M68ICS08GR datasheet(262/408 Pages) FREESCALE

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Serial Communications Interface (SCI)

Technical Data

MC68HC908GR8 — Rev 4.0

262

Serial Communications Interface (SCI)

MOTOROLA

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 18-13 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.

NF — Receiver Noise Flag Bit

This clearable, read-only bit is set when the SCI detects noise on the

PE1/RxD pin. NF generates an NF 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 = Noise detected

0 = No noise detected

FE — Receiver Framing Error Bit

This clearable, read-only bit is set when a logic 0 is accepted as the

stop bit. FE generates an SCI error CPU interrupt request if the FEIE

bit in SCC3 also is set. Clear the FE bit by reading SCS1 with FE set

and then reading the SCDR. Reset clears the FE bit.

1 = Framing error detected

0 = No framing error detected

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