MC68HC908GR16 Datasheet, PDF (171/310 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Enhanced Serial Communications Interface (ESCI) Module
Functional Description
14.4.3.4 Framing Errors
If the data recovery logic does not detect a logic 1 where the stop bit should be in
an incoming character, it sets the framing error bit, FE, in SCS1. A break character
also sets the FE bit because a break character has no stop bit. The FE bit is set at
the same time that the SCRF bit is set.
14.4.3.5 Baud Rate Tolerance
A transmitting device may be operating at a baud rate below or above the receiver
baud rate. Accumulated bit time misalignment can cause one of the three stop bit
data samples to fall outside the actual stop bit. Then a noise error occurs. If more
than one of the samples is outside the stop bit, a framing error occurs. In most
applications, the baud rate tolerance is much more than the degree of
misalignment that is likely to occur.
As the receiver samples an incoming character, it resynchronizes the RT clock on
any valid falling edge within the character. Resynchronization within characters
corrects misalignments between transmitter bit times and receiver bit times.
Slow Data Tolerance
Figure 14-8 shows how much a slow received character 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.
MSB
STOP
RECEIVER
RT CLOCK
DATA
SAMPLES
Figure 14-8. Slow Data
For an 8-bit character, data sampling of the stop bit takes the receiver
9 bit times × 16 RT cycles + 10 RT cycles = 154 RT cycles.
With the misaligned character shown in Figure 14-8, the receiver counts 154
RT cycles at the point when the count of the transmitting device is 9 bit
times × 16 RT cycles + 3 RT cycles = 147 RT cycles.
The maximum percent difference between the receiver count and the
transmitter count of a slow 8-bit character with no errors is:
1----5---4-----–_{-----1---4---7-- × 100 = 4.54%}
154
For a 9-bit character, data sampling of the stop bit takes the receiver
10 bit times × 16 RT cycles + 10 RT cycles = 170 RT cycles.
MC68HC908GR16 — Rev. 1.0
MOTOROLA
Enhanced Serial Communications Interface (ESCI) Module
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Data Sheet
171