MC68HC705JP7 Datasheet, PDF (139/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Analog Subsystem
Port B Interaction with Analog Inputs
by a single V_SS return pin. This offset also provides a means to measure
the internal V_SS level regardless of the comparator offset to determine
N_OFF as described in 8.7 Voltage Measurement Methods. In either
case the OPT bit must be set in the COPR located at $1FF0 as in
Figure 8-12 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 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
be measured, but the logic state of the port B pins can be read from
location $0002.
8.14 Port B Pulldowns
All the port B pins have internal software programmable pulldown
devices available dependent on the state of the SWPDI bit in the mask
option register (MOR).
If the pulldowns are enabled, they will create an approximate 100 µA
load to any analog source connected to the pin. In some cases, the
analog source may be able to supply this current without causing any
MC68HC705JJ7 • MC68HC705JP7 — REV 4
MOTOROLA
Analog Subsystem
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