Public API for pin-ctrl

  (:require [
             [protocols :as p]
             [implementation :as impl]
             [state-wrapper :as sw]]
            [clojure.core.async :as async :refer [>!! <!! >! <! go go-loop chan]]))

The philosophy behind pin-ctrl is that physical computing shouldn't require different API's for different devices. A set of routines involving monitoring inputs, and controlling outputs should be independent of what device it runs on, within the bounds of the capabilities and features of those devices.

For this reason, pin-ctrl aims to establish itself as a common device programming API, with multiple implementations available for different devices. These implementations are separate libraries depending on the core pin-ctrl library which implement the underlying protocols.

Board functions

The starting point for this library is the board abstraction. Board objects carry with them information about what pins are available, what modes are available for those pins, and the current mode settings for those pins.

(declare board? board-dispatch board-apply chan?)

We start by creating board objects with the create-board function, which returns a board object based on the specified board-type, which points towards the implementation key for the device being used (e.g. :rpi, :bbb, :firmata, etc.). The full set of configuration options will be detailed elsewhere soon (XXX Todo).

Create a new board instance. For documentation on the available options, see the implementation library documentation.

(defn create-board
   ; XXX - let's eventually register documentation for each of the implementations
   (create-board type {}))
  ([type config]
     (sw/new-board-wrapper (impl/instantiate type config)))))

What follows then is a suite of simple tools for dealing with the boards.

Returns a map of pin numbers to possible modes for the entire board, or the available modes of a specific pin. Arguments should be [board & [pin-n]] or [pin].

(defmulti pin-modes
(defmethod pin-modes true
  [board & [pin-n]]
  (if pin-n
    (get (pin-modes board) pin-n)
    (p/pin-modes board)))
(defmethod pin-modes false
  (board-apply pin-modes pin))

Returns a map of pin numbers to their corresponding current modes.

(defn pin-modes
  (p/pin-modes board))

Return the current mode of a specific pin.

(defn current-pin-mode
  ([board pin-n]
   (get (pin-modes board) pin-n))
   (board-apply current-pin-mode pin)))

Set the default board implementation

(defn set-default-board!
  (println "Not yet implemented"))

Pin functions

The following functions are provided for controlling and monitoring pin states.

In addition to the board abstraction, we offer the Pin abstraction for dealing with the control and state of individual pins. This abstraction is "thin", in that it is really just a reference to a board and pin number. In the functions that follow, you will see that the pin control/access functions can typically operate either directly on the board given a pin-n, or on a pin abstraction. Whether you use the pin abstraction or the board objects directly is up to you.

(defrecord Pin
  [board pin-n])

Return a new pin object based on a given board, and optionally set the mode.

(defn get-pin
  ([board pin-n]
   (Pin. board pin-n))
  ([board pin-n mode]
   (p/set-mode! board pin-n mode)
   (Pin. board pin-n)))

Many pins have multiple available modes. This function lets us switch between modes, when possible, and activate modes that are not activated by default, such as GPIO pins.

Set the mode of the pin. Must be a value supported by the pin.

(defn set-mode!
  ([pin mode] (board-apply p/set-mode! pin mode))
  ([board pin-n mode] (p/set-mode! board pin-n mode)))

Next we have our reader function.

Read a single value from the pin. If an analog input, returns a value between 0 and 1.

(defn read-value
  ([pin] (board-apply p/stateful-read-value pin))
  ([board pin-n] (p/stateful-read-value board pin-n)))

And last but not least, control functions:

Write a value to a writable pin. The kind of value supported and what it means is entirely dependent on what kind of pin is being used.

(defn write-value!
  ([pin val] (board-apply p/stateful-write-value! pin val))
  ([board pin-n val] (p/stateful-write-value! board pin-n val)))

XXX Should we try to worry about race conditions here? Could make this lower level so read/write messages don't have anything inbetween interfere.

Toggle a GPIO pin between high and low.

(defn toggle!
  ([pin] (board-apply toggle! pin))
  ([board pin-n]
   (let [current-val (read-value board pin-n)
         new-val (if (= current-val :low) :high :low)]
     (write-value! board pin-n new-val))))

Edge detection functionality

Certain applications, such as monitoring of a button push or other input trigger, require consistent monitoring of a pin value. The naive tight-loop approach here can be quite inefficient. Fortunately, GPIO pins typically support edge detection functionality, which allows the system to efficiently monitor for changes in pin state, and send those changes as interrupts within the program.

This library exposes these interrupts -- naturally -- via core.async channels. The usage pattern involves first setting the edge detection mode of the pin, then tapping into a central channel and using that tap to convey messages of state changes throughout the app.

Set the edge channel for the specified pin. If there is an existing channel for the pin, it will be closed.

(defn set-edge-chan!
  ([board pin-n ch]
   (p/set-edge-chan! board pin-n ch))
  ([pin ch]
   (board-apply p/set-edge-chan! pin ch)))

Get the current edge channel for the specified pin.

(defn get-edge-chan
  ([board pin-n]
   (p/get-edge-chan board pin-n))
   (board-apply p/get-edge-chan pin)))

Set the edge direction of a pin. Accepts :falling, :rising, :both and :none edges. Args should either be [pin edge & [ch-or-buffer]] or [board pin-n edge & [ch-or-buffer]]. Edge events will be placed on the specified channel or one that is created with the given buffer. If ch-or-buffer is not specified, an unbeffered chan is returned. The channel can be changed with set-edge-chan! and retrieved with get-edge-chan.

(defmulti set-edge!
(defmethod set-edge! true
  [board pin-n edge & [ch-or-buffer]]
  (let [current-ch (get-edge-chan board pin-n)
        ch (cond
             (chan? ch-or-buffer) ch-or-buffer
             ch-or-buffer         (chan ch-or-buffer)
             current-ch           current-ch
             :else                (chan))]
    (p/set-edge-chan! board pin-n ch)
    (p/set-edge! board pin-n edge ch)))
(defmethod set-edge! false
  [pin edge & [ch-or-buffer]]
  (board-apply set-edge! pin edge ch-or-buffer))

Some helper functions for navigating through boards and pins.

Nothing to see here...

(defn- board?
  (satisfies? p/PBoard obj))
(defn- board-dispatch
  [x & args] (board? x))
(defn- board-apply
  [f pin & args]
  (apply f (:board pin) (:pin-n pin) args))
(defn- chan?
  (and (satisfies? clojure.core.async.impl.protocols/ReadPort c)
       (satisfies? clojure.core.async.impl.protocols/WritePort c)))