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The initial level state of pull-up/pull-down input mode is high level if pull-up, and
low level if pull-down; when connecting the equipment, it is determined by the
external input level and load impedance.
Figure 17 I/O Structure in Input Mode
Read
V
DD
Input data
register
TTL Schmitt
trigger
I/O pin
V
SS
Output Mode
In the output mode, it can be set as push-pull output and open-drain output.
When GPIO is configured as the output pin, the output speed of the port can be
configured and the output drive mode (push-pull / open-drain) can be selected.
In output mode:
Schmitt trigger is opened,
Activate output buffer
By configuring the pull-up/pull-down register GPIOx_PUPD, select
whether to use pull-up/pull-down resistors
Push-pull mode:
-
Double MOS transistor works by turns and the output data register
can control the high and low level of I/O output
-
Read the finally written value through the output data register
GPIOx_ODATA
Open-drain mode:
-
Only N-MOS works, and the output data register can control I/O
output high resistance state or low level
-
The input data register GPIOx_IDATA captures the data on I/O pin
in each AHB clock cycle
-
Read the actual I/O state through the input data register
GPIOx_IDATA