iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC22/36
The Correction Configuration controls how the auto
adaption corrections are applied to the AB signal
path parameters during operation. First, select linear
or exponential correction mode. Linear correction
mode is the recommended selection and provides
the smoothest correction; parameter corrections are
applied one increment per correction cycle. This
results in the least disturbance to the interpolator
output when the auto adaption corrections are made.
FAULT Pin Configuration
In the FAULT Pin Configuration tab, select whether
the FAULT output (pin 20) is active high or active
low. Active low is the recommended since this is the
startup default of the iC-TW8. If active high is cho-
sen, the FAULT output will be active (high) at
startup until configured to active low polarity during
the startup process.
The FAULT output can be configured to stay active
for a time after a fault condition has cleared to en-
hance observation of transient fault conditions. The
amount of time by which the output is prolonged is
fixed and inversely proportional to crystal frequen-
cy. Next, choose whether or not the AB outputs
should be stopped when the FAULT output is ac-
tive.
Faster correction is available in exponential correc-
tion mode which allows selection of the desired cor-
rection step size. Small correction steps correct 25%
of the parameter error every correction cycle; medi-
um correction steps correct 50% of the parameter
error every correction cycle; large correction steps
correct 75% of the parameter error every correction
cycle. Experimentation may be required to deter-
mine the optimal correction configuration.
iC-TW8 fault conditions can be configured to either
activate or latch the FAULT output. Faults which do
not latch the FAULT output only activate the
FAULT output for the time during which the condi-
tion is active (subject to prolonging, as explained
above). Faults which latch the FAULT output are
cleared at restart or may be cleared by writing to the
STAT_SP, STAT_EE, or FLT_STAT registers us-
ing one of the serial ports (see Programmer’s Refer-
ence).
The correction timebase determines the rate at
which auto adaption corrections are applied. In gen-
eral, enter 0 for the desired correction timebase and
the design tool confirms the closest possible actual
timebase using the current crystal. This results in
auto adaption corrections being applied as quickly
as possible. Larger timebase values provide slower
response.
Lastly, select whether or not the auto adapted pa-
rameter values are written to the EEPROM during
operation (auto store) and used at the next restart. If
auto store is enabled, enter the desired digital offset
and gain match thresholds; the design tool confirms
the closest available (actual) values. These thresh-
olds are the levels of change in the respective pa-
rameters that must be achieved by auto adaption to
cause new values to be written to EEPROM. Since
most EEPROMS are only guaranteed for a finite
number of write cycles, these thresholds must be set
high enough to minimize unnecessary EEPROM
writes. If auto store is disabled, these thresholds are
ignored.
A crystal fault is active if the iC-TW8 is configured
to use an external crystal or clock source (see page
16) and the expected external signal is not present.
In this case, the iC-TW8 reverts to using its internal
oscillator. Once an external clock signal becomes
available, the iC-TW8 switches back to using the
external oscillator and clears the crystal fault. Clear-
ing the crystal fault also de-activates the FAULT
output if the crystal fault is not configured to latch.
An EEPROM fault is active if the EEPROM has not
been initialized, there is a hardware or communica-
tion problem with the EEPROM, or if any of the
internal checksums are invalid. It is recommended
to latch EEPROM faults as they can result in un-
defined startup conditions.
A fatal operational fault is active if the instantane-
ous sensor input velocity is greater than the fatal
fault speed shown on the AB configuration tab (see
page 18), the filter lag is too large (see page 19), or
the AB output is more than half an input cycle be-
hind the sensor input position due to prolonged op-
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