MC68HC908AS60ACFN FREESCALE [Freescale Semiconductor, Inc], MC68HC908AS60ACFN Datasheet - Page 199

• Manufacturer Part Number: MC68HC908AS60ACFN
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

OR — Receiver Overrun Bit
Software latency may allow an overrun to occur between reads of SCS1 and SCDR in the flag-clearing
sequence.
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.
NF — Receiver Noise Flag Bit
Freescale Semiconductor
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.
This clearable, read-only bit is set when the SCI detects noise on the RxD pin. NF generates an SCI
error CPU interrupt request if the NEIE bit in SCC3 is also set. Clear the NF bit by reading SCS1 and
then reading the SCDR. Reset clears the NF bit.
1 = Receive shift register full and SCRF = 1
0 = No receiver overrun
1 = Noise detected
0 = No noise detected
Figure 18-15
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
BYTE 1
BYTE 1
shows the normal flag-clearing sequence and an example of an overrun caused
READ SCS1
READ SCDR
SCRF = 1
Figure 18-15. Flag Clearing Sequence
BYTE 1
OR = 0
READ SCDR
READ SCS1
DELAYED FLAG CLEARING SEQUENCE
NORMAL FLAG CLEARING SEQUENCE
BYTE 2
BYTE 2
SCRF = 1
BYTE 1
OR = 0
READ SCDR
READ SCS1
SCRF = 1
BYTE 2
OR = 0
BYTE 3
BYTE 3
READ SCDR
READ SCDR
READ SCS1
READ SCS1
SCRF = 1
SCRF = 1
BYTE 3
BYTE 3
OR = 1
OR = 0
BYTE 4
BYTE 4
I/O Registers
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