ST20-GP1 datasheet(75/116 Pages) STMICROELECTRONICS

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ST20-GP1

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Figure 13.3 9-bit data frames

Transmission

Transmission begins at the next overflow of the divide-by-16 counter (see Figure 13.3 above),

provided that the Run bit is set and data has been loaded into the ASCTxBuffer. The transmitted

data frame consists of three basic elements:

•

the start bit

•

the data ﬁeld (8 or 9 bits, least signiﬁcant bit (LSB) ﬁrst, including a parity bit, if selected)

•

the stop bits (0.5, 1, 1.5 or 2 stop bits)

Data transmission is double buffered. When the transmitter is idle, the transmit data written into the

transmit buffer is immediately moved to the transmit shift register, thus freeing the transmit buffer

for the next data to be sent. This is indicated by the transmit buffer empty flag (TxBufEmpty) being

set. The transmit buffer can be loaded with the next data, while transmission of the previous data is

still going on.

The transmitter empty flag (TxEmpty) will be set at the beginning of the last data frame bit that is

transmitted, i.e. during the first system clock cycle of the first stop bit shifted out of the transmit shift

Reception

Reception is initiated by a falling edge on the data input pin (RXD), provided that the Run and

RxEnable bits are set. The RXD pin is sampled at 16 times the rate of the selected baud rate. A

majority decision of the first, second and third samples of the start bit determines the effective bit

value. This avoids erroneous results that may be caused by noise.

If the detected value is not a 0 when the start bit is sampled, the receive circuit is reset and waits

for the next falling edge transition of the RXD pin. If the start bit is valid, the receive circuit

continues sampling and shifts the incoming data frame into the receive shift register. For

subsequent data and parity bits, the majority decision of the seventh, eighth and ninth samples in

each bit time is used to determine the effective bit value.

For 0.5 stop bits, the majority decision of the third, fourth, and fifth samples during the stop bit is

used to determine the effective stop bit value.

For 1 and 2 stop bits, the majority decision of the seventh, eighth, and ninth samples during the

stop bits is used to determine the effective stop bit values.

1st

stop

bit

2nd

stop

bit

start

bit

(LSB)

9th

bit

• Data bit (D8)

• Parity bit

• Wake-up bit

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