COP8CCR9IMT7SLASHNOPB datasheet(65/111 Pages) TI1

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COP8CBR9, COP8CCR9, COP8CDR9

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SNOS535I – OCTOBER 2000 – REVISED MARCH 2013

5.13.7 BAUD CLOCK GENERATION

The clock inputs to the transmitter and receiver sections of the USART can be individually selected to

come either from an external source at the CKX pin (port L, pin L1) or from a source selected in the PSR

and BAUD registers. Internally, the basic baud clock is created from the MCLK through a two-stage divider

chain consisting of a 1-16 (increments of 0.5) prescaler and an 11-bit binary counter (Figure 5-18). The

divide factors are specified through two read/write registers shown in Figure 5-19. 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 5-17, a Prescaler Factor of 0 corresponds to NO CLOCK. This 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 5-17.

There are 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 5-

16). Other baud rates may be created by using appropriate divisors. The 16x clock is then divided by 16 to

provide the rate for the serial shift registers of the transmitter and receiver.

Table 5-16. Baud Rate Divisors

(1.8432 MHz Prescaler Output)(1)

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

(1)

The entries in Table 5-16 assume a prescaler output of 1.8432 MHz.

In asynchronous mode the baud rate could be as high as 1250k.

Figure 5-18. USART BAUD Clock Generation

Functional Description

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