Steppers on A4988 and DRV8825, on an Arduino Uno

// Steppers on A4988 and DRV8825: STEP, DIR, and nothing else.
//
//   pin 3  the A4988's STEP     pin 4  its DIR
//   pin 5  the DRV8825's STEP   pin 6  its DIR
//   both   VMOT on 12 V, the grounds to the Uno's GND, RST linked to SLP,
//          all three mode pins on 5 V
//   A4988  VDD on 5 V; its motor on 1A/1B and 2A/2B
//   DRV8825 its motor on A1/A2 and B1/B2
//
// A stepper driver takes two wires from a sketch. DIR says which way; each
// rising edge on STEP moves the motor one step. Everything else, the two coil
// currents and the order they go in, is the driver's job. That is the whole
// reason these boards exist, and why a sketch for one is a loop that toggles
// a pin. This one toggles two, one for each driver, at the same moment, so
// both get exactly the same pulses.
//
// # One pulse, two different steps
//
// Both drivers have their three mode pins tied high. On the A4988 that is
// 1/16 step; on the DRV8825 the same three pins mean 1/32. So 3200 pulses turn
// the top motor exactly once and the bottom one exactly half a turn. Moving a
// sketch from one board to the other without reading the table is a classic
// way to build a printer that prints at half size.
//
// # The link from RST to SLP
//
// RST floats on these boards and a floating reset holds the driver off. Take
// the link away and nothing moves: the outputs stay off and the board shows
// "off".
//
// # How fast
//
// 400 us a pulse at 1/16 is 156 full steps a second for the top motor, well
// under the thousand a NEMA 17 follows from a standstill. Try 20 us and watch
// the rotors fall behind and slip.

const int STEP_A = 3;
const int DIR_A = 4;
const int STEP_B = 5;
const int DIR_B = 6;

const unsigned int PULSES = 3200;   // one turn at 1/16, half a turn at 1/32
const unsigned int PERIOD_US = 400;

void turn(bool forward) {
  digitalWrite(DIR_A, forward ? HIGH : LOW);
  digitalWrite(DIR_B, forward ? HIGH : LOW);
  for (unsigned int i = 0; i < PULSES; i++) {
    digitalWrite(STEP_A, HIGH);
    digitalWrite(STEP_B, HIGH);
    delayMicroseconds(4);
    digitalWrite(STEP_A, LOW);
    digitalWrite(STEP_B, LOW);
    delayMicroseconds(PERIOD_US - 4);
  }
}

void setup() {
  pinMode(STEP_A, OUTPUT);
  pinMode(DIR_A, OUTPUT);
  pinMode(STEP_B, OUTPUT);
  pinMode(DIR_B, OUTPUT);
  Serial.begin(115200);
  Serial.println("Mokxi Uno: 3200 pulses to an A4988 and a DRV8825");
  // The drivers ignore STEP for a millisecond or two after they wake.
  delay(10);
}

void loop() {
  Serial.println("forward: A4988 one turn, DRV8825 half a turn");
  turn(true);
  delay(800);
  Serial.println("and back");
  turn(false);
  delay(800);
}