MC68HC908AS32A Datasheet, PDF (184/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Serial Communications Interface (SCI)
receive a valid character that sets the SCRF bit before an idle condition can set the IDLE bit. Also, after
the IDLE bit has been cleared, a valid character must again set the SCRF bit before an idle condition
can set the IDLE bit. Reset clears the IDLE bit.
1 = Receiver input idle
0 = Receiver input active (or idle since the IDLE bit was cleared)
OR — Receiver Overrun Bit
This clearable, read-only bit is set when software fails to read the SCDR before the receive shift
register receives the next character. The OR bit generates an SCI error CPU interrupt request if the
ORIE bit in SCC3 is also set. The data in the shift register is lost, but the data already in the SCDR is
not affected. Clear the OR bit by reading SCS1 with OR set and then reading the SCDR. Reset clears
the OR bit.
1 = Receive shift register full and SCRF = 1
0 = No receiver overrun
Software latency may allow an overrun to occur between reads of SCS1 and SCDR in the flag-clearing
sequence. Figure 14-13 shows the normal flag-clearing sequence and an example of an overrun caused
by a delayed flag-clearing sequence. The delayed read of SCDR does not clear the OR bit because OR
was not set when SCS1 was read. Byte 2 caused the overrun and is lost. The next flag-clearing sequence
reads byte 3 in the SCDR instead of byte 2.
In applications that are subject to software latency or in which it is important to know which byte is lost
due to an overrun, the flag-clearing routine can check the OR bit in a second read of SCS1 after reading
the data register.
NORMAL FLAG CLEARING SEQUENCE
BYTE 1
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 1
BYTE 2
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 2
BYTE 3
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 3
BYTE 4
DELAYED FLAG CLEARING SEQUENCE
BYTE 1
BYTE 2
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 1
BYTE 3
READ SCS1
SCRF = 1
OR = 1
READ SCDR
BYTE 3
BYTE 4
Figure 14-13. Flag Clearing Sequence
MC68HC908AS32A Data Sheet, Rev. 2.0
184
Freescale Semiconductor