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R01AN0168ED0101 Rev. 01.01 43
Application Note
Chapter 7 Performing Zero Point Detection
For each channel, one of its four ports can be assigned by ZISn for the
measurement input of its individual analogue comparator. Common to all
comparators are two selectable and adjustable reference sources
GRV, which
are defining the two different reference levels that can be assigned by
ZRSn.
Behind this analogue stuff, the digital filters are attached, one for each channel
with common settings
GFLn and GFDn.
7.2.2 ZPD Measurement Cycles
The ZPD units within ISM have their own clocking scheme. Therefore, the
clocking of ZPD is set independently from other ISM functionality by using the
settings of
GZF, GCS and GFD.
With these parameters, the Measurement Cycle of ZPD is defined as a
frequency
f
m
.
In detail, the
Measurement Cycle of ZPD looks like this:
Figure 7-8 The ZPD Measurement Cycle
Whenever the ZPD measurement is active (ZMTn is set for a channel), the
Measurement Cycles are continuously repeated. If at least one cycle detects
that the ZP is not reached, this flag is kept stored and will cause that after
stopping the measurement, the status of ZPD is “ZP not yet reached”.
During the time of
GCS, no measurements are taken, because this is the
analogue hardware settling time, where voltage jumps by possible port
switchings before the measurement start are skipped.
(1) Example how to set up the Measurement Cycle
In the previous examples of ZPD measurement, certain settings have been
assumed. Within this paragraph, the way to define these settings is described.
The example is defining a
Measurement Cycle of 144 µs with 16
measurements.
1. Having one
Measurement Cycle in the measurement window
– We select to use a
GCS delay of 1, and a filter depth GFD of 15.
This yields an amount of 16 prescaled clocks of ZPD.
– To fit the 16 clocks into 144 µs, a ZPD prescaled clock of 111 kHz is
required.
– If we assume a system clock of 80 MHz, a divider of 720 would be
fm
f(MACRO_CLK)
GZF FactorGCS GFD+
--------------------------------------------------------------------------------=
Delay GCS (here set to 6)
Filter depth GFD (here set to 3)
System Clock
GZF prescaled clock (here set to 0)
Measurement
sequence
Measurement Cycle = 1/f
m