Add new driver for HX711 load cell ADC

[tve]

The moddable SDK already has a driver for the HX711 but I found it to be very, very slow. E.g. over 300ms per reading, in part due to averaging that can't be disabled. This new driver is very simple and optimized for speed: it essentially just provides one read() function to read the current ADC value without performing any averaging. There is a second function readable() to check that the chip can be read (it typically samples at 10Hz). This driver implements the read in C using modGPIO and performs a read in a couple of milliseconds.

Conveniently the existing driver is under modules/drivers and this PR places code in modules/drivers/sensors so there is no clash.

This driver is intended to conform to ECMA-419.

[phoddie]

configure() can be called more than once. If it is, the first HX711c instance will be replaced with another. But, there's also no guarantee that options.sensor will be present when configure is called, since that property is for the constructor. The simplest solution is to move these two lines to the constructor and leave configure empty.

[phoddie]

Is this function necessary? It isn't in the standard. The usual behavior is to have sample() return undefined if no reading is available. (It looks like this is what you implemented in your native read function already)

[phoddie]

The destructor can be called with hostData set to NULL. This happens when the virtual machine is deleted. It is rare. ;) It can also happen if an exception occurs in the constructor before the host data is set. So, the best practice is to always check for NULL

[phoddie]

unused. what was the idea?

[phoddie]

If any of these throw, data is orphaned (even the call to xsmcToInteger can call in type coercion cases). Error clean-up isn't pretty, but here's what I'd probably do here:

  int clk_pin = xsmcToInteger(xsArg(0));
  int din_pin = xsmcToInteger(xsArg(1));
  hx711_data d; 
  d.gain = xsmcToInteger(xsArg(2));

  if (modGPIOInit(&d.clk, NULL, clk_pin, kModGPIOOutput))
    xsUnknownError("can't init clk pin");
  if (modGPIOInit(&d.din, NULL, din_pin, kModGPIOOutput)) {
    modGPIOUninit(&d.clk);
    xsUnknownError("can't init din pin");
  }
  hx711_data *data = c_malloc(sizeof(hx711_data));
  if (data == NULL) {
    modGPIOUninit(&d.clk);
    modGPIOUninit(&d.din);
    xsUnknownError("can't allocate data");
  }
  data = d;

[phoddie]

This check isn't strictly necessary. Using xsArg on an argument that doesn't exist will throw. All this guarantees is that there aren't more than 3 arguments. No harm in the check, but it is common for JavaScript functions to simply ignore extra arguments.