iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC16/36
Rotary Encoder Design Tool
General Configuration
A design tool in the form of an Excel spreadsheet is
available to simplify configuration of the iC-TW8 in
rotary encoder applications. The design tool requires
answers to questions and input of desired applica-
tion specifications (in engineering units) to produce
the desired iC-TW8 configuration (CFG block regis-
ter values). These values can then be written directly
to the iC-TW8 over the SPI or 1-wire serial inter-
faces to configure the device.
Click on the General Configuration tab and start by
entering the operating voltage (Vdd) as 5.0 or 3.3. If
you are using an external crystal or other clock
source (recommended), enter its frequency. The op-
erating voltage and crystal frequency affect the cur-
rent drawn by the iC-TW8, which is calculated and
shown for reference. The signal path clock frequen-
cy (fcore)—which affects the maximum encoder
speed and the filter response—is shown for refer-
ence. In general, use the highest possible frequency
unless there is a need to minimize current.
In the design tool spreadsheet, cells with a blue
background are for user entry. Cells without a blue
background are fixed or calculated by the spread-
sheet and cannot be edited. Depending on answers
to previous questions, certain blue background cells
may be labeled “Ignore”. In this case, the entered
value has no effect on device operation. Register
values and internal variables calculated from user
input are shown in column F or I (typically) on the
spreadsheets. These correlate directly to the register
and variable descriptions in the iC-TW8 Program-
mer’s Reference document.
If you are not using an external oscillator or clock
source, enter the desired Clock Divider and Clock
Tuning Value for the internal oscillator.
Internal
Oscillator
Clock
Divider
fcore
Clock
Tuning
Value
When possible, legal responses to questions are pro-
vided (in parentheses). Entering values other than
the given responses may cause “Error” to appear for
the corresponding variable value in column H (typi-
cally). In this case, the Excel error “#VALUE!” is
displayed in the corresponding CFG register in the
CFG Block tab.
Figure 18: Internal Oscillator Configuration
The nominal frequency of the internal oscillator,
shown for reference, depends on the operating volt-
age. The actual frequency, however, varies from
unit to unit and over temperature. To compensate
for this, the oscillator frequency may be reduced by
a factor of 2 or 4 using the Clock Divider and also
tuned (raised) using the Clock Tuning Value (see
page 33). For initial configuration, enter the desired
divider and a tuning value of 0. The resulting signal
path clock frequency (fcore) is shown for reference.
Changing the Clock Tuning Value changes the nom-
inal calculated fcore. The actual fcore in any given
chip with the given Clock Tuning Value will vary
from the nominal calculated value due to normal
production tolerances.
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