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AT85C51SND3Bx

7632A–MP3–03/06

Table 238. Stop Bit Number Selection

Guard Field The guard field is not part of a character and is an optional inter-character spacing com-

posed of 0 to 3 bits transmitted at high level by programming GBIT1:0 bits in SCON

according to

Table 239. The guard field allows transmitter to be compliant with con-

nected host (overrun avoiding) and is emitted after the last stop bit of a character.

Table 239. Guard Field Size Selection

Baud Rate Generator The Baud Rate Generator is fed by the SIO clock as detailed in Section “SIO Clock Gen-

erator”, page 32. The maximum baud rate can be achieved by selecting the high

frequency issued by a division of the PLL clock. The clock generated is an oversampling

clock. The oversampling factor is programmable using OVRSF3:0 bits in SFCON with

oversampling factor equal to OVRSF3:0 + 1 (e.g.: OVRSF3:0= 11 for a 12x

oversampling).

Baud Rate Calculation As shown in Figure 109, the baud rate generator is composed of an integer divider fol-

lowed by a fractional divider. The baud rate formula is given by Figure 108. In this

formula, variables must be chosen as followed:

– OVRSF

The oversampling factor depends mainly on the frequency and the quality of

the medium transporting the data. In any case, OVSF3:0 must not be less

than 4 for proper majority vote in bit reception.

–ADIV

Must be greater than BDIV and less than (K ⋅ OVRSF). K being the number

of bit in a character (from 9 to 11).

–BDIV

Must be greater than 1/

ε according to the tolerance on the real baud rate

compare to the theoretical baud rate BR

ε being the error: ε= K ⋅ |1/BB

- 1/BR

Table 240 shows some programming values depending on the SIO frequency and con-

sidering an oversampling factor of 12 (OVERSF3:0= 11).

Figure 108. Baud Rate Formula

STOP Description

0 1 Stop Bit.

1 2 Stop Bits.

GBIT1 GBIT0 Description

0 0 0 guard bit inserted (default).

0 1 1 guard bit inserted.

1 0 2 guard bits inserted.

1 1 3 guard bits inserted.

Baud_Rate=

ADIV ⋅ CDIV ⋅ OVRSF

SIO

⋅ BDIV

1 2 ... 210 211 212 213 214 215 216 217 218 219 220 ... 262 263

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