
received. In Mode 0, SM2 should be 0.
Enables serial reception. Set by software to enable reception. Clear by software to disable reception.
The 9th data bit that will be transmitted in Modes 2 and 3. Set or clear by software as desired.
In Modes 2 and 3, is the 9th data bit that was received. In Mode 1, it SM2=0, RB8 is the stop bit that was received. In Mode 0,
RB8 is not used.
Transmit interrupt flag. Set by hardware at the end of the 8th bit time in Mode 0, or at the beginning of the stop bit in the other
modes, in any serial transmission. Must be cleared by software.
Receive interrupt flag. Set by hardware at the end of the 8th bit time in Mode 0, or halfway through the stop bit time in the other
modes, in any serial reception (except see SM2). Must be cleared by software.
Mode 0 Max: 1.67MHz
Mode 1, 3 Max: 104.2k
Mode 2 Max: 625k
Ba d Rate
Baud Rate
19.2k
137.5
9.6k
4.8k
2.4k
1.2k
110
110
2
SMOD
64
(Oscillator Frequency)
SM0
Where SM0, SM1 specify the serial port mode, as follows:
Figure 2. Timer 1 Generated Commonly Used Baud Rates
0
0
1
1
SM1
SM0
MSB
0
1
0
1
SM1 SM2 REN TB8
Mode
3
0
1
2
Description
11.059MHz
11.059MHz
11.059MHz
11.059MHz
11.059MHz
11.986MHz
shift register
8-bit UART
9-bit UART
9-bit UART
20MHz
20MHz
20MHz
12MHz
Figure 1.
6MHz
f
f
OSC
OSC
6
Using Timer 1 to Generate Baud Rates
When Timer 1 is used as the baud rate generator, the baud rates in
Modes 1 and 3 are determined by the Timer 1 overflow rate and the
value of SMOD as follows:
The Timer 1 interrupt should be disabled in this application. The
Timer itself can be configured for either “timer” or “counter”
operation, and in any of its 3 running modes. In the most typical
applications, it is configured for “timer” operation, in the auto-reload
mode (high nibble of TMOD = 0010B). In that case the baud rate is
given by the formula:
One can achieve very low baud rates with Timer 1 by leaving the
Timer 1 interrupt enabled, and configuring the Timer to run as a
16-bit timer (high nibble of TMOD = 0001B), and using the Timer 1
interrupt to do a 16-bit software reload. Figure 2 lists various
commonly used baud rates and how they can be obtained from
Timer 1.
RB8
Mode 1, 3 Baud Rate
Mode 1, 3 Baud Rate
TI
Baud Rate
LSB
f
variable
f
variable
RI
OSC
OSC
SMOD
SMOD
/ 12
/64 or f
X
1
1
1
0
0
0
0
0
0
0
OSC
/32
C/T
X
X
0
0
0
0
0
0
0
0
0
2
2
SMOD
SMOD
32
32
83C754/87C754
Mode
X
X
2
2
2
2
2
2
2
2
1
(Timer 1 Overflow Rate)
Oscillator Frequency
12
Timer 1
Preliminary specification
[256
Reload Value
FEEBH
FDH
FDH
1DH
FFH
FAH
F4H
E8H
72H
(TH1)]
X
X
SU00120