Siemens EF 88H Series Manual de usuario descargar pdf (Pagina 66)

Semiconductor Group 6-32

On-Chip Peripheral Components

C501

6.3.3 Baud Rates Generation

There are several possibilities to generate the baud rate clock for the serial port depending on the

mode in which it is operating.

For clarification some terms regarding the difference between “baud rate clock” and “baud rate”

should be mentioned. The serial interface requires a clock rate which is 16 times the baud rate for

internal synchronization. Therefore, the baud rate generators have to provide a “baud rate clock” to

the serial interface which - there divided by 16 - results in the actual “baud rate”. However, all

formulas given in the following section already include the factor and calculate the final baud rate.

Further, the abrevation

OSC

refers to the external clock frequency (oscillator or external input clock

operation).

The baud rate of the serial port is controlled by bit SMOD which is located in the special function

Special Function Register PCON (Address 87

) Reset Value : 0XXX0000

Mode 0

The baud rate in mode 0 is fixed:

Mode 0 baud rate = oscillator frequency/12 =

OSC

/12

Mode 2

The baud rate in mode 2 depends on the value of bit SMOD in special function register PCON. If

SMOD = 0 (which is the value on reset), the baud rate is

OSC

/64. If SMOD = 1, the baud rate is f

OSC

32.

Mode 2 baud rate = 2

SMOD

/64×(f

OSC

)

Modes 1 and 3

The baud rates in mode 1 and 3 are determined by the timer overflow rate. These baud rates can

be determined by timer 1 or by timer 2 or by both (one for transmit and the other for receive).

Bit Function

SMOD Double baud rate

When set, the baud rate of serial interface in modes 1, 2, 3 is doubled. After reset

this bit is cleared.

SMOD–––87

PCON

Bit No. MSB LSB

GF1 GF0 PDE IDLE

76543210

The shaded bits are not used for controlling the baud rate.

1 2 ... 61 62 63 64 65 66 67 68 69 70 71 ... 120 121

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Siemens EF 88H Series Manual de usuario Pagina 66

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