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MC9S12VR-Family - Data Sheet - Freescale Semiconductor

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<strong>MC9S12VR</strong> <strong>Family</strong> Reference Manual, Rev. 2.8<br />

Serial Communication Interface (S12SCIV5)<br />

Figure 10-26 shows a burst of noise near the beginning of the start bit that resets the RT clock. The sample<br />

after the reset is low but is not preceded by three high samples that would qualify as a falling edge.<br />

Depending on the timing of the start bit search and on the data, the frame may be missed entirely or it may<br />

set the framing error flag.<br />

RXD<br />

Samples<br />

RT Clock<br />

RT Clock Count<br />

Reset RT Clock<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

Figure 10-26. Start Bit Search Example 5<br />

In Figure 10-27, a noise burst makes the majority of data samples RT8, RT9, and RT10 high. This sets the<br />

noise flag but does not reset the RT clock. In start bits only, the RT8, RT9, and RT10 data samples are<br />

ignored.<br />

RXD<br />

Samples<br />

RT Clock<br />

RT Clock Count<br />

Reset RT Clock<br />

10.4.6.4 Framing Errors<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT2<br />

RT3<br />

Figure 10-27. Start Bit Search Example 6<br />

If the data recovery logic does not detect a logic 1 where the stop bit should be in an incoming frame, it<br />

sets the framing error flag, FE, in SCI status register 1 (SCISR1). A break character also sets the FE flag<br />

because a break character has no stop bit. The FE flag is set at the same time that the RDRF flag is set.<br />

<strong>Freescale</strong> <strong>Semiconductor</strong> 335<br />

RT4<br />

RT5<br />

RT6<br />

Start Bit<br />

No Start Bit Found<br />

1 1 1 1 1 1 1 1 1 0 0 1 1 0 0 0 0 0 0 0 0<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT1<br />

RT2<br />

RT3<br />

RT4<br />

RT5<br />

RT6<br />

RT7<br />

RT7<br />

RT1<br />

Start Bit<br />

1 1 1 1 1 1 1 1 1 0 0 0 0 1 0<br />

1<br />

RT8<br />

RT1<br />

RT9<br />

RT1<br />

RT10<br />

RT1<br />

RT11<br />

RT1<br />

RT12<br />

RT1<br />

RT13<br />

RT1<br />

RT14<br />

RT1<br />

RT15<br />

RT1<br />

RT16<br />

RT1<br />

RT1<br />

LSB<br />

RT1<br />

LSB<br />

RT2<br />

RT1<br />

RT3

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