Datasheet

REV. 0–58–
ADuC831
Timer 3 Generated Baud Rates
The high integer dividers in a UART block mean that high speed
baud rates are not always possible using some particular crystals.
For example, using a 12 MHz crystal, a baud rate of 115200 is
not possible. To address this problem, the ADuC831 has added
a dedicated baud rate timer (Timer 3) specifically for generating
highly accurate baud rates.
Timer 3 can be used instead of Timer 1 or Timer 2 for generating
very accurate high speed UART baud rates including 115200
and 230400. Timer 3 also allows a much wider range of baud
rates to be obtained. In fact, every desired bit rate from 12 bit/s
to 393216 bit/s can be generated to within an error of ±0.8%.
Timer 3 also frees up the other three timers, allowing them to
be used for different applications. A block diagram of Timer 3 is
shown in Figure 54 below.
(1 + T3FD/64)
2
T3 RX/TX
CLOCK
CORE
CLK
T3EN
RX CLOCK
TX CLOCK
TIMER 1/TIMER 2
RX CLOCK (FIG 53)
FRACTIONAL
DIVIDER
0
0
1
1
TIMER 1/TIMER 2
TX CLOCK (FIG 53)
16
2
DIV
Figure 54. Timer 3, UART Baud Rates
Two SFRs (T3CON and T3FD) are used to control Timer 3.
T3CON is the baud rate control SFR, allowing Timer 3 to be
used to set up the UART baud rate, and setting up the binary
divider (DIV).
Table XXVI. T3CON SFR Bit Designations
Bit Name Description
7 T3BAUDEN T3UARTBAUD Enable
Set to enable Timer 3 to generate
the baud rate. When set, PCON.7,
T2CON.4 and T2CON.5 are ignored.
Cleared to let the baud rate be
generated as per a standard 8052.
6 –
5 –
4 –
3 –
2 DIV2 Binary Divider Factor.
1 DIV1 DIV2 DIV1 DIV0 Bin Divider
0 DIV0 0 0 0 1
0 0 1 1
0 1 0 1
0 1 1 1
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 1
The appropriate value to write to the DIV2-1-0 bits can be calcu-
lated using the following formula where f
CORE
is the crystal
frequency:
Note: The DIV value must be rounded down.
DIV
f
32 Baud Rate
CORE
=
×






log
log( )2
T3FD is the fractional divider ratio required to achieve the
required baud rate. We can calculate the appropriate value for
T3FD using the following formula.
Note: T3FD should be rounded to the nearest integer.
TFD
f
Baud Rate
CORE
DIV
3
2
2
=
×
×
Once the values for DIV and T3FD are calculated the actual
baud rate can be calculated using the following formula.
Actual Baud Rate =
2f
2 (T3FD+ 64)
CORE
DIV
×
×
For example, to get a baud rate of 115200 while operating at
11.0592 MHz:
DIV LOG LOG
TFD H
=×
()
()
==
=×
()
×
()
==
11059200 32 115200 2 1 58 1
3211059200 2 115200 64 32 20
1
//.
/–
Therefore, the actual baud rate is 115200 bit/s.
Table XXVII. Commonly Used Baud Rates Using Timer 3
Ideal %
Baud Crystal DIV T3CON T3FD Error
230400 11.0592 0 80H 20H 0.0
115200 11.0592 1 81H 20H 0.0
57600 11.0592 2 82H 20H 0.0
38400 11.0592 3 83H 08H 0.0
19200 11.0592 4 84H 08H 0.0
9600 11.0592 5 85H 08H 0.0
230400 12 0 80H 28H 0.16
115200 12 1 81H 28H 0.16
57600 12 2 82H 28H 0.16
38400 12 3 83H 0EH 0.16
19200 12 4 84H 0EH 0.16
9600 12 5 85H 0EH 0.16
230400 14 0 80H 3AH 0.39
115200 14 1 81H 3AH 0.39
57600 14 2 82H 3AH 0.39
38400 14 3 83H 1BH 0.16
19200 14 4 84H 1BH 0.16
9600 14 5 85H 1BH 0.16
230400 16 1 81H 05H 0.64
115200 16 2 82H 05H 0.64
57600 16 3 83H 05H 0.64
38400 16 3 83H 28H 0.16
19200 16 4 84H 28H 0.16
9600 16 5 85H 28H 0.16