IS80LV51 Datasheet, PDF (18/43 Pages) Integrated Silicon Solution, Inc – CMOS SINGLE CHIP LOW VOLTAGE 8-BIT MICROCONTROLLER

IS80LV51
IS80LV31
ISSI ^®
Using the Timer 1 to Generate Baud Rates
When Timer 1 is 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 according to the following
equation.
Mode 1, 3
Baud Rate
=
2^SMOD
32
X (Timer 1 Overflow Rate)
The Timer 1 interrupt should be disabled in this application.
The Timer itself can be configured for either timer or
counter operation in any of its three running modes. In the
most typical applications, it is configured for timer operation
in auto-reload mode (high nibble of TMOD = 0010B). In
this case, the baud rate is given by the following formula.
Mode 1,3 ₌
Baud Rate
2^SMOD
Oscillator Frequency
X
32
12x [256-(TH1)]
Programmers can achieve very low baud rates with Timer
1 by leaving the Timer 1 interrupt enabled, 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.
Table 7 lists commonly used baud rates and how they can
be obtained from Timer 1.
More About Mode 0
Serial data enters and exits through RXD. TXD outputs
the shift clock. Eight data bits are transmitted/received,
with the LSB first. The baud rate is fixed at 1/12 the
oscillator frequency.
Figure 12 shows a simplified functional diagram of the
serial port in Mode 0 and associated timing.
Transmission is initiated by any instruction that uses
SBUF as a destination register. The "write to SBUF"
signal at S6P2 also loads a 1 into the ninth position of the
transmit shift register and tells the TX Control block to
begin a transmission. The internal timing is such that one
full machine cycle will elapse between "write to SBUF"
and activation of SEND.
SEND transfer the output of the shift register to the
alternate output function line of P3.0, and also transfers
SHIFT CLOCK to the alternate output function line of
P3.1. SHIFT CLOCK is low during S3, S4, and S5 of every
machine cycle, and high during S6, S1, and S2. At S6P2
of every machine cycle in which SEND is active, the
contents of the transmit shift register are shifted one
position to the right.
As data bits shift out to the right, 0s come in from the left.
When the MSB of the data byte is at the output position of
the shift register, the 1 that was initially loaded into the
ninth position is just to the left of the MSB, and all positions
to the left of that contain 0s. This condition flags the TX
Control block to do one last shift, then deactivate SEND
and set TI. Both of these actions occur at S1P1 of the tenth
machine cycle after "write to SBUF."
Table 7. Commonly Used Baud Rates Generated by Timer 1
Baud Rate
Mode 0 Max: 1 MHz
Mode 2 Max: 375K
Modes 1, 3: 62.5K
19.2K
9.6K
4.8K
2.4K
1.2K
137.5
110
110
fOSC
12 MHz
12 MHz
12 MHz
11.059 MHz
11.059 MHz
11.059 MHz
11.059 MHz
11.059 MHz
11.986 MHz
6 MHz
12 MHz
SMOD
X
1
1
1
0
0
0
0
0
0
0
Timer 1
C/T
Mode
Reload Value
X
X
X
X
X
X
0
2
FFH
0
2
FDH
0
2
FDH
0
2
FAH
0
2
F4H
0
2
E8H
0
2
1DH
0
2
72H
0
1
FEEBH
18
Integrated Silicon Solution, Inc.
ADVANCE INFORMATION MC018-0A
10/01/98