COP888EB Datasheet, PDF (60/75 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 8k Memory, CAN Interface, 8-Bit A/D, and USART

Baud Clock Generation (Continued)
write registers shown in Figure 47. Note that the 11-bit Baud
Rate Divisor spills over into the Prescaler Select Register
(PSR). PSR is cleared upon reset.
As shown in Table 15, a Prescaler Factor of 0 corresponds to
NO CLOCK. NO CLOCK condition is the USART power
down mode where the USART clock is turned off for power
saving purpose. The user must also turn the USART clock
off when a different baud rate is chosen.
The correspondences between the 5-bit Prescaler Select
and Prescaler factors are shown in Table 15. There are
many ways to calculate the two divisor factors, but one par-
ticularly 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 program-
mable baud rate counter to create a x16 clock for the follow-
ing baud rates: 110, 134.5, 150, 300, 600, 1200, 1800, 2400,
3600, 4800, 7200, 9600, 19200 and 38400 (Table 16). Other
baud rates may be created by using appropriate divisors.
The x16 clock is then divided by 16 to provide the rate for the
serial shift registers of the transmitter and receiver.
TABLE 15. Prescaler Factors
Prescaler
Select
00000
00001
00010
00011
00100
00101
00110
Prescaler
Factor
NO CLOCK
1
1.5
2
2.5
3
3.5
Prescaler
Select
00111
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
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
15
15.5
16
FIGURE 46. USART BAUD Clock Generation
DS012837-50
FIGURE 47. USART BAUD Clock Divisor Registers
DS012837-51
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