s908gz60cfa Freescale Semiconductor, Inc, s908gz60cfa Datasheet - Page 200

• Manufacturer Part Number: s908gz60cfa
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Enhanced Serial Communications Interface (ESCI) Module
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
200
Figure 14-8
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
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:
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
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:
shows how much a slow received character can be misaligned without causing a noise
RECEIVER
RT CLOCK
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
MSB
Figure 14-8. Slow Data
154 147
------------------------- -
170 163
------------------------- -
154
170
Figure
Figure
–
–
14-8, the receiver counts 154 RT cycles at the point
×
14-8, the receiver counts 170 RT cycles at the point
×
100
100
SAMPLES
=
=
DATA
4.54%
4.12%
STOP
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