MC908MR16CFUE Datasheet, PDF (175/282 Pages) Freescale Semiconductor, Inc – On-chip programming firmware for use with host personal computer, Clock generator module (CGM)

I/O Registers
IDLE — Receiver Idle Bit
This clearable, read-only bit is set when 10 or 11 consecutive 1s appear on the receiver input. IDLE
generates an SCI error CPU interrupt request if the ILIE bit in SCC2 is also set. Clear the IDLE bit by
reading SCS1 with IDLE set and then reading the SCDR. After the receiver is enabled, it must 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 13-12 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.
NORMAL FLAG CLEARING SEQUENCE
BYTE 1
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 1
BYTE 2
BYTE 3
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 2
DELAYED FLAG CLEARING SEQUENCE
READ SCS1
SCRF = 1
OR = 0
READ SCDR
BYTE 3
BYTE 4
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 13-12. Flag Clearing Sequence
BYTE 4
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor
175