MC68HC908GZ8 FREESCALE [Freescale Semiconductor, Inc], MC68HC908GZ8 Datasheet - Page 202

• Manufacturer Part Number: MC68HC908GZ8
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Enhanced Serial Communications Interface (ESCI) Module
OR — Receiver Overrun Bit
202
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.
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 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.
Software latency may allow an overrun to occur between reads of SCS1 and SCDR in the flag-clearing
sequence.
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.
1 = Receiver input idle
0 = Receiver input active (or idle since the IDLE bit was cleared)
1 = Receive shift register full and SCRF = 1
0 = No receiver overrun
Figure 15-14
BYTE 1
BYTE 1
shows the normal flag-clearing sequence and an example of an overrun
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
READ SCS1
READ SCDR
SCRF = 1
Figure 15-14. 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
Freescale Semiconductor