Freescale Semiconductor, Inc.
Serial Communications Interface (SCI)
5.9.1 Receiver flags
The SCI receiver has seven status flags, three of which can generate
interrupt requests. The status flags are set by the SCI logic in response
to specific conditions in the receiver. These flags can be read (polled) at
any time by software. Refer to Figure 5-3, which shows SCI interrupt
arbitration.
When an overrun takes place, the new character is lost, and the
character that was in its way in the parallel receive data register (RDR)
is undisturbed. RDRF is set when a character has been received and
transferred into the parallel RDR. The OR flag is set instead of RDRF if
overrun occurs. A new character is ready to be transferred into the RDR
before a previous character is read from the RDR.
The NF, FE and PF flags provide additional information about the
character in the RDR, but do not generate interrupt requests.
The receiver active flag (RAF) indicates that the receiver is busy.
The last receiver status flag and interrupt source come from the IDLE
flag. The RXD line is idle if it has constantly been at logic one for a full
character time. The IDLE flag is set only after the RXD line has been
busy and becomes idle. This prevents repeated interrupts for the time
RXD remains idle.
Technical Data
MC68HC11P2 — Rev 1.0
Serial Communications Interface (SCI)
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