MC68HC908QC16_07 Datasheet, PDF (142/274 Pages) Freescale Semiconductor, Inc – Microcontrollers

Enhanced Serial Communications Interface (ESCI) Module
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.
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 ESCI error 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.
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 13-13 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
BYTE 2
BYTE 3
BYTE 4
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 1
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 2
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 3
DELAYED FLAG CLEARING SEQUENCE
BYTE 1
BYTE 2
BYTE 3
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 1
READ SCS1
SCRF = 1
OR = 1
READ SCDR
BYTE 3
Figure 13-13. Flag Clearing Sequence
BYTE 4
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 3
142
Freescale Semiconductor