Square wave generator example code added

It generates 7.5 10 12 16 Hz square waves on pin 21 26 12 6
simultaneously

Example Code/square wave generator (4 channels).py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python
+
+# 2014-11-15
+# tarte.py
+# Public Domain
+
+import time
+
+import pigpio
+
+LED1=21
+LED2=26
+LED3=12
+LED4=6
+
+"""
+7.5 10 12 16
+
+Find the number of cycles needed for an integral switch on/off for each LED.
+
+That's 2*7.5 which is 2 seconds worth.
+
+In 2 seconds there will be this many cycles of on/off
+
+15  20 24 32
+
+How many micros for each cycle?
+
+15 66666 on 66667 off = 1999995
+20 50000 on 50000 off = 2000000
+24 41666 on 41667 off = 1999992
+32 31250 on 31250 off = 2000000
+
+There will be a slight error which will not be detectable by most means.
+"""
+
+def wave(pi, gpio, hz, secs, on=1, offset=0):
+   """
+   Generate a hz cycles per second square wave on gpio for
+   secs seconds.  The first transition is to level on at
+   offset microseconds from the start.
+   """
+   micros_left = int(secs * 1000000)
+   transitions = int(2 * hz * secs)
+   micros = micros_left / transitions
+
+   if (offset < 0) or (offset > micros):
+      print("Illegal offset {} for hz {}".format(offset, hz))
+      exit()
+
+   wf = [] # Empty waveform.
+
+   if offset:
+      wf.append(pigpio.pulse(0, 0, offset))
+      micros_left -= micros
+      last_micros = micros - offset
+      transitions -= 1
+
+   for t in range(transitions, 0, -1):
+      micros = micros_left / t
+      if (t & 1) == (on & 1):
+         wf.append(pigpio.pulse(0, 1<<gpio, micros))
+      else:
+         wf.append(pigpio.pulse(1<<gpio, 0, micros))
+      micros_left -= micros
+
+   if offset:
+      if on:
+         wf.append(pigpio.pulse(1<<gpio, 0, last_micros))
+      else:
+         wf.append(pigpio.pulse(0, 1<<gpio, last_micros))
+
+   pi.wave_add_generic(wf)
+
+pi = pigpio.pi() # Connect to local host.
+
+pi.wave_clear()
+
+wave(pi, LED1, 7.5, 2, 0, 23456)
+wave(pi, LED2, 10,  2)
+wave(pi, LED3, 12,  2, 1,17123)
+wave(pi, LED4, 16,  2)
+
+wid = pi.wave_create()
+
+pi.set_mode(LED1, pigpio.OUTPUT)
+pi.set_mode(LED2, pigpio.OUTPUT)
+pi.set_mode(LED3, pigpio.OUTPUT)
+pi.set_mode(LED4, pigpio.OUTPUT)
+
+pi.wave_send_repeat(wid)
+
+pi.stop()
+