COP8AME9_14 Datasheet, PDF (55/102 Pages) Texas Instruments – COP8AME9 8-Bit CMOS Flash Microcontroller with 8k Memory, Dual Op Amps, Virtual EEPROM, Temperature Sensor, 10-Bit A/D and Brownout Reset

COP8AME9, COP8ANE9
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SNOS930F – MARCH 2001 – REVISED MARCH 2013
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 25). The divide factors are specified
through two read/write registers shown in Figure 26. 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 22, 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 22. 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 21). 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.
Baud Rate
110 (110.03)
134.5 (134.58)
150
300
600
1200
1800
2400
3600
4800
7200
9600
19200
38400
Table 21. Baud Rate Divisors
(1.8432 MHz Prescaler Output)
Baud Rate
Divisor − 1 (N-1)
1046
855
767
383
191
95
63
47
31
23
15
11
5
2
NOTE
The entries in Table 21 assume a prescaler output of 1.8432 MHz. In asynchronous mode
the baud rate could be as high as 625k.
Figure 25. USART BAUD Clock Generation
Copyright © 2001–2013, Texas Instruments Incorporated
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