Datasheet
Functional Description
MC68HC908GR16A Data Sheet, Rev. 1.0
Freescale Semiconductor 153
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.
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
+10RTcycles=154RTcycles.
• 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:
• For a 9-bit character, data sampling of the stop bit takes the receiver 10 bit times × 16 RT cycles
+10RTcycles=170RTcycles.
• With the misaligned character shown in Figure 14-8, 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:
Fast Data Tolerance
Figure 14-9 shows how much a fast received character can be misaligned without causing a noise error
or a framing error. The fast stop bit ends at RT10 instead of RT16 but is still there for the stop bit data
samples at RT8, RT9, and RT10.
MSB STOP
RT1
RT2
RT3
RT4
RT5
RT6
RT7
RT8
RT9
RT10
RT11
RT12
RT13
RT14
RT15
RT16
DATA
SAMPLES
RECEIVER
RT CLOCK
154 147–
154
--------------------------
100× 4.54%=
170 163–
170
--------------------------
100× 4.12%=