ST72C171 datasheet(31/152 Pages) STMICROELECTRONICS

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ST72C171

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I/O PORTS (Cont’d)

7.1.2.4 Analog Alternate Function

When the pin is used as an ADC input the I/O must

be configured as input, floating. The analog multi-

plexer (controlled by the ADC registers) switches

the analog voltage present on the selected pin to

the common analog rail which is connected to the

ADC input.

It is recommended not to change the voltage level

or loading on any port pin while conversion is in

progress. Furthermore it is recommended not to

have clocking pins located close to a selected an-

alog pin.

Warning: The analog input voltage level must be

within the limits stated in the Absolute Maximum

Ratings.

7.1.2.5 I/O Port Implementation

The hardware implementation on each I/O port de-

pends on the settings in the DDR and OR registers

and specific feature of the I/O port such as ADC In-

put (see Figure 26) or true open drain. Switching

these I/O ports from one state to another should

be done in a sequence that prevents unwanted

side effects. Recommended safe transitions are il-

lustrated in Figure 25. Other transitions are poten-

tially risky and should be avoided, since they are

likely to present unwanted side-effects such as

spurious interrupt generation.

Figure 25. Recommended I/O State Transition Diagram

with interrupt

INPUT

OUTPUT

no interrupt

INPUT

push-pull

open-drain

OUTPUT

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