ST72754 datasheet(31/144 Pages) STMICROELECTRONICS

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ST72774/ST727754/ST72734

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I/O PORTS (Cont’d)

4.1.4 Port B

The alternate functions are the I/O pins of the on-

chip DDC SCLD & SCDAD for PB0:1, the I/O pins

of the on-chip I2C SCLI & SCDAI for PB2:3, and 4

bits of port B bit can be used as the Analog source

to the Analog to Digital Converter.

Only one I/O line must be configured as an analog

input at any time. The user must avoid any

situation in which more than one I/O pin is selected

as an analog input simultaneously to avoid device

malfunction.

When the analog function is selected for an I/O pin,

the pull-up of the respective pin of Port B is

disconnected and the digital input is off.

All unused I/O lines should be tied to an

appropriate logic level (either VDD or VSS)

Since the ADC is on the same chip as the

microprocessor, the user should not switch heavily

loaded signals during conversion, if high precision

is required. Such switching will affect the supply

voltages used as analog references. the accuracy

of the conversion depends on the quality of the

power supplies (VDD and VSS). The user must take

special care to ensure that a well regulated

reference voltage is present on the VDD and VSS

pins (power supply variations must be less than

5V/ms). This implies, in particular, that a suitable

decoupling capacitor is used at the VDD pin.

Table 9. Port B Description

PORT B

I/O

Alternate Function

Input*

Output

Signal

Condition

PB0

Without pull-up

Open-drain

SCLD (input with CMOS schmitt trigger or

open drain output)

DDC enable

PB1

Without pull-up

Open-drain

SDAD (input with CMOS schmitt trigger or

open drain output)

DDC enable

PB2

Without pull-up

Open-drain

SCLI (input with CMOS schmitt trigger or

open drain output)

I2C enable

PB3

Without pull-up

Open-drain

SDAI (input with CMOS schmitt trigger or

open drain output)

I2C enable

PB4

With pull-up

Push-pull

Analog input (ADC) (without pull-up)

CH[2:0]=000 (ADCCSR)

PB5

With pull-up

Push-pull

Analog input (ADC) (without pull-up)

CH[2:0]=001 (ADCCSR)

PB6

With pull-up

Push-pull

Analog input (ADC) (without pull-up)

CH[2:0]=010 (ADCCSR)

10-bit output 1 (PWM)

OE0=1 (PWMOE)

PB7

With pull-up

Push-pull

Analog input (ADC) (without pull-up)

CH[2:0]=011 (ADCCSR)

10-bit output 2 (PWM)

OE1=1 (PWMOE)

*Reset state

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