ST7285C datasheet(27/117 Pages) STMICROELECTRONICS

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ST7285C

I/O PORTS (Cont’d)

4.1.3 I/O Port Implementation

On the ST7285C, the pull down is always absent,

the pull up exists only where an interrupt facility is

present (Ports C4, C5, D4, D5, F0, F1, F2, F3,

G3). On port A, the analog inputs are directly con-

trolled by the ADC.The I/O port register configura-

tions are reduced to the following.

4.1.3.1 Ports A0-A7, B0-B7, C0-C3, C6, C7, D0-

D3, D6, D7,E0-E7, F4-F7, G0-G2, G4-G7, H0-H5

These ports do not offer interrupt capabilities.

Note: Open drain I/O is implemented on I2C pins

(pins 19 and 20) and high voltage pins (PH3/4/5).

The design uses special I/O devices without P

channel, thus forbidding the push pull configura-

tion.

In this case there is neither pull up register nor op-

tion register. These registers do not exist and so

cannot be read or written to.

4.1.3.2 Ports C4, C5, D4, D5, F0-F3, G3

These ports offer interrupt capabilities.

In this case there is no pull up register since the

pull-up is present only when the interrupt feature is

selected. This register does not exist and so can-

not be read or written to.

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

er should be done in such a sequence as to pre-

vent unwanted side effects. Recommended safe

transitions are illustrated in Figure 14. Other tran-

sitions are potentially risky and should be avoided,

since they are likely to present unwanted side-ef-

fects such as spurious interrupt generation.

Figure 14. Recommended I/O State Transition Diagram

DDR

MODE

OPTION

input

no pull-up, no pull-down, no interrupt

output

push-pull (or open drain: see note)

DDR

MOD

OPTION

input

no pull-up, no pull-down,

no interrupt

input

interrupt, pull-up

output

open-drain, no pull-up

output

push-pull, no pull-up,

no pull-down

Interrupt

pull-up

push-pull

no pull-up

open-drain

no pull-up

no pull-down

INPUT

OUTPUT

no pull-up

no interrupt

INPUT

no pull-down

(Reset state)

= DDR, OR

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