Although I’m interested in making an integrated board that incorporates an AVR microcontroller and one or more stepper drivers, it’s also a valid choice to design around a microcontroller that people already have.

For the Easy Driver-style boards, hookup varies depending on whether you want to control the microstep resolution with the Arduino (which requires 3 more pins).

The simplest hookup just uses 2 pins to control step and direction, with power coming from a separate supply:

There is also an example given for multiple steppers, for which you need to use a library:

Again, this just uses 2 pins per motor. If I want to use two steppers, and also use all the control pins on the drivers, this may cause problems down the line…

The Adafruit board uses I2C, making it very easy to control multiple motors:

Instead of using a latch and the Arduino’s PWM pins, we have a fully-dedicated PWM driver chip onboard. This chip handles all the motor and speed controls over I2C. Only two data pins (SDA & SCL in addition to the power pins GND & 5V) are required to drive the multiple motors, and since it’s I2C you can also connect any other I2C devices or shields to the same pins. This also makes it drop-in compatible with any Arduino, such as the Uno, Due, Leonardo and Mega R3.

Either way, for now, I should be able to get away with a standard Arduino Uno, and may be able to specify a smaller board like the Pro Mini, or maybe, for version 2, an ATtiny-based DIY board if I need only a few pins.