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TC1796
Peripheral Units (Vol. 2 of 2)
Asynchronous/Synchronous Serial Interface (ASC)
User’s Manual
19-15
V2.0, 2007-07
ASC, V2.0
Using the Fractional Divider
When the fractional divider is selected, the input clock
f
DIV
for the baud rate timer is
derived from the module clock
f
ASC
by a programmable fractional divider. If
CON.FDE = 1, the fractional divider is activated. It divides
f
ASC
by a fraction of n/512 for
any value of n from 0 to 511. If n = 0, the divider ratio is 1, which means that
f
DIV
=
f
ASC
.
In general, the fractional divider allows the baud rate to be programmed with much better
accuracy than with the two fixed prescaler divider stages.
Note: In fractional divider mode, the clock
f
DIV
can have a maximum period jitter of one
f
ASC
clock period.
BG represents the content of the reload register bit field BG.BR_VALUE, taken as an
unsigned 13-bit integer. FDV represents the contents of the fractional divider register bit
field FDV.FD_VALUE, taken as an unsigned 9-bit integer.
Table 19-3
Asynchronous Baud Rate Formulas using the Fractional Input Clock
Divider
FDE
BRS
BG
FDV
Formula
1
–
0 … 8191 1 … 511
0
Table 19-4
Typical Asynchronous Baud Rates using the Fractional Input Clock
Divider
f
ASC
Desired
Baud Rate
BG
FDV
Resulting
Baud Rate
Deviation
75 MHz
115.2 kbit/s
17
H
12E
H
115.204 kbit/s
< 0.01%
57.6 kbit/s
2F
H
12E
H
57.602 kbit/s
< 0.01%
38.4 kbit/s
23
H
097
H
38.401 kbit/s
< 0.01%
19.2 kbit/s
AE
H
16F
H
19.200 kbit/s
< 0.01%
Baud rate
FDV
512
------------
f
ASC
16
BG 1
+
(
)
×
------------------------------------
×
=
Baud rate
f
ASC
16
BG 1
+
(
)
×
------------------------------------
=