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I/O ports

ST7FOXU0

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I/O ports

9.1

Introduction

The I/O ports allow data transfer. An I/O port can contain up to 8 pins. Each pin can be

programmed independently either as a digital input or digital output. In addition, specific pins

may have several other functions. These functions can include external interrupt, alternate

signal input/output for on-chip peripherals or analog input.

9.2

Functional description

A Data register (DR) and a Data Direction register (DDR) are always associated with each

port. The Option register (OR), which allows input/output options, may or may not be

implemented. The following description takes into account the OR register. Refer to the Port

Configuration table for device specific information.

An I/O pin is programmed using the corresponding bits in the DDR, DR and OR registers: bit

x corresponding to pin x of the port.

Figure 27 shows the generic I/O block diagram.

9.2.1

Input modes

Clearing the DDRx bit selects input mode. In this mode, reading its DR bit returns the digital

value from that I/O pin.

If an OR bit is available, different input modes can be configured by software: floating or pull-

up. Refer to I/O Port Implementation section for configuration.

Note:

Writing to the DR modifies the latch value but does not change the state of the input pin.

Do not use read/modify/write instructions (BSET/BRES) to modify the DR register.

External interrupt function

Depending on the device, setting the ORx bit while in input mode can configure an I/O as an

input with interrupt. In this configuration, a signal edge or level input on the I/O generates an

interrupt request via the corresponding interrupt vector (eix).

Falling or rising edge sensitivity is programmed independently for each interrupt vector. The

External Interrupt Control register (EICR) or the Miscellaneous register controls this

sensitivity, depending on the device.

Each external interrupt vector is linked to a dedicated group of I/O port pins (see pinout

description and interrupt section). If several I/O interrupt pins on the same interrupt vector

are selected simultaneously, they are logically combined. For this reason if one of the

interrupt pins is tied low, it may mask the others.

External interrupts are hardware interrupts. Fetching the corresponding interrupt vector

automatically clears the request latch. Changing the sensitivity of a particular external

interrupt clears this pending interrupt. This can be used to clear unwanted pending

interrupts.

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