XC886/888CLM
Serial Interfaces
User’s Manual
12-15
V1.3, 2010-02
Serial Interfaces, V 1.0
shows the operation in fractional divider mode with a reload value of
STEP = 8D
H
(factor of 141/256 = 0.55).
Figure 12-4
Fractional Divider Mode Timing
Note: In fractional divider mode, f
MOD
will have a maximum jitter of one f
DIV
clock period.
In general, the fractional divider mode can be used to generate an average output clock
frequency with higher accuracy than the normal divider mode.
Normal Divider Mode
Setting bit FDM in register FDCON to 1 configures the fractional divider to normal divider
mode, while at the same time disables baud rate generation (see
). Once the
fractional divider is enabled (FDEN = 1), it functions as an 8-bit auto-reload timer (with
no relation to baud rate generation) and counts up from the reload value with each input
clock pulse. Bit field RESULT in register FDRES represents the timer value, while bit
field STEP in register FDSTEP defines the reload value. At each timer overflow, an
overflow flag (FDCON.NDOV) will be set and an interrupt request generated. This gives
an output clock f
MOD
that is 1/n of the input clock f
DIV
, where n is defined by 256 - STEP.
The output frequency in normal divider mode is derived as follows:
(12.5)
shows the operation in normal divider mode with a reload value of
STEP = FD
H
. In order to get f
MOD
=
f
DIV
, STEP must be programmed with FF
H
.
RESULT
70
f
DIV
STEP = 8D
H
:
f
MOD
= 0.55 x
f
DIV
f
MOD
FD
8A
+8D
17
A4
31
BE
4B
D8
65
F2
7F
0C
+8D
+8D
+8D
+8D
+8D
+8D
+8D
+8D
+8D
+8D
+8D
+8D
STEP
-
256
1
DIV
f
MOD
f
x
=
*
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