P912XDP512F0MPVR datasheet(507/1348 Pages) FREESCALE

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Chapter 11 Serial Communication Interface (S12SCIV5)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

507

Figure 11-26 shows a burst of noise near the beginning of the start bit that resets the RT clock. The sample

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

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

set the framing error ﬂag.

Figure 11-26. Start Bit Search Example 5

In Figure 11-27, a noise burst makes the majority of data samples RT8, RT9, and RT10 high. This sets the

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

ignored.

Figure 11-27. Start Bit Search Example 6

11.4.6.4

Framing Errors

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

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

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

Reset RT Clock

Samples

RT Clock

RT Clock Count

Start Bit

RXD

LSB

No Start Bit Found

Reset RT Clock

Samples

RT Clock

RT Clock Count

Start Bit

RXD

LSB

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