don't set stepper freq if it's already OK

.travis.yml

@@ -23,7 +23,8 @@ deploy:
     tags: true
 
 install:
-  - pip install pylint circuitpython-build-tools Sphinx sphinx-rtd-theme pytest
+  - pip install --force-reinstall pylint==1.9.2
+  - pip install circuitpython-build-tools Sphinx sphinx-rtd-theme pytest
 
 script:
   - py.test

adafruit_motor/stepper.py

@@ -75,7 +75,8 @@ def __init__(self, ain1, ain2, bin1, bin2, *, microsteps=16):
 
 	# set a safe pwm freq for each output
         for i in range(4):
-            self._coil[i].frequency = 2000
+            if self._coil[i].frequency < 1500:
+                self._coil[i].frequency = 2000
 
         self._current_microstep = 0
         if microsteps < 2:

tests/test_stepper.py

@@ -14,6 +14,10 @@ class Coil:
     def __init__(self):
         self._duty_cycle = 0
 
+    @property
+    def frequency(self):
+        return 1500
+
     @property
     def duty_cycle(self):
         return self._duty_cycle