August 27, 1998
GENERAL RELEASE SPECIFICATION
15.9.1 Port B Pins As Inputs
All the Port B pins will power up as inputs or return to inputs following 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 to be read from location $0002.
15.10 NOISE SENSITIVITY
In addition to the normal effects of electrical noise on the analog input signal there
can also be other noise related effects caused by the digital-to-analog interface.
Since there is only one V return for both the digital and the analog subsystems
SS
on the device, currents in the digital section may affect the analog ground refer-
ence within the device. This can add voltage offsets to measured inputs or cause
channel-to-channel crosstalk.
In order to reduce the impact of these effects, there should be no switching of
heavy I/O currents to or from the device while there is a critical analog conversion
or voltage comparison in process. Limiting switched I/O currents to 2 to 4 mA dur-
ing these times is recommended.
A noise reduction benefit can be gained with 0.1µF bypass capacitors from each
analog input (PB7:4) to the V pin. Also, try to keep all the digital power supply or
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load currents from passing through any conductors which are the return paths for
an analog signal.
MC68HC05SB7
REV 2.1
ANALOG SUBSYSTEM
MOTOROLA
15-31
FREESCALE [ Freescale ]
