iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC18/36
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.
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 the number of AB cycles per revolu-
tion (CPR). This is also sometimes called PPR
(pulses per revolution) and is equivalent to the num-
ber of “lines” in a non-interpolated optical encoder.
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, AB edge frequency, and as minimum time be-
tween edges. Exceeding the actual speed limit can
be configured to activate the FAULT output (pin
20). See Fault Configuration on page 22 for more
information.
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
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.
4Desired Output Resolution
4
16,384
Input Resolution
and
4Desired Output Resolution
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.
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
416,384
2048
32
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
at the selected input resolution using the current
crystal. Exceeding the fatal fault speed always acti-
vates the FAULT output. See Fault Configuration
on page 22 for more information.
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.
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
416,384
2730.66
24
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