
Analog Subsystem
Port B Interaction with Analog Inputs
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
General Release Specification
MOTOROLA
Analog Subsystem
135
NON-DISCLOSURE
AGREEMENT
REQUIRED
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 VSS pin such that it may appear to leak down or
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 VSS to be sampled and
then shifted up past the comparator offset and the device offset caused
by a single VSS return pin. This offset also provides a means to measure
the internal VSS level regardless of the comparator offset in order to
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 sample and hold circuit.
8.12 Port B Interaction with Analog Inputs
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 Inputs
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