Designing and building a clock from scratch
Arduino Programming
Need to integrate the following parts:
- Nema 17 style stepper motor, 200 steps per rotation
- Adafruit motor shield
- DS3231 RTC
After I figured out that I could execute a function every certain number of milliseconds using the arduino-timer.h library, I got the stepper making one revolution every minute using the stepper’s interleave stepping and making one step every 150ms. Now, I have read that the built-in timer on the Arduino is not very accurate, so I got an RTC (Real Time Clock) to keep accurate time. I went down a few dead-ends trying to figure out how to adjust the number of steps based on the actual time from the RTC. Initially I was reading the time from the clock and checking the number of steps every minute/hour/day.
I started looking into using the SQW (square wave output) pin from the RTC. The example code I found for using the square wave function of the RTC used that signal to step the system time on the Arduino and keep it synced with the RTC. But, me being me, I ignored the obvious solution and insisted on trying to keep track of seconds myself using a counter. The DS3231 can only output a 1Hz, 1.024 kHz, 4.096 kHz, or 8.192 kHz signal. The only one that made sense was the 1 Hz signal, and make 20 steps every three seconds for a nice integer number of steps per rotation, which is a weird way for a clock to move.
Then I realized, duh, if the Arduino’s time-keeping was being kept in sync with the RTC, then I could go back to using the arduino-timer library to step the motor every 150ms, and it should be accurate because the Arduino’s internal time was being kept accurate by the RTC. Done!
Eventually, I’ll add some routines to automatically set the clock to the exact time by quickly rotating the stepper motor by some calculated number of steps based on the time set on the RTC.
Here is my code, and the test output for 53 minutes.
//Set Board to Arduino Uno
//Set Port to SLAB_USB to UART
#include <Wire.h>
#include <Adafruit_MotorShield.h>
#include <arduino-timer.h>
#include "RTClib.h"
#include <time.h>
RTC_DS3231 rtc;
auto timer = timer_create_default();
// Pin receiving the one-pulse-per-second signal from the RTC.
// This should be an interrupt-capable pin.
const uint8_t pin1pps = 2;
// Create the motor shield object with the default I2C address
// Connect a stepper motor with 200 steps per revolution (1.8 degree)
// to motor port #2 (M3 and M4)
Adafruit_MotorShield AFMS = Adafruit_MotorShield();
Adafruit_StepperMotor *myMotor = AFMS.getStepper(200, 2);
unsigned long target_minute;
long target_minute_steps;
long total_steps = 0L;
long start_steps;
int interval = 150;
void time_check(int expected_steps) {
int x;
int steps;
Serial.println("time check!");
Serial.print("expected ");
Serial.println(expected_steps);
Serial.print("steps ");
Serial.println(total_steps);
}
bool one_step(void *) {
myMotor->onestep(FORWARD, INTERLEAVE);
total_steps++;
// From here on, we only use the standard C timing functions.
// time() returns the current time as a single number of type time_t,
// this is the number of seconds elapsed since a reference "epoch".
time_t now = time(nullptr);
// Serial.print("now ");
// Serial.println(now);
// Serial.print("target_minute ");
// Serial.println(target_minute);
// Serial.print("target_minute_steps ");
// Serial.println(target_minute_steps);
// Serial.print("total_steps ");
// Serial.println(total_steps);
if (now == target_minute) {
target_minute = now + 60ul;
time_check(target_minute_steps);
target_minute_steps = total_steps + (60000/interval);
}
// // gmtime() converts the time to a broken-down form (year, month...)
// // similar to the DateTime class. Unlike localtime(), it doesn't
// // attempt timezone conversions.
// struct tm *broken_down_time = gmtime(&now);
//
// // asctime() returns a textual representation of the date and time as
// // a C string (pointer to a character array). The format is similar to
// // the DateTime::toString() format "DDD MMM DD hh:mm:ss YYYY".
// Serial.println(asctime(broken_down_time));
return true; // to repeat the action - false to stop
}
void setup() {
Serial.begin(9600); // set up Serial library at 9600 bps
Wire.begin();
rtc.begin();
AFMS.begin(); // create with the default frequency 1.6KHz
// following line sets the RTC to the date & time this sketch was compiled
rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
// Initialize the system time from the RTC time. Both avr-libc and
// DateTime::secondstime() use the start of year 2000 as their
// reference "epoch".
set_system_time(rtc.now().secondstime());
// Keep the time in sync using the one-pulse-per-second output of the
// RTC as an interrupt source and calling system_tick() from the
// interrupt service routine.
pinMode(pin1pps, INPUT_PULLUP);
rtc.writeSqwPinMode(0x00);
attachInterrupt(digitalPinToInterrupt(pin1pps), system_tick, FALLING);
time_t now = time(nullptr);
target_minute = now + 60ul;
target_minute_steps = total_steps + (60000/interval);
// call the one_step function every n millis
timer.every(150, one_step);
}
void loop() {
timer.tick();
}time check!
expected 400
steps 401
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