9 I/O PORTS
The I/O ports offer different functional modes:
– transfer of data through digital inputs and outputs
and for specific pins:
– external interrupt generation
– alternate signal input/output for the on-chip pe-
An I/O port contains up to 8 pins. Each pin can be
programmed independently as digital input (with or
without interrupt generation) or digital output.
9.2 FUNCTIONAL DESCRIPTION
Each port has 2 main registers:
– Data Register (DR)
– Data Direction Register (DDR)
and one optional register:
– Option Register (OR)
Each I/O pin may be programmed using the corre-
sponding register bits in the DDR and OR regis-
ters: bit X corresponding to pin X of the port. The
same correspondence is used for the DR register.
The following description takes into account the
OR register, (for specific ports which do not pro-
vide this register refer to the I/O Port Implementa-
tion section). The generic I/O block diagram is
shown in Figure 29
9.2.1 Input Modes
The input configuration is selected by clearing the
corresponding DDR register bit.
In this case, reading the DR register returns the
digital value applied to the external I/O pin.
Different input modes can be selected by software
through the OR register.
1. Writing the DR register modifies the latch value
but does not affect the pin status.
2. When switching from input to output mode, the
DR register has to be written first to drive the cor-
rect level on the pin as soon as the port is config-
ured as an output.
3. Do not use read/modify/write instructions (BSET
or BRES) to modify the DR register
External interrupt function
When an I/O is configured as Input with Interrupt,
an event on this I/O can generate an external inter-
rupt request to the CPU.
Each pin can independently generate an interrupt
request. The interrupt sensitivity is independently
programmable using the sensitivity bits in the
Each external interrupt vector is linked to a dedi-
cated group of I/O port pins (see pinout description
and interrupt section). If several input pins are se-
lected simultaneously as interrupt sources, these
are first detected according to the sensitivity bits in
the EICR register and then logically ORed.
The external interrupts are hardware interrupts,
which means that the request latch (not accessible
directly by the application) is automatically cleared
when the corresponding interrupt vector is
fetched. To clear an unwanted pending interrupt
by software, the sensitivity bits in the EICR register
must be modified.
9.2.2 Output Modes
The output configuration is selected by setting the
corresponding DDR register bit. In this case, writ-
ing the DR register applies this digital value to the
I/O pin through the latch. Then reading the DR reg-
ister returns the previously stored value.
Two different output modes can be selected by
software through the OR register: Output push-pull
DR register value and output pin status:
9.2.3 Alternate Functions
When an on-chip peripheral is configured to use a
pin, the alternate function is automatically select-
ed. This alternate function takes priority over the
standard I/O programming.
When the signal is coming from an on-chip periph-
eral, the I/O pin is automatically configured in out-
put mode (push-pull or open drain according to the
When the signal is going to an on-chip peripheral,
the I/O pin must be configured in input mode. In
this case, the pin state is also digitally readable by
addressing the DR register.
Note: Input pull-up configuration can cause unex-
pected value at the input of the alternate peripheral
input. When an on-chip peripheral use a pin as in-
put and output, this pin has to be configured in in-
put floating mode.