68HC705SP7 datasheet(135/230 Pages) FREESCALE

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Analog Subsystem

Port B Interaction with Analog Inputs

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

General Release Specification

Analog Subsystem

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

determine NOFF as described in 8.7 Voltage Measurement Methods.

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

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

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