MC68HC05JJ6 Datasheet, PDF (135/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

Analog Subsystem
NOTE:
The sample capacitor can be affected by excessive noise created with
respect to the device’s V_SS 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 V_SS to be sampled and
then shifted up past the comparator offset and the device offset caused
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 proper mask option must be selected 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.
MC68HC05JJ6/MC68HC05JP6 — Rev. 3.2
Freescale Semiconductor
Analog Subsystem
General Release Specification
135