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SCI COMMUNICATION BETWEEN ST7 AND PC

1.3 FRAMES

Any transmission is Least Significant Bit first. A data word can be 8 or 9 bits long. A data frame

begins with a «start bit», which is a ’0’ bit and ends with a «stop bit», which is a ’1’ bit. See

Figure 2.

Two special frames are also managed :

An Idle character is interpreted as an entire frame of ’1’ bits followed by the start bit of the next

frame which contains data.

A Break character is detected when receiving ’0’ bits for some multiple of the frame period. At

the end of the last break, the transmitter inserts an extra ’1’ bit to acknowledge the next start

bit.

Figure 2. Frames and word length

Bit0

Bit1

Bit2

Bit3

Bit4

Bit5

Bit6

Bit7

Bit8

Start

Bit

Stop

Bit

Next

Start

Bit

Idle Frame

Bit0

Bit1

Bit2

Bit3

Bit4

Bit5

Bit6

Bit7

Start

Bit

Stop

Bit

Next

Start

Bit

Start

Bit

Idle Frame

Start

Bit

9-bit Word length (M bit of SCICR1 is set)

8-bit Word length (M bit of SCICR1 is reset)

Break Frame

Start

Bit

Extra

’1’

Data Frame

Break Frame

Start

Bit

Extra

’1’

Data Frame

Next Data Frame

Next Data Frame