AN2710

STR75x general-purpose I/O ports (GPIO)

13/30

2.1.1

General purpose I/O (GPIO)

During and just after reset the alternate functions are not active and the I/O ports are

configured in Input Floating mode (PxC2=0, PxC1=0, PxC0=1).

When configured as output, the value written to the I/O Data register is loaded into the

output latch. The output latch holds the data to be output. It is possible to use the output

driver in Push-Pull or Open-Drain mode (only the N-MOS is activated when outputting 0).

The input latch captures the data present on the I/O pin at every APB clock cycle.

A read access to the I/O Data register reads the input latch or the output latch depending on

whether the port bit is configured as input or output Open-Drain or Push-Pull.

All GPIO pins features weak internal pull-up and pull-down resistors which can or not be

activated when configured as inputs.

In all low-power modes, except for the Standby mode, GPIO states are preserved. In

Standby mode, all GPIOs are put in high impedance with the exception of the WKP_STDBY

pin which is kept in input mode.

Note:

Care must be taken when configuring an I/O port from one mode to another, because an

unexpected intermediate state could disturb the application. Program the registers using

only intermediate states that do not disturb your application. For instance, it is important to

know that in "analog input" mode, the Schmitt trigger output is forced to '0'.

2.1.2

Alternate functions (AF)

It is necessary to program the Port Bit Configuration Register before using a default

alternate function.

●

For alternate function inputs, the port must be configured in Input mode (floating, pull-

up or pull-down) and the input pin must be driven externally

Note:

It is also possible to emulate the AFI input pin by firmware by programming the GPIO

controller. In this case, the port should be configured in Alternate Function Output mode.

Table 3.

STR75x Port bit configuration table

Configuration mode

Input buffer

PxD Register

PxC2

Register

PxC1

Register

PxC0

Register

Read

access

Write

access

Input

Input floating (reset state)

Input floating

I/O pin

don’t care

0

0

1

Input floating

Input Floating

I/O pin

don’t care

0

1

0

Input pull-down

TTL Pull-Down

I/O pin

0

0

1

1

Input pull-up

TTL Pull-Up

I/O pin

1

0

1

1

Analog input

AIN

0

don’t care

0

0

0

Output

Output open-drain

TTL floating

I/O pin

0 or 1

1

0

0

Output push-pull

not used

last value

written

0 or 1

1

0

1

Alternate function open-drain

TTL floating

I/O pin

don’t care

1

1

0

Alternate function push-pull

TTL floating

I/O pin

don’t care

1

1

1