COP8-REF-AM datasheet(47/83 Pages) NSC

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14.0 USART (Continued)

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.

TABLE 19. Baud Rate Divisors

(1.8432 MHz Prescaler Output)

Baud Rate

Divisor − 1 (N-1)

110 (110.03)

1046

134.5 (134.58)

855

150

767

300

383

600

191

1200

1800

2400

3600

4800

7200

9600

19200

38400

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.

TABLE 20. Prescaler Factors

Prescaler

Select

Factor

00000

NO CLOCK

00001

00010

1.5

00011

00100

2.5

00101

00110

3.5

00111

Prescaler

Select

Factor

01000

4.5

01001

01010

5.5

01011

01100

6.5

01101

01110

7.5

01111

10000

8.5

10001

10010

9.5

10011

10100

10.5

10101

10110

11.5

10111

11000

12.5

11001

11010

13.5

11011

11100

14.5

11101

11110

15.5

11111

As an example, considering Asynchronous Mode and a crys-

tal frequency of 4.608 MHz, the prescaler factor selected is:

(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.

N−1=5(N−1isthe value from Table 19)

N=6(Nisthe 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.

The actual Baud Rate may be found from:

BR=(F

C x 2)/(16xNxP)

Where:

BR is the Baud Rate

C is the crystal frequency

N is the Baud Rate Divisor (Table 19)

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.

Example:

Asynchronous Mode:

Crystal Frequency=5MHz

Desired baud rate = 19200

20006327

FIGURE 23. USART BAUD Clock Generation

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