MC68HC908KX2 MOTOROLA [Motorola, Inc], MC68HC908KX2 Datasheet - Page 208

• Manufacturer Part Number: MC68HC908KX2
• Description: Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

Serial Communications Interface Module (SCI)
Technical Data
208
Software latency may allow an overrun to occur between reads of SCS1
and SCDR in the flag-clearing sequence.
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.
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.
Serial Communications Interface Module (SCI)
1 = Receive shift register full and SCRF = 1
0 = No receiver overrun
BYTE 1
BYTE 1
READ SCS1
READ SCDR
Figure 14-13. Flag Clearing Sequence
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 — Rev. 1.0
SCRF = 1
BYTE 1
OR = 0
READ SCDR
READ SCS1
BYTE 2
BYTE 2
SCRF = 1
BYTE 1
OR = 0
READ SCDR
READ SCS1
SCRF = 1
BYTE 2
OR = 0
Figure 14-13
BYTE 3
BYTE 3
READ SCDR
READ SCDR
READ SCS1
READ SCS1
SCRF = 1
SCRF = 1
BYTE 3
BYTE 3
OR = 1
OR = 0
shows the
BYTE 4
BYTE 4
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