cop8cde9 National Semiconductor Corporation, cop8cde9 Datasheet - Page 43

• Manufacturer Part Number: cop8cde9
• Description: 8-bit Cmos Flash Microcontroller With 8k Memory, Virtual Eeprom, 10-bit A/d And Brownout Reset
• Manufacturer: National Semiconductor Corporation

8.0 USART
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 18 ). 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.
Note: The entries in Table 18 assume a prescaler output of 1.8432 MHz. In
asynchronous mode the baud rate could be as high as 1250k.
FIGURE 24. USART BAUD Clock Generation
134.5 (134.58)
110 (110.03)
Baud Rate
TABLE 18. Baud Rate Divisors
(1.8432 MHz Prescaler Output)
TABLE 19. Prescaler Factors
19200
38400
1200
1800
2400
3600
4800
7200
9600
Prescaler
150
300
600
Select
00000
00001
00010
00100
00101
00011
00110
00111
(Continued)
Divisor − 1 (N-1)
NO CLOCK
Prescaler
Baud Rate
Factor
1.5
2.5
3.5
1046
855
767
383
191
1
2
3
4
95
63
47
31
23
15
11
5
2
20022527
43
As an example, considering Asynchronous Mode and a crys-
tal frequency of 4.608 MHz, the prescaler factor selected is:
The 5 entry is available in Table 19 . The 1.8432 MHz pres-
caler output is then used with proper Baud Rate Divisor
( Table 18 ) to obtain different baud rates. For a baud rate of
19200 e.g., the entry in Table 18 is 5.
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.
The actual Baud Rate may be found from:
Where:
BR is the Baud Rate
F
N is the Baud Rate Divisor ( Table 18 )
P is the Prescaler Divide Factor selected by the value in the
Prescaler Select Register ( Table 19 )
Note: In the Synchronous Mode, the divisor 16 is replaced
by two.
Example:
Asynchronous Mode:
C
N − 1 = 5 (N − 1 is the value from Table 18 )
N = 6 (N is the Baud Rate Divisor)
Baud Rate = 1.8432 MHz/(16 x 6) = 19200
BR = (F
Crystal Frequency = 5 MHz
Desired baud rate = 19200
is the crystal frequency
C
x 2)/(16 x N x P)
Prescaler
Select
01000
01001
01010
10000
10001
10010
10100
10101
01011
01100
01101
01110
10011
10110
10111
11000
11001
11010
11011
11100
11101
(4.608 x 2)/1.8432 = 5
01111
11110
11111
Prescaler
Factor
10.5
12.5
13.5
14.5
15.5
11.5
4.5
5.5
6.5
7.5
8.5
9.5
10
12
13
14
15
16
11
5
6
7
8
9
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