ST72E121 datasheet(52/92 Pages) STMICROELECTRONICS

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ST72E121 ST72T121

SERIAL COMMUNICATIONS INTERFACE (Cont’d)

4.4.4 Functional Description

The block diagram of the Serial Control Interface,

is shown in Figure 32. It contains 6 dedicated reg-

isters:

– Two control registers (CR1 & CR2)

– A status register (SR)

– A baud rate register (BRR)

– An extended prescaler receiver register (ERPR)

– Anextended prescaler transmitter register (ETPR)

Refer to the register descriptions in Section

4.4.7for the definitions of each bit.

4.4.4.1 Serial Data Format

Word length may be selected as being either 8 or 9

bits by programming the M bit in the CR1 register

(see Figure 32).

The TDO pin is in low state during the start bit.

The TDO pin is in high state during the stop bit.

An Idle character is interpreted as an entire frame

of “1”s followed by the start bit of the next frame

which contains data.

A Break character is interpreted on receiving “0”s

for some multiple of the frame period. At the end of

the last break frame the transmitter inserts an ex-

tra “1” bit to acknowledge the start bit.

Transmission and reception are driven by their

own baud rate generator.

Figure 33. Word length programming

Bit0

Bit1

Bit2

Bit3

Bit4

Bit5

Bit6

Bit7

Bit8

Start

Bit

Stop

Bit

Start

Bit

Idle Frame

Bit0

Bit1

Bit2

Bit3

Bit4

Bit5

Bit6

Bit7

Start

Bit

Stop

Bit

Start

Bit

Start

Bit

Idle Frame

Start

Bit

9-bit Word length (M bit is set)

8-bit Word length (M bit is reset)

Possible

Parity

Bit

Possible

Parity

Bit

Break Frame

Start

Bit

Extra

’1’

Data Frame

Break Frame

Start

Bit

Extra

’1’

Data Frame

Next Data Frame

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