MC68HC05JJ6 Datasheet, PDF (136/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

Analog Subsystem
8.14 Port B Pulldowns
All the port B pins have internal software programmable pulldown
devices available dependent on the software pulldown inhibit mask
option.
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
error due to the analog source output impedance. Since this may not
always be true, it is therefore best to disable port B pulldowns on those
pins used for analog input sources.
8.15 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_SS return for both the
digital and the analog subsystems on the device, currents in the digital
section may affect the analog ground reference within the device. This
can add voltage offsets to measured inputs or cause channel-to-channel
crosstalk.
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 during these times is recommended.
A noise reduction benefit can be gained with 0.1 µF bypass capacitors
from each analog input (PB4:PB1) to the V_SS pin. Also, try to keep all the
digital power supply or load currents from passing through any
conductors which are the return paths for an analog signal.
General Release Specification
136
Analog Subsystem
MC68HC05JJ6/MC68HC05JP6 — Rev. 3.2
Freescale Semiconductor