Chapter 11 Serial Communication Interface (S12SCIV5)
MC9S12XDP512 Data Sheet, Rev. 2.17
508
Freescale Semiconductor
11.4.6.5
Baud Rate Tolerance
A transmitting device may be operating at a baud rate below or above the receiver baud rate. Accumulated
bittimemisalignmentcancauseoneofthethreestopbitdatasamples(RT8,RT9,andRT10)tofalloutside
the actual stop bit. A noise error will occur if the RT8, RT9, and RT10 samples are not all the same logical
values. A framing error will occur if the receiver clock is misaligned in such a way that the majority of the
RT8, RT9, and RT10 stop bit samples are a logic zero.
As the receiver samples an incoming frame, it re-synchronizes the RT clock on any valid falling edge
within the frame. Re synchronization within frames will correct a misalignment between transmitter bit
times and receiver bit times.
11.4.6.5.1
Slow Data Tolerance
Figure 11-28
shows how much a slow received frame can be misaligned without causing a noise error or
a framing error. The slow stop bit begins at RT8 instead of RT1 but arrives in time for the stop bit data
samples at RT8, RT9, and RT10.
Figure 11-28. Slow Data
Let’s take RTras receiver RT clock and RTt as transmitter RT clock.
For an 8-bit data character, it takes the receiver 9 bit times x 16 RTr cycles +7 RTr cycles = 151 RTr cycles
to start data sampling of the stop bit.
Withthemisalignedcharactershownin
Figure 11-28
,thereceivercounts151RTrcyclesatthepointwhen
the count of the transmitting device is 9 bit times x 16 RTt cycles = 144 RTt cycles.
The maximum percent difference between the receiver count and the transmitter count of a slow 8-bit data
character with no errors is:
((151 – 144) / 151) x 100 = 4.63%
For a 9-bit data character, it takes the receiver 10 bit times x 16 RTr cycles + 7 RTr cycles = 167 RTr cycles
to start data sampling of the stop bit.
Withthemisalignedcharactershownin
Figure 11-28
,thereceivercounts167RTrcyclesatthepointwhen
the count of the transmitting device is 10 bit times x 16 RTt cycles = 160 RTt cycles.
The maximum percent difference between the receiver count and the transmitter count of a slow 9-bit
character with no errors is:
((167 – 160) / 167) X 100 = 4.19%
MSB
Stop
R
R
R
R
R
R
R
R
R
Data
Samples
R
R
R
R
R
R
R
Receiver
RT Clock