Z51F6412
Product Specification
PS030302-0212
P R E L I M I N A R Y
101
The USART Receiver has three error flags which are Frame Error (FE), Data OverRun (DOR) and
Parity Error (PE). These error flags can be read from the USTATx register. As data received are
stored in the 2-level receive buffer, these error flags are also stored in the same position of receive
buffer. So, before reading received data from UDATAx register, read the USTATx register first which
contains error flags.
The Frame Error (FE) flag indicates the state of the first stop bit. The FE flag is zero when the stop
bit was correctly detected as one, and the FE flag is one when the stop bit was incorrect, ie detected
as zero. This flag can be used for detecting out-of-sync conditions between data frames.
The Data OverRun (DOR) flag indicates data loss due to a receive buffer full condition. A DOR
occurs when the receive buffer is full, and another new data is present in the receive shift register
which are to be stored into the receive buffer. After the DOR flag is set, all the incoming data are lost.
To prevent data loss or clear this flag, read the receive buffer.
The Parity Error (PE) flag indicates that the frame in the receive buffer had a Parity Error when
received. If Parity Check function is not enabled (UPM[1]=0), the PE bit is always read zero.
Note) The error flags related to receive operation are not used when USART is in spi mode.
11.7.9.3 Parity Checker
If Parity Bit is enabled (UPM[1]=1), the Parity Checker calculates the parity of the data bits in
incoming frame and compares the result with the parity bit from the received serial frame.
11.7.9.4 Disabling Receiver
In contrast to Transmitter, disabling the Receiver by clearing RXE bit makes the Receiver inactive
immediately. When the Receiver is disabled the Receiver flushes the receive buffer and the remaining
data in the buffer is all reset. The RXD pin is not overridden the function of USART, so RXD pin
becomes normal GPIO or primary function pin.
11.7.9.5 Asynchronous Data Reception
To receive asynchronous data frame, the USART includes a clock and data recovery unit. The Clock
Recovery logic is used for synchronizing the internally generated baud-rate clock to the incoming
asynchronous serial frame on the RXD pin.
The Data recovery logic samples and low pass filters the incoming bits, and this removes the noise
of RXD pin.
The next figure illustrates the sampling process of the start bit of an incoming frame. The sampling
rate is 16 times the baud-rate for normal mode, and 8 times the baud rate for Double Speed mode
(U2X=1). The horizontal arrows show the synchronization variation due to the asynchronous sampling
process. Note that larger time variation is shown when using the Double Speed mode.
