analog write frequency too high

[Makeblock-official]

the analogWrite output pwm frequency(48M/256 = 187.5k) is too high for most h-bridge based motor drive, for example TB6612, which may cause the chip over heat. A reasonable driving frequency is around 20k for most motors.

[sandeepmistry]

Now that #151 is merged the PWM frequency is now 732.4Hz.