Nuts and Volts - November 2011

SMILEY’S WORKSHOP ☺

chops. If you want to take a more humane and neuron-saving approach, then use macros with sensible names. We will take the latter approach and write the macros port_pin_activate_pullup(), port_pin_deactivate_pullup(), and port_pin_toggle() that we will discuss below.

Switching Between Input and Output

It is possible to get problematic intermediate states when changing between certain states: • From tri-state to output HIGH.

We will take care of this in the digitalio pin_mode() macro which will always deactivate the pull-up and set the output to 0 before setting the data direction to output.

■ FIGURE 7. Arduino ATmega328 pin mapping.

Setting a Pin to Tri-state

This is only odd because it indicates that there is a third state for a binary system which — by definition — only has two states. Tri-state is considered a third state for a pin; the first two being HIGH (Vcc) or LOW (GND). Tri-state means that the pin is disconnected from the voltage and can be considered no longer in the circuit to which it is attached. When the AVR system is reset, the digital I/O pins are in the tri-state condition.

Expanded Digitalio Functional Requirements Specification

The digitalio toolset will provide functions or macros to do the following: • Set a port pin mode to input or output.

Warning Arduino Pin 13 is Funky

While we are cataloging funkiness, let’s not forget to mention that digital I/O is a bit off on the Arduino pin 13 which is attached to an LED and resistor. This doesn’t have much effect if you want to use it as an output pin, but it does if you use it for input. If you activate the internal pull-up, then the high voltage will be about 1.7V rather than the expected 5V. The Arduino reference states “If you must use pin 13 as a digital input, use an external pull-down resistor.”

Expanding the avrtoolbox Digitalio Library

Now that we have a good idea about the underlying AVR digital I/O architecture, let’s expand our avrtoolbox libavr digitalio toolset to include some more useful functions and macros. We’ll demonstrate this with a chaser light application that reads the digital input from an eight-bit DIP switch and writes the digital output to eight LEDs configured as shown in the figures from last month’s workshop, except that now we will be using ports rather than the Arduino style pins.