Freescale Semiconductor, Inc.
Analog Subsystem
Port B Interaction with Analog Inputs
the channel selection are changed on the same write cycle, the sample
may be corrupted during the switching transitions.
NOTE: The sample capacitor can be affected by excessive noise created with
respect to the device’s V pin such that it may appear to leak down or
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charge up depending on the voltage level stored on the sample
capacitor. It is recommended to avoid switching large currents through
the port pins while a voltage is to remain stored on the sample capacitor.
The additional option of adding an offset voltage to the bottom of the
sample capacitor allows unknown voltages near V to be sampled and
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then shifted up past the comparator offset and the device offset caused
by a single V return pin. This offset also provides a means to measure
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the internal V level regardless of the comparator offset in order to
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determine N
as described in 8.7 Voltage Measurement Methods.
OFF
In either case the OPT bit must be set in the COPR located at $1FF0 and
the VOFF bit must be set in the ASR. It is not necessary to switch the
VOFF bit during conversions, since the offset is controlled by the HOLD
and DHOLD bits when the VOFF is active. Refer to 8.3 Analog
Multiplex Register for more details on the design and decoding of the
sample and hold circuit.
8.12 Port B Inte ra c tion with Ana log Inp uts
The analog subsystem is connected directly to the port B I/O pins without
any intervening gates. It is, therefore, possible to measure the voltages
on port B pins set as inputs or to have the analog voltage measurements
corrupted by port B pins set as outputs.
8.13 Port B Pins As Inp uts
All the port B pins will power up as inputs or return to inputs after a reset
of the device since the bits in the port B data direction register will be
reset.
If any port B pins are to be used for analog voltage measurements, they
should be left as inputs. In this case, not only can the voltage on the pin
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
General Release Specification
Analog Subsystem
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