ST72311 Datasheet, PDF (52/94 Pages) STMicroelectronics – 8-BIT MCU WITH 8 TO 16K ROM/OTP/EPROM, 384 TO 512 BYTES RAM, ADC, WDG, SCI, SPI AND 2 TIMERS

ST72311
SERIAL COMMUNICATIONS INTERFACE (Cont’d)
4.4.4.3 Receiver
Overrun Error
The SCI can receive data words of either 8 or 9
bits. When the M bit is set, word length is 9 bits
and the MSB is stored in the R8 bit in the CR1 reg-
ister.
Character reception
During a SCI reception, data shifts in least signifi-
cant bit first through the RDI pin. In this mode, DR
register consists in a buffer (RDR) between the in-
ternal bus and the received shift register (see Fig-
ure 32).
Procedure
– Select the M bit to define the word length.
– Select the desired baud rate using the BRR and
the ERPR registers.
– Set the RE bit, this enables the receiver which
begins searching for a start bit.
When a character is received:
– The RDRF bit is set. It indicates that the content
of the shift register is transferred to the RDR.
– An interrupt is generated if the RIE bit is set and
the I bit is cleared in the CCR register.
– The error flags can be set if a frame error, noise
or an overrun error has been detected during re-
ception.
Clearing the RDRF bit is performed by the following
software sequence done by:
1. An access to the SR register
2. A read to the DR register.
The RDRF bit must be cleared before the end of the
reception of the next character to avoid an overrun
error.
Break Character
When a break character is received, the SPI han-
dles it as a framing error.
Idle Character
When a idle frame is detected, there is the same
procedure as a data received character plus an in-
terrupt if the ILIE bit is set and the I bit is cleared in
the CCR register.
An overrun error occurs when a character is re-
ceived when RDRF has not been reset. Data can
not be transferred from the shift register to the
TDR register as long as the RDRF bit is not
cleared.
When a overrun error occurs:
– The OR bit is set.
– The RDR content will not be lost.
– The shift register will be overwritten.
– An interrupt is generated if the RIE bit is set and
the I bit is cleared in the CCR register.
The OR bit is reset by an access to the SR register
followed by a DR register read operation.
Noise Error
Oversampling techniques are used for data recov-
ery by discriminating between valid incoming data
and noise.
When noise is detected in a frame:
– The NF is set at the rising edge of the RDRF bit.
– Data is transferred from the Shift register to the
DR register.
– No interrupt is generated. However this bit rises
at the same time as the RDRF bit which itself
generates an interrupt.
The NF bit is reset by a SR register read operation
followed by a DR register read operation.
Framing Error
A framing error is detected when:
– The stop bit is not recognized on reception at the
expected time, following either a de-synchroni-
zation or excessive noise.
– A break is received.
When the framing error is detected:
– the FE bit is set by hardware
– Data is transferred from the Shift register to the
DR register.
– No interrupt is generated. However this bit rises
at the same time as the RDRF bit which itself
generates an interrupt.
The FE bit is reset by a SR register read operation
followed by a DR register read operation.
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