ST72C254G2 datasheet(33/140 Pages) STMICROELECTRONICS

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I/O PORTS (Cont’d)

CAUTION: The alternate function must not be ac-

tivated as long as the pin is configured as input

with interrupt, in order to avoid generating spurious

interrupts.

Analog alternate function

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

must be configured as floating input. The analog

multiplexer (controlled by the ADC registers)

switches the analog voltage present on the select-

ed pin to the common analog rail which is connect-

ed 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 maxi-

mum ratings.

10.3 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 or true open drain.

Switching these I/O ports from one state to anoth-

er should be done in a sequence that prevents un-

wanted side effects. Recommended safe transi-

tions are illustrated in Figure 22 Other transitions

are potentially risky and should be avoided, since

they are likely to present unwanted side-effects

such as spurious interrupt generation.

Figure 22. Interrupt I/O Port State Transitions

The I/O port register configurations are summa-

rized as follows.

Interrupt Ports

PA7, PA5, PA3:0, PB7:0, PC5:0 (with pull-up)

True Open Drain Interrupt Ports

PA6, PA4 (without pull-up)

Table 8. Port Configuration

MODE

DDR

floating input

pull-up interrupt input

open drain output

push-pull output

MODE

DDR

floating input

floating interrupt input

open drain (high sink ports)

floating/pull-up

interrupt

INPUT

floating

(reset state)

INPUT

open-drain

OUTPUT

push-pull

OUTPUT

= DDR, OR

Port

Pin name

Input (DDR = 0)

Output (DDR = 1)

OR = 0

OR = 1

OR = 0

OR = 1

High-Sink

Port A

PA7

floating

pull-up interrupt

open drain

push-pull

Yes

PA6

floating

floating interrupt

true open-drain

PA5

floating

pull-up interrupt

open drain

push-pull

PA4

floating

floating interrupt

true open-drain

PA3:0

floating

pull-up interrupt

open drain

push-pull

Port B

PB7:0

floating

pull-up interrupt

open drain

push-pull

Port C

PC7:0

floating

pull-up interrupt

open drain

push-pull

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