None of these are yet frequently asked, and perhaps they never will be… But they are still questions, and they relate to pylibftdi.
Using pylibftdi - General¶
Can I use pylibftdi with device XYZ?¶
If the device XYZ is (or uses as it’s ) an FTDI device, then possibly. A large number of devices will work, but won’t be recognised due to the limited USB Vendor and Product IDs which pylibftdi checks for.
To see the vendor / product IDs which are supported, run the following:
>>> from pylibftdi import USB_VID_LIST, USB_PID_LIST >>> print(', '.join(hex(pid) for pid in USB_VID_LIST)) 0x403 >>> print(', '.join(hex(pid) for pid in USB_PID_LIST)) 0x6001, 0x6010, 0x6011, 0x6014, 0x6015
If a FTDI device with a VID / PID not matching the above is required, then the device’s values should be appended to the appropriate list after import:
>>> from pylibftdi import USB_PID_LIST, USB_VID_LIST, Device >>> USB_PID_LIST.append(0x1234) >>> >>> dev = Device() # will now recognise a device with PID 0x1234.
Which devices are recommended?¶
While I used to do a lot of soldering, I prefer the cleaner way of breadboarding nowadays. As such I can strongly recommend the FTDI DIP modules which plug into a breadboard nice and easy, can be self-powered from USB, and can be re-used for dozens of different projects.
I’ve used (and test against) the following, all of which have 0.1” pin spacing in two rows 0.5” or 0.6” apart, so will sit across the central divide of any breadboard:
- a small 8 pin device with mini-USB port; serial and CBUS bit-bang.
- a 24-pin device with parallel FIFO modes. Full-size USB type B socket.
- a 24-pin device with serial and bit-bang modes. Full-size USB type B socket.
- this contains a more modern FT232H device, and libftdi support is fairly recent (requires 0.20 or later). Supports USB 2.0 Hi-Speed mode though, and lots of interesting modes (I2C, SPI, JTAG…) which I’ve not looked at yet. Mini-USB socket.
Personally I’d go with the UM232R device for compatibility. It works great with both UART and bit-bang IO, which I target as the two main use-cases for pylibftdi. The UM232H is certainly feature-packed though, and I hope to support some of the more interesting modes in future.
Using pylibftdi - Programming¶
How do I set the baudrate?¶
In both serial and parallel mode, the internal baudrate generator (BRG) is
set using the
baudrate property of the
Device instance. Reading this
will show the current baudrate (which defaults to 9600); writing to it
will attempt to set the BRG to that value.
On failure to set the baudrate, it will remain at its previous setting.
In parallel mode, the actual bytes-per-second rate of parallel data is 16x the programmed BRG value. This is an effect of the FTDI devices themselves, and is not hidden by pylibftdi.
How do I send unicode over a serial connection?¶
Device instance is created with
mode='t', then text-mode is
activated. This is analogous to opening files; after all, the API is
intentionally modelled on file objects whereever possible.
When text-mode is used, an encoding can be specified. The default is
latin-1 for the very practical reason that it is transparent to 8-bit
binary data; by default a text-mode serial connection looks just like a
binary mode one.
An alternative encoding can be used provided in the same constructor call
used to instantiate the
Device class, e.g.:
>>> dev = Device(mode='t', encoding='utf-8')
Read and write operations will then return / take unicode values.
Whether it is sensible to try and send unicode over a ftdi connection is a separate issue… At least consider doing codec operations at a higher level in your application.
How do I use multiple-interface devices?¶
Some FTDI devices have multiple interfaces, for example the FT2232H has 2 and the FT4232H has four. In terms of accessing them, they can be considered as independent devices; once a connection is established to one of them, it is isolated from the other interfaces.
To select which interface to use when opening a connection to a specific
interface on a multiple-interface device, use the
parameter of the Device (or BitBangDevice) class constructor.
The value should be one of the following values. Symbolic constants are
provided in the pylibftdi namespace.
Meaning INTERFACE_ANY (0) Any interface INTERFACE_A (1) INTERFACE A INTERFACE_B (2) INTERFACE B INTERFACE_C (3) INTERFACE C INTERFACE_D (4) INTERFACE D
You should be able to open multiple
Devices with different
Thanks to Daniel Forer for testing multiple device support.
What is the difference between the
latch BitBangDevice properties?¶
latch reflects the current state of the output latch (i.e. the last value
written to the port), while
port reflects input states as well. Writing to
latch has an identical effect, so when pylibftdi is used
only for output, there is no effective difference, and
port is recommended
for simplicity and consistency.
The place where it does make a difference is during read-modify-write operations. Consider the following:
>>> dev = BitBangDevice() # 1 >>> dev.direction = 0x81 # 2 # set bits 0 and 7 are output >>> dev.port = 0 # 3 >>> for _ in range(255): # 4 >>> dev.port += 1 # 5 # read-modify-write operation
In this (admittedly contrived!) scenario, if one of the input lines D1..D6
were held low, then they would cause the counter to effectively ‘stop’. The
+= 1 operation would never actually set the bit as required (because it is
an input at 0), and the highest output bit would never get set.
dev.latch in lines 3 and 5 above would resolve this, as the
read-modify-write operation on line 5 is simply working on the in-memory
latch value, rather than reading the inputs, and it would simply count up from
0 to 255 in steps of one, writing the value to the device (which would be
ignored in the case of input lines).
Similar concepts exist in many microcontrollers, for example see http://stackoverflow.com/a/2623498 for a possibly better explanation, though in a slightly different context :)
If you aren’t using read-modify-write operations (e.g. augmented assignment),
or you have a direction on the port of either ALL_INPUTS (0) or ALL_OUTPUTS
(1), then just ignore this section and use
What is the purpose of the
While libftdi is performing I/O to the device, it is not really running Python code at all, but C library code via ctypes. If there is a significant amount of data, especially at low baud-rates, this can be a significant delay during which no Python bytecode is executed. The most obvious result of this is that no signals are delivered to the Python process during this time, and interrupt signals (Ctrl-C) will be ignored.
Try the following:
>>> dev = Device() >>> dev.baudrate = 120 # nice and slow! >>> dev.write('helloworld' * 1000)
This should take approximately 10 seconds prior to returning, and crucially,
Ctrl-C interruptions will be deferred for all that time. By setting
chunk_size on the device (which may be set either as a keyword parameter
Device instantiation, or at a later point as an attribute of the
Device instance), the I/O operations are performed in chunks of at most
the specified number of bytes. Setting it to 0, the default value, disables
Repeat the above command but prior to the write operation, set
dev.chunk_size = 10. A Ctrl-C interruption should now kick-in almost
instantly. There is a performance trade-off however; if using
required, set it as high as is reasonable for your application.
Using pylibftdi - Interfacing¶
How do I control an LED?¶
pylibftdi devices generally have sufficient output current to sink or source the 10mA or so which a low(ish) current LED will need. A series resistor is essential to protect both the LED and the FTDI device itself; a value between 220 and 470 ohms should be sufficient depending on required brightness / LED efficiency.
How do I control a higher current device?¶
FTDI devices will typically provide a few tens of milli-amps, but beyond that things either just won’t work, or the device could be damaged. For medium current operation, a standard bipolar transistor switch will suffice; for larger loads a MOSFET or relay should be used. (Note a relay will require a low-power transistor switch anyway). Search online for something like ‘mosfet logic switch’ or ‘transistor relay switch’ for more details.
What is the state of an unconnected input pin?¶
This depends on the device and the EEPROM configuration values. Most devices will have weak (typ. 200Kohm) pull-ups on input pins, so there is no harm leaving them floating. Consult the datasheet for your device for definitive information, but you can always just leave an (unconnected) device and read it’s pins when set as inputs; chances are they will read 255 / 0xFF:
>>> dev = BitBangDevice(direction=0) >>> dev.port 255
While not recommended for anything serious, this does allow the possibility of reading a input switch state by simply connecting a switch between an input pin and ground (possibly with a low value - e.g. 100 ohm - series resistor to prevent accidents should it be set to an output and set high…). Note that with a normal push-to-make switch, the value will read ‘1’ when the switch is not pressed; pressing it will set the input line value to ‘0’.