ST72E331 datasheet(61/107 Pages) STMICROELECTRONICS

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ST72E331 ST72T331

SERIAL COMMUNICATIONS INTERFACE (Cont’d)

5.5.4.2 Transmitter

The transmitter can send data words of either 8 or

9 bits depending on the M bit status. When the M

bit is set, word length is 9 bits and the 9th bit (the

MSB) has to be stored in the T8 bit in the CR1 reg-

ister.

Character Transmission

During an SCI transmission, data shifts out least

significant bit first on the TDO pin. In this mode,

the DR register consists of a buffer (TDR) between

the internal bus and the transmit shift register (see

Figure 1.).

Procedure

– Select the M bit to define the word length.

– Select the desired baud rate using the BRR and

the ETPR registers.

– Set the TE bit to assign the TDO pin to the alter-

nate function and to send a idle frame as first

transmission.

– Access the SR register and write the data to

send in the DR register (this sequence clears the

TDRE bit). Repeat this sequence for each data to

be transmitted.

Clearing the TDRE bit is always performed by the

following software sequence:

1. An access to the SR register

2. A write to the DR register

The TDRE bit is set by hardware and it indicates:

– The TDR register is empty.

– The data transfer is beginning.

– The next data can be written in the DR register

without overwriting the previous data.

This flag generates an interrupt if the TIE bit is set

and the I bit is cleared in the CCR register.

When a transmission is taking place, a write in-

struction to the DR register stores the data in the

TDR register and which is copied in the shift regis-

ter at the end of the current transmission.

When no transmission is taking place, a write in-

struction to the DR register places the data directly

in the shift register, the data transmission starts,

and the TDRE bit is immediately set.

When a frame transmission is complete (after the

stop bit or after the break frame) the TC bit is set

and an interrupt is generated if the TCIE is set and

the I bit is cleared in the CCR register.

Clearing the TC bit is performed by the following

software sequence:

1. An access to the SR register

2. A write to the DR register

Note: The TDRE and TC bits are cleared by the

same software sequence.

Break Characters

Setting the SBK bit loads the shift register with a

break character. The break frame length depends

on the M bit (see Figure 2.).

As long as the SBK bit is set, the SCI send break

frames to the TDO pin. After clearing this bit by

software the SCI insert a logic 1 bit at the end of

the last break frame to guarantee the recognition

of the start bit of the next frame.

Idle Characters

Setting the TE bit drives the SCI to send an idle

frame before the first data frame.

Clearing and then setting the TE bit during a trans-

mission sends an idle frame after the current word.

Note: Resetting and setting the TE bit causes the

data in the TDR register to be lost. Therefore the

best time to toggle the TE bit is when the TDRE bit

is set i.e. before writing the next byte in the DR.

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