Wednesday, 9 February 2011

pylibftdi 0.7 - multiple device support

pylibftdi has always been about minimalism, which means that if you wanted to do something it didn't support, things got tricky. One of it's glaring deficiencies until now was that it only supported a single FTDI device - if you had multiple devices plugged in, it would pick one - seemingly - at random.

With pylibftdi 0.7, that has finally changed, and devices can now be opened by name. Or at least by serial number, which is nearly as good. A new example script (which I've just remembered is hideously raw and lacks any tidying up at all) examples/ in the source distribution will enumerate the attached devices, displaying the manufacturer (which should be FTDI in all cases), description, and serial number.

The API has changed slightly to cope with this; whereas previously there was just a single Driver class, now the primary interface is the Device class. Driver still exists, and holds the CDLL reference, as well as supporting device enumeration and providing backwards compatibility.

(As an aside, using ftdi_usb_find_all was (not) fun - it sets a pointer to pointer which is then used to traverse a linked list. Trivial in C, an hour of frustration in ctypes. Anyway, I got there in the end).

>>> from pylibftdi import Device
>>> import time
>>> # make some noise
>>> with Device('FTE4FFVQ', mode='b') as midi_dev:
...     midi_dev.baudrate = 31250
...     for count in range(3):
...         midi_dev.write(b'\x90\x3f\x7f')
...         time.sleep(0.5)
...         midi_dev.write(b'\x90\x3f\x00')
...         time.sleep(0.25)

Both Device() and BitBangDevice take device_id as the (optional) first parameter to select the target device. If porting from an earlier version, one of the first changes is probably to use named parameters for options when instantiating these classes. My intention is that device_id will always be the first parameter, but the order and number of subsequent parameters could change.

Another change is that Devices are now opened implicitly on instantiation unless told not to (see the docstrings). Previously the Driver class only opened automatically when used as a context manager. There is no harm in opening devices multiple times though - subsequent open()s have no effect.

I've also finally figured out that I need to set long_description in to get documentation to appear on the PyPI front page. After all, without docs, it doesn't exist.

It's only been a few days since 0.6, but I wanted to get this release out - I think it is a big improvement since 0.5, and It'll probably be a while till the next release. In the mean time, I'll try and get a vaguely useful example going - which will probably involve MIDI and an LCD...

No comments:

Post a Comment