IC-TW8_16 Datasheet, PDF (49/67 Pages) IC-Haus GmbH – 16-BIT SIN/COS INTERPOLATOR

iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Aug 16, 2016 Page SC18/36
AB Configuration
In the AB Configuration tab, enter the input resolu-
tion produced by the wheel or magnet and sensor.
This value should be the number of Sin/Cos cycles
per mechanical revolution of the wheel, magnet, or
optical disc produced by the sensor connected to the
iC-TW8. Next enter the desired output resolution of
the encoder as an integer equal to the number of AB
cycles per revolution (CPR). This is also sometimes
called PPR (pulses per revolution) and is equivalent
to the number of “lines” in a non-interpolated opti-
cal encoder. Fractional output resolutions are possi-
ble using the iC-TW8, but are not supported by the
design tool.
With these values entered, the design tool calculates
and displays the minimum required iC-TW8 inter-
polation factor, the minimum post-AB divider, and
the effective overall interpolation factor. In most
applications, the post-AB divider is not required,
and the Minimum Divider value should be 1. In
general, this will be true if the Desired Output Reso-
lution is an integer multiple of the Input Resolution.
Specifically, Minimum Divider = 1 if
4 ≤ ^{4 ⋅ Desired Output Resolution} ≤ 16, 384
Input Resolution
and
4 ⋅ Desired Output Resolution _{is an integer.}
Input Resolution
For example, with an input resolution of 32 Sin/Cos
cycles per revolution, a desired output resolution of
16,384 CPR results in a minimum interpolation fac-
tor of 512 and a minimum divider of 1 since
4 ⋅16,384 _{= 2048}
32
In this case enter 1 for the divider override factor
and then actual interpolation = effective interpola-
tion = minimum interpolation and actual output res-
olution = desired output resolution. This is the usual
case.
4 ⋅16,384 _{= 2730.66}
24
If minimum divider ≠ 1, it is not possible to syn-
chronize the Z output with the states of the A and B
outputs. See Using the Post-AB Divider on page 32.
Impossible combinations of input and output resolu-
tion are flagged with red warnings.
Next, enter the desired maximum speed (speed lim-
it) for the encoder in RPM. The spreadsheet calcu-
lates and displays the closest actual speed limit that
can be implemented for the chosen output resolution
using the current crystal. The calculated actual
speed limit is shown in RPM, AB channel frequen-
cy, and AB edge frequency. Exceeding the actual
speed limit can be configured to activate the
FAULT output (pin 20). See Fault Configuration on
page 22 for more information.
For an enclosed encoder the maximum speed is usu-
ally dictated by the bearings used. For a modular
encoder (without bearings) or to use the full speed
capability of the iC-TW8, enter an arbitrarily large
value (such as 1,000,000 RPM) and the design tool
will default to the highest possible speed. The calcu-
lated actual speed limit is affected by the crystal
frequency and output resolution.
Because edge separation (the time between two con-
secutive AB edges) is the reciprocal of the AB out-
put frequency, the design tool also calculates this
value. Note that if the post-AB divider is used, the
minimum edge separation at standstill (dither) is
different than the minimum edge separation when
running at speed.
Next, enter the desired maximum adaption speed in
RPM. This is the speed above which auto adaption
of the AB signal path parameters ceases. The actual
maximum adaption speed depends on the input reso-
lution and crystal frequency. In general, set the de-
sired maximum adaption speed to an arbitrarily high
value (such as 1,000,000 RPM) to enable auto adap-
tion at all speeds. See Auto Adaption Configuration
on page 21 for more information.
If it is desired to achieve the same output resolution
using an input resolution of 24 cycles per revolu-
tion, however, a minimum interpolation of 2048 and
a minimum divider of 3 are required since
The warning fault speed depends on crystal fre-
quency and input resolution and can be configured
to activate the FAULT output. The warning fault
speed is always half the fatal fault speed. The fatal
fault speed is the ultimate speed limit of the iC-TW8
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