STR75x general-purpose I/O ports (GPIO)

AN2710

14/30

And obviously, the corresponding port should not be driven externally as it will be driven by

the firmware using the GPIO controller.

●

For alternate function outputs, the port must be configured in Alternate Function Output

mode (push-pull or open-drain).

●

For bidirectional alternate functions, the port bit must be configured in Alternate

Function Output mode (push-pull or open-drain). In this case the input driver is

configured in input floating mode

Configuring a port bit as Alternate Function Output will disconnect the output latch and

connect the pin to the output signal of an on-chip peripheral.

If firmware configures a GPIO pin as Alternate Function Output, but no peripheral output

alternate function exists for that pin (refer to the datasheet pin description table), its output is

not specified.

Special case of SSP bidirectional alternate functions

When using the SSP, the MISO, MOSI, NSS and SCK alternate functions consist of

bidirectional alternate functions. They must be configured as alternate function output

through the Port Configuration register:

●

When configuring the SSP in master mode, the MISO pin is automatically used as an

alternate function input and the output driver is automatically disabled (even if still

programmed as alternated function output in the Port Configuration registers).

In addition, when configured in master mode, the MOSI pin is always driven (never left

Hi-Z) even if the SSP is in idle mode (no transmission)

●

When configuring the SSP in slave mode, the MOSI, SCK and NSS pins are

automatically configured as alternate functions inputs and the output drivers are

automatically disabled (even if still programmed as alternated function outputs in the

Port Configuration registers).

In addition, when configured in slave mode, the MISO pin is left Hi-Z when the NSS pin

is high or when the SOD control bit (Slave Output Disable) is set.

reference manual)

Consequently, care must be taken when configuring SDA and SCL as alternate function

open-drain in order not to create parasitic falling edges.

The states to avoid are:

●

output 0

●

input pull-down

●

analog input (because the output of the Schmitt trigger goes to 0)