MC68HC908GR8A_07 Datasheet, PDF (142/260 Pages) Freescale Semiconductor, Inc – Microcontrollers

Serial Communications Interface (SCI) Module
13.4.3.5 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 13-8 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.
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 13-8, 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.
MSB
STOP
RECEIVER
RT CLOCK
DATA
SAMPLES
Figure 13-8. Slow Data
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.
With the misaligned character shown in Figure 13-8, the receiver counts 170 RT cycles at the point when
the count of the transmitting device is 10 bit times × 16 RT cycles + 3 RT cycles = 163 RT cycles.
The maximum percent difference between the receiver count and the transmitter count of a slow 9-bit
character with no errors is
-1---7---0-----^–----1----6---3-- ^{× 100 = 4.12%}
170
MC68HC908GR8A • MC68HC908GR4A Data Sheet, Rev. 5
142
Freescale Semiconductor