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Files of check-in [f5c0015044] in the top-level directory


Drivers for the various devices in my ongoing robotics work.

PWM

The pwm2 file uses the pins_af.py file built for each board to automatically select and configure timer for the given pin. It allows you to pick alternative timers if the pin uses them and specify the pulse length in microseconds.

You can get and set the pulse width in microseconds as well as duty cycle. Changes to the pulse width can be made to occure over a period of time.

HCSR04

Driver for an HCSR04 ultrasonic rangefinder.

This interrupt-driven driver allows concurrent operations while ranging. It should be timed out, though, as the device will return garbage if the maximum distance is exceeded.

Motors

A collection of motor drivers.

Layers

Motor drivers have three layers.

Circuits

This is the physical circuit used. It determines the types of motors controlled and how to do so.

Implementation

A chip or breakout that implements a specific circuit, usually with some support hardware for specific behavior, possibly tweaking the behavior or providing a specific option.

An implementation class will be created with parameters specific to that implementation, then provide a method to connect pins to the appropriate circuit objects.

Shields

Sheilds are implementations for specific connections. Since the connections are hard wired, the instance objects will have methods to return the circuit objects by name, returning the circuit object with the connections used by the sheild.

Servo circuits

A Python vesion of the PyBoard's Servo driver, but a bit more flexible. Servos allow you to rotate the motor shaft to a specific angle, or at a specific speed (a "continuous rotation" servo), depending on the hardware, and to do so over a period of time.

The PyBoard servo driver's "raw" pulse widths are replaced by width measured in microseconds.

H-bridge circuits

An h-bridge circuit is four switches controlling inputs to a motor. While four switches imply 16 different states, some of them are uninteresting duplicates of other states, and some are to interesting, shorting power across the motor and destroying your hardware. So there are four interesting states, controlled by in1 and in2.

H-bridge inputs

The two inputs in1 and in2 select between four different states:

  1. Both inputs low turns off the drive so the motor coasts.
  2. One input high and one low drives the motor in one direction.
  3. Swapping the state of the inputs reverses the motor direction.
  4. Both inputs high will apply a braking force to the motor.

H-bridge controls

The speed of a motor can be controlled by using a pwm signal on one of the inputs. If the high input is a pwm signal, then the motor switches between run and coast modes, with a higher duty cycle being higher speed. If the low input is a pwm signal, the motor switches between run and brake modes, with a higher duty cycle being lower speed. Run/coast mode is a bit easier on the electronics, but the ouput power falls off very quickly as the duty cycle drops. Run/brake mode doesn't have that problem, and is often preferable because of it. In tests with the DRV8835, run/brake mode could move my platform with a duty cycle below 10%, but run/coast wouldn't budget it until the duty cycle was over 50%.

The downside of this approach is that having variable speed in both directions requires pwm signals on both inputs. So a common alternative input scheme replaces those two inputs, only one of which needs to be pwm. One is direction (or phase), and is sent to the two inputs to the h-bridge, one inverted. The motor runs in one direction when it is high, and the other when low. The other is normally called enable, and switches the inputs between the state selected by direction and both being in the same state, either on or off. Using pwm on this input makes it a speed control, providing a bi-directional variable speed motor control with a single pwm signal.

Fundumoto - L298-based Arduino motor/servo/sensor shield

This is a very basic shield, using a single L289 with a pair of h-bridges, wired as run/coast motors with enable and direction inputs. Due to it's simple nature, it just uses the h-bridge driver directly.

It also provides a buzzer and servo connectors.

DRV8835 - Driver for the TI DRV8835 on the Polulu carrier.

This chip supports both a run/brake mode with speed and direction inputs, or the two pwm inputs passed through to the h-bridge. A single mode input selects the mode for both h-bridges on the chip.

There is an Arduino shield version of this from Pololu. The mode input is connected to ground or vcc, and not under software control.