14.0 USART (Continued)
Prescaler
Select
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
11110
11111
Prescaler
Factor
4.5
5
many ways to calculate the two divisor factors, but one
particularly effective method would be to achieve a 1.8432
MHz frequency coming out of the first stage. The 1.8432
MHz prescaler output is then used to drive the software
programmable baud rate counter to create a 16x clock for
the following baud rates: 110, 134.5, 150, 300, 600, 1200,
1800, 2400, 3600, 4800, 7200, 9600, 19200 and 38400
(Table 19). Other baud rates may be created by using ap-
propriate divisors. The 16x clock is then divided by 16 to
provide the rate for the serial shift registers of the transmitter
and receiver.
5.5
6
6.5
7
7.5
8
8.5
9
TABLE 19. Baud Rate Divisors
(1.8432 MHz Prescaler Output)
9.5
10
Baud Rate
Baud Rate
Divisor − 1 (N-1)
10.5
11
110 (110.03)
134.5 (134.58)
150
1046
855
767
383
191
95
11.5
12
300
12.5
13
600
1200
13.5
14
1800
63
2400
47
14.5
15
3600
31
4800
23
15.5
16
7200
15
9600
11
As an example, considering Asynchronous Mode and a crys-
tal frequency of 4.608 MHz, the prescaler factor selected is:
19200
38400
5
2
(4.608 x 2)/1.8432 = 5
The 5 entry is available in Table 20. The 1.8432 MHz pres-
caler output is then used with proper Baud Rate Divisor
(Table 19) to obtain different baud rates. For a baud rate of
19200 e.g., the entry in Table 19 is 5.
Note: The entries in Table 19 assume a prescaler output of 1.8432 MHz. In
asynchronous mode the baud rate could be as high as 625k.
N − 1 = 5 (N − 1 is the value from Table 19)
N = 6 (N is the Baud Rate Divisor)
Baud Rate = 1.8432 MHz/(16 x 6) = 19200
The divide by 16 is performed because in the asynchronous
mode, the input frequency to the USART is 16 times the
baud rate. The equation to calculate baud rates is given
below.
20006327
FIGURE 23. USART BAUD Clock Generation
The actual Baud Rate may be found from:
BR = (FC x 2)/(16 x N x P)
Where:
TABLE 20. Prescaler Factors
Prescaler
Select
00000
00001
00010
00011
00100
00101
00110
00111
Prescaler
BR is the Baud Rate
Factor
FC is the crystal frequency
N is the Baud Rate Divisor (Table 19)
NO CLOCK
1
1.5
2
P is the Prescaler Divide Factor selected by the value in the
Prescaler Select Register (Table 20)
Note: In the Synchronous Mode, the divisor 16 is replaced
by two.
2.5
3
Example:
Asynchronous Mode:
Crystal Frequency = 5 MHz
Desired baud rate = 19200
3.5
4
47
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