Charging pad

A charging pad on the bus says what it’s doing, and when a robot comes onto it and leaves.

In mcos

/dev/charger0 is read only: a line saying what it’s doing, the robot on it (its board’s id, or -), and its buffer:

charging 4f0c2a9e-... 1200/26400
idle - 26400/26400

The state is taken at the first read after opening or seeking to the start. Opening it to write fails with EACCES. Its events (a robot docked or undocked) make the file ready for sys.poll’s PRI, and the pull ioctl takes them. The file’s ioctls are the device’s methods, by name, in any open mode.

Methods

get() → {charging, robot, buffer, capacity}

Whether it’s charging a robot now, the robot’s board id (nil: none on it), and its buffer’s level and capacity, in power units.

pull() → event or nil

Its oldest event, or nil.

Events

Its interrupt is raised when an event comes, and stays raised while one waits. With no program running on the board that has it, it takes no data: its events go.

Example

Log every robot that comes to the pad and leaves it:

-- Keep a log of the robots that come to a charging pad and leave it, and how full its buffer was.
--
--   lua charger-log.lua [log]   (run it with & to keep it going)
--
-- The pad's file says what it's doing ("charging <robot> 26400/26400"); its events (docked,
-- undocked) come as poll's PRI and the pull ioctl. sys.poll blocks until there's news, so the
-- program uses no time while nothing comes or goes.
local sys = require("sys")

local LOG = arg[1] or "/home/charger.log"

local pad = assert(sys.open("/dev/charger0"))

-- The pad's state now: its file read from its start.
local function state()
  sys.lseek(pad, 0, "set")
  return (sys.read(pad, 256) or ""):gsub("\n$", "")
end

while true do
  local event = sys.ioctl(pad, "pull")
  if event then
    local log = assert(io.open(LOG, "a"))
    log:write(event.event, " ", event.robot, " (", state(), ")\n")
    log:close()
  else
    sys.poll({[pad] = {PRI = true}})
  end
end