185
AT90PWM216/316 [DATASHEET]
7710H–AVR–07/2013
The receive function example reads all the I/O Registers into the Register File before any computation is done.
This gives an optimal receive buffer utilization since the buffer location read will be free to accept new data as early
as possible.
17.7.3
Receive Complete Flag and Interrupt
The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXC) flag indicates if there are unread data present in the receive buffer. This flag is one
when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e., does not contain any
unread data). If the Receiver is disabled (RXEN = 0), the receive buffer will be flushed and consequently the RXC
bit will become zero.
When the Receive Complete Interrupt Enable (RXCIE) in UCSRB is set, the USART Receive Complete interrupt
will be executed as long as the RXC flag is set (provided that global interrupts are enabled). When interrupt-driven
data reception is used, the receive complete routine must read the received data from UDR in order to clear the
RXC flag, otherwise a new interrupt will occur once the interrupt routine terminates.
17.7.4
Receiver Error Flags
The USART Receiver has three error flags: Frame Error (FE), Data OverRun (DOR) and Parity Error (UPE). All
can be accessed by reading UCSRA. Common for the error flags is that they are located in the receive buffer
together with the frame for which they indicate the error status. Due to the buffering of the error flags, the UCSRA
must be read before the receive buffer (UDR), since reading the UDR I/O location changes the buffer read location.
Another equality for the error flags is that they can not be altered by software doing a write to the flag location.
However, all flags must be set to zero when the UCSRA is written for upward compatibility of future USART imple-
mentations. None of the error flags can generate interrupts.
The Frame Error (FE) flag indicates the state of the first stop bit of the next readable frame stored in the receive
buffer. The FE flag is zero when the stop bit was correctly read (as one), and the FE flag will be one when the stop
bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions, detecting break conditions and
protocol handling. The FE flag is not affected by the setting of the USBS bit in UCSRC since the Receiver ignores
all, except for the first, stop bits. For compatibility with future devices, always set this bit to zero when writing to
UCSRA.
The Data OverRun (DOR) flag indicates data loss due to a receiver buffer full condition. A Data OverRun occurs
when the receive buffer is full (two characters), it is a new character waiting in the Receive Shift Register, and a
new start bit is detected. If the DOR flag is set there was one or more serial frame lost between the frame last read
from UDR, and the next frame read from UDR. For compatibility with future devices, always write this bit to zero
when writing to UCSRA. The DOR flag is cleared when the frame received was successfully moved from the Shift
Register to the receive buffer.
The following example (See
Figure 17-5.) represents a Data OverRun condition. As the receive buffer is full with
CH1 and CH2, CH3 is lost. When a Data OverRun condition is detected, the OverRun error is memorized. When
the two characters CH1 and CH2 are read from the receive buffer, the DOR bit is set (and not before) and RxC
remains set to warn the application about the overrun error.