libftdi on Mac OS X

Update 2011/02/02: In trying to port pylibftdi to Python3, I found that the libraries which get built following the instructions below are 64-bit. All well and good, but the Python3.1 installation on Mac OS X 10.6 is 32-bit only (unlike the Python2.6 install).

While it's possible to get both 32-bit and 64-bit universal libraries, I haven't tried that yet. The solution to built 32-bit libraries was to use the following:

CFLAGS="-arch i386" ./configure && make && sudo make install

(Note I omitted libusb and sudo (for make install) in previous instructions - I've updated below). I also found I needed to do:

export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig/

prior to building libusb-compat - I can't remember if this was a problem or not the first time round. Anyway, hopefully I'll have pylibftdi working with Python3 soon.
End of Update

I've not written much about my ftdi projects recently, so here's an update. I've played around with reading values from an encoder and outputting to a RGB LED with PWM, but both of these require a fairly high constant poll rate (or in the case of reading the encoder, ideally interrupt-on-change). The jitter is quite annoying, and decent PWM seems tricky, even when following FTDI's instructions (pdf link) for maxmising the buffer size. There's always going to be a gap when one buffer has finished and the next starts.

On a different note, I'm now mostly using my new Mac mini, so I've spent a few moments getting things to work there. Prerequisites:

• libusb (tested with version 1.0.8)
• pkg-config (tested with version 0.25)
• libusb-compat (test with version 0.1.3)
• libftdi (tested with version 0.18)

These should be installed in the order listed; for each file, download it, untar it (tar -xf filename), then run './configure && make && sudo make install' in the new directory which tar has created.

On the Python side, there were a couple of issues when I tried to run the old code. First was that the library could not be found at all. This was due to the library extension (.so) being hardcoded. For a cross-platform solution, it seems the best way is to use the following:

from ctypes import *
from ctypes.util import find_library

dll = CDLL(find_library('ftdi'))

Rather than specifying 'libftdi.so', we have stripped off the 'lib' prefix and the .so extension. The simple 'ftdi' string would be used if we were linking this library with gcc using the -l flag. Try 'gcc -lftdi' and it should report only that it can't find _main, not that the library is missing (try gcc -lmadeupname and it should complain about being unable to find 'madeupname')

Once this was done, the program would some times run (especially under pdb, which lead me up the garden path of thinking it was a timing issue...), but other times would cause a segfault. This was tracked down to another hard-coded value - the size of the context structure. The following will report the size of the ftdi_context structure:

echo -e '#include \nint main(){return sizeof(struct ftdi_context);}' | gcc -xc - && (./a.out; echo $?)

This is somewhat fragile, as it will fail if the structure ever gets larger than 255 bytes, but this seems unlikely for the time being. On this Mac this gives 112 bytes; on my Linux boxes it gives 84 - though they are running on the previous (0.17) version of libftdi. There is also the issue that the Mac library is x86_64 (i.e. 64-bit), while the Linux libraries are 32-bit.

One solution, though not exactly a purist's, is to allocate far more memory than will be needed. It won't slow anything down, as only a pointer to the start of the block is passed around, and probably won't make a difference to the memory consumption as applications will always use whole numbers of pages (4KB minimum) anyway. So for now, an allocation of 1KB seems a good solution.

The result of this is that the changes needed to the earlier code for compatibility with both Mac OS X and Linux are as follows:

...

from ctypes.util import find_library

def ftdi_start():
    global ctx, fdll  # frown... :P
    fdll = CDLL(find_library('libftdi'))
    # size of ftdi_context varies
    # seen 112 (x86_64, v0.18). 84 (i386, v0.17)
    # over-allocate to 1KB.
    ctx = create_string_buffer(1024)
    fdll.ftdi_init(byref(ctx))
    fdll.ftdi_usb_open(byref(ctx), 0x0403, 0x6001)
    fdll.ftdi_set_bitmode(byref(ctx), 0xFF, 0x01)

...

Me loves os.name == 'posix' :-)

New website - codedstructure.net

I've not got round to putting much on here for a while, but I have been busy, honest! I've finally got my own website after far too many years of not bothering: codedstructure.net.

I'm probably not the first to use scrolling like that, but I wasn't copying anyone and I think it's quite funky, especially the nice text shadows. Me likes the way web development is going, after years of shunning it...

Oh, and while I'm at it, I've got a twitter account, and hope to be churning out iPhone apps in the near future - though Objective C looks horribly like some terrible collision of Java and Smalltalk - in a bad way. Hope I can grow to like it.

A Python LCD status display

This is the third in my series of blog posts about doing IO with FTDI's Async BitBang mode on the UM245R / UM232R eval boards, and this time we're actually going to do something useful - have an LCD update with interesting system information. The LCD in question is based on the ubiquitous HD44780 controller, which is interfaced to microcontrollers throughout the world...

Anyway, with only 8 IO lines on the UM2xxR boards, we need to use the 4bit interface mode (two extra IO lines are needed beyond the 'data' path - one to act as a 'data-ready' strobe, and the other to select between 'data' and 'commands'). The wiring up is basically DB0-DB3 on the FTDI device going to D4-D7 on the LCD, with DB6 on the FTDI going to the 'RS' (register select) line on the LCD, and DB7 to the 'E' strobe. If that makes no sense, then hopefully the photo makes things slightly clearer...

I've also got an LED backlight for my display, which makes it look a whole lot cooler :-)

I'll present the code in chunks and try to explain it as I go along. First the initialisation and shutdown code, which are fundamentally unchanged from the previous examples, though they are now in functions to make them just a little tidier. (Note there is still no error checking here...)

"""
Write a string (argv[1] if run from command line) to a HD44780
LCD module connected via a FTDI UM232R/245R module

example usage:

# while true;
>   do python lcd.py $( awk '{print $1}' /proc/loadavg);
>   sleep 5;
> done
"""

from ctypes import *
import time, sys

def ftdi_start():
    global ctx, fdll  # frown... :P
    fdll = CDLL('libftdi.so')
    ctx = create_string_buffer(84)
    fdll.ftdi_init(byref(ctx))
    fdll.ftdi_usb_open(byref(ctx), 0x0403, 0x6001)
    fdll.ftdi_set_bitmode(byref(ctx), 0xFF, 0x01)

def ftdi_end():
    fdll.ftdi_usb_close(byref(ctx))
    fdll.ftdi_deinit(byref(ctx))

The following class is an abstraction of a bus - a collection of one (probably two, technically...) or more electrical lines which should be treated as a single unit. The aim here is to be able to program in a similar style to the embedded programming on a microcontroller, where registers are typically memory mapped and writing to a bus is simply writing into a bitfield. The parameters of this abstraction are the width of the bus (in bits), and the offset from the LSB of the entire port being accessed. It also needs a reference to a driver which allows it to do the reading and writing to the port. By using this as a descriptor, we can define buses within classes representing the various devices we are using; in this case the LCD.

class Bus(object):
    """
    This class is a descriptor for a bus of a given width starting
    at a given offset (0 = LSB).  It needs a driver which does the
    actual reading and writing - see FtdiDriver below
    """
    def __init__(self, driver, offset, width=1):
        self.offset = offset
        self.width = width
        self._mask = ((1<<width)-1)
        self.driver = driver

    def __get__(self):
        val = self.driver.read()
        return (val >> offset) & self._mask

    def __set__(self, obj, value):
        value = value & self._mask
        # in a multi-threaded environment, would
        # want to ensure following was locked, eg
        # by acquiring a driver lock
        val = self.driver.latch()
        val &= ~(self._mask << self.offset)
        val |= value << self.offset
        self.driver.write(val)

The following is the driver which will be used to do the actual data access. Note the use of the latch to store the last value written to the port, which cannot generally be read from the device after having been written (Latch registers for the IO ports was a big advance for the PIC18F series over the earlier 16F series, which needed the application to store this separately in order to do read-modify-write operations properly on the IO ports).

class FtdiDriver(object):
    def __init__(self):
        self._latch = 0

    def read(self):
        z = c_byte()
        fdll.ftdi_read(byref(ctx), byref(z), 1)
        return z.value

    def latch(self):
        return self._latch

    def write(self, val):
        self._latch = val
        z = c_byte(val)
        fdll.ftdi_write_data(byref(ctx), byref(z), 1)
        # the following is a hack specifically to allow
        # me to ignore all the timing constraints of the
        # LCD.  For more advanced LCD usage, this wouldn't
        # be acceptable...
        time.sleep(0.005)

Now we've got a Bus class and a driver to use with it, we can define the LCD module interface. I'm not going to cover the details of the interface, but the wikipedia HD44780 page has some pointers.

# need to instantiate this in global context so LCD
# class can be defined. Could tidy this up...
ftdi_driver = FtdiDriver()

class LCD(object):
    """
    The UM232R/245R is wired to the LCD as follows:
       DB0..3 to LCD D4..D7 (pin 11..pin 14)
       DB6 to LCD 'RS' (pin 4)
       DB7 to LCD 'E' (pin 6)
    """
    data = Bus(ftdi_driver, 0, 4)
    rs = Bus(ftdi_driver, 6)
    e = Bus(ftdi_driver, 7)

    def init_four_bit(self):
        """
        set the LCD's 4 bit mode, since we only have
        8 data lines and need at least 2 to strobe
        data into the module and select between data
        and commands.
        """
        self.rs = 0
        self.data = 3
        self.e = 1; self.e = 0
        self.e = 1; self.e = 0
        self.e = 1; self.e = 0
        self.data = 2
        self.e = 1; self.e = 0

    def _write_raw(self, rs, x):
        # rs determines whether this is a command
        # or a data byte. Write the data as two
        # nibbles. Ahhh... nibbles. QBasic anyone?
        self.rs = rs
        self.data = x >> 4
        self.e = 1; self.e = 0
        self.data = x & 0x0F
        self.e = 1; self.e = 0

    def write_cmd(self, x):
        self._write_raw(0, x)

    def write_data(self, x):
        self._write_raw(1, x)

All that remains is to initialise the FTDI device, initialise the LCD module, and write some data to it.

def display(string):
    ftdi_start()

    lcd = LCD()
    lcd.init_four_bit()

    # 001xxxxx - function set
    lcd.write_cmd(0x20)
    # 00000001 - clear display
    lcd.write_cmd(0x01)
    # 000001xx - entry mode set
    # bit 1: inc(1)/dec(0)
    # bit 0: shift display
    lcd.write_cmd(0x06)
    # 00001xxx - display config
    # bit 2: display on
    # bit 1: display cursor
    # bit 0: blinking cursor
    lcd.write_cmd(0x0C)

    for i in string:
        lcd.write_data(ord(i))

    ftdi_end()


if __name__ == '__main__':
    # note blatant lack of error checking...
    display(sys.argv[1])

and there we have it; a slightly cumbersome but also slightly cool and slightly useful little display utility. In the spirit of UNIX programming, this only does one thing - display the command line argument on the LCD. Obviously it needs major error handling if robustness is required...

while true; do python lcd.py $( awk '{print $1}' /proc/loadavg); sleep 5; done

Mandelbrot Canvas...

This Mandelbrot Plotter is something I worked on a little while ago. One of the things I want to do in any language I'm learning is know how to get a pixel on a page. A long time ago it was function 0Ch of INT 10h, then memory mapped displays, and now the wonderful ImageData object underlying HTML5's Canvas element. It's so much more fun displaying a picture than 'Hello World' in text...

I'm also fascinated by fractals, and the Mandelbrot set in particular (since I sort-of understand the maths behind it). It makes me wonder whether it is an invention or a discovery, and just why it is what it is - and whether other inscrutable patterns are just waiting to have some new visual representation cast a whole new light on them...

Of course with it being hosted (albeit slightly oddly on Blogger), I can now claim to have written a 'web app', for better or worse. Anyway, it's nice to be able to put random JavaScript up within Blogger...

Let there be LEDs

"In the beginning, [...] God said 'let there be light'".

Some time later, Man discovered Gallium Arsenide, and eye-piercing LEDs have blighted us ever-since.  (Although they used to be quite dim, actually).  Nevertheless, the 'hello world' of the embedded world is to make an LED flash.

In part 1 I 'covered' installation and test of the libftdi module, but the program didn't actually do anything.  The UM232R module is primarily a UART module (unlike the DLP USB245M, which is a FIFO), but as with most recent FTDI devices it supports a 'BitBang' mode.  BitBang is embedded jargon for running a protocol at the digital bit level, specifying in firmware at what point each of the (potentially many) IO lines goes high or low.

To put these devices in to this mode, an API call is needed to the FTDI driver to establish this mode.  The ftdi_set_bitmode function takes three parameters; the FTDI control context (by reference), a direction bitmap, and a mode indicator (which should be 1 for async bitbang mode).  The interesting is the direction bitmap.  For each of the 8 IO lines, the corresponding bit in this mask can be a '1' to indicate an output, or a '0' for an input.  I'm starting with output, so...

if ((ret = ftdi_set_bitmode(&ftdic, 0xFF, 0x01)) < 0) {
        fprintf(stderr, "unable to set bitmode\n");
        return EXIT_FAILURE;
    }

(this fits into the 'DO STUFF HERE' part of the code from last time)

Once this is done, we can start writing things out to the data port.

unsigned char x = 0;
    while (1) {
        ftdi_write_data(&ftdic, &x, 1);
        x ^= 0xFF;      /* toggle all lines */
        usleep(100000)  /* pause for 100ms  */
    }

This is hackish - it never exits for a start - but it should enable an LED to flash!

In terms of wiring things up, the 'GND' pin needs to go to the shorter leg of the LED (the cathode), and a data line (it doesn't matter which in this example, use DB0 to be safe) goes to the longer lead (anode).  A 1K resistor needs to be in either line to limit the current and prevent burning out the LED. As supplied, the jumpers on the UM232R/UM245R module configure it to be self powered. Anyway, it should look something like this, and the LED should be flashing at approx 5Hz.

To do anything more interesting, let's move to Python.  This is where the fun begins - with ctypes. I remember the first time I used ctypes, using puts from the C standard library on a Solaris machine, and the fact that it just worked. Occasionally there is a need to write the equivalent of a function prototype, but most of the time things are incredibly easy.

First up: loading the shared library. Lets guess its name - libftdi.so?

[GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-20)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> from ctypes import *
>>>
>>> fdll = CDLL('libftdi.so')
>>>
>>> fdll
<CDLL 'libftdi.so', handle 81da028 at b7cfd4ac>
>>>

So far so good :) Next thing is this ftdi_context struct which is referenced by each function in the C code. It starts as an unpopulated structure, and presumably the first functions (init and usb_open) set it up as required.  While the D2XX interface just gives an integer handle, in libftdi we have to pass pointers to this structure around.  So, how do we define this in ctypes?
First we need its size:

#include "ftdi.h"
#include "stdio.h"
int main()
{
    printf ("%d\n", sizeof(struct ftdi_context));
}

Now compile and run...

/home/user/ftdi> gcc get_size.c
/home/user/ftdi> ./a.out
84

So, we need 84 bytes of storage for this context. ctypes will do that for us:

>>> ftdic = create_string_buffer(84) 

Done that. Now we can rattle it off: only two other things: first, where in C you give &var to give the address of a variable (e.g. as a parameter to a function taking *var), in Python with ctypes, it's byref(var). Second, to create a something which has existence in the C world, (e.g. so can have its address taken), there are a whole bunch of c_XXX functions - we'll use c_byte, which is equivalent to an unsigned char.

from ctypes import *

fdll = CDLL('libftdi.so')

ftdic = create_string_buffer(84)

fdll.ftdi_init(byref(ftdic))

fdll.ftdi_usb_open(byref(ftdic), 0x0403, 0x6001)

fdll.ftdi_set_bitmode(byref(ftdic), 0xFF, 0x01)

import time
x = c_byte(0)
try:
    while True:
        fdll.ftdi_write_data(byref(ftdic), byref(x), 1)
        time.sleep(0.1)
        x.value ^= 0xFF # toggle all bits
except KeyboardInterrupt:
    pass

fdll.ftdi_usb_close(byref(ftdic))

fdll.ftdi_deinit(byref(ftdic))

And we're done! It's surprising how easy it is to translate basic C code using libraries into Python using ctypes - normally it just works.

Next time we'll look at interfacing to an LCD module...

Should I teach my children machine code?

I waited till primary school to read this book, but it seems that 'modern' kids can understand these things sooner - or at least Amazon thinks so...

The outside world...

I'm going to do a series of blog posts on using FTDI devices to access the outside world. There are probably a dozen other similar series out there, so I hope I can introduce enough novelty to make it interesting

I've been interested in low-level programming for as long as I've been programming (anyone else remember this book? - yes, a "children's" book on Machine Code...)

In terms of 'physical computing', things like Arduino are really taking off at the moment, but I'm going to take a step back to the simplicity of simple digital IO based on the BitBang mode of FTDI's latest devices.  There are two reasons for this: firstly it came seem laborious writing two sets of software (for both the host computer and the target micro-controller), and secondly that even if the eventual application is going to be a standalone micro based system, it is still generally quicker to prototype things using only the host computer, avoiding the cross-compile and firmware upload cycle.

Hardware-wise, I'm using a FTDI UM232R (Farnell link) device.  (I've also used one of these, and this either will be usable) This is a DIL module which plugs nicely into a breadboard which can be used to interface with stuff.  If you want to follow along, get a breadboard to plug it into, some LEDs, 1Kohm resistors, and some connecting wire (I like these, but they are waaay over priced).  In a couple of posts I'll be using one of these (N26AZ), too...

On the software side, I'm using Linux on a EEEPC 701 (stock Xandros) with the libftdi drivers, compiled from source found here.  The FTDI supplied drivers are similar and might be a better choice on Windows (not sure if libftdi works on Windows), but some of the function names and interfaces differ somewhat.

First step is getting the simple.c program compiled and working.  The program outline - slightly edited for length - looks something like this.

/* see http://www.intra2net.com/en/developer/libftdi/documentation/ */ 
#include "stdio.h"
#include "ftdi.h"

int main(void)
{
    int ret;
    struct ftdi_context ftdic;
    if (ftdi_init(&ftdic) < 0) {
        return EXIT_FAILURE;
    }

    if ((ret = ftdi_usb_open(&ftdic, 0x0403, 0x6001)) < 0) {
        return EXIT_FAILURE;
    }

    /* DO STUFF HERE */

    if ((ret = ftdi_usb_close(&ftdic)) < 0) {
        return EXIT_FAILURE;
    }

    ftdi_deinit(&ftdic);

    return EXIT_SUCCESS;
}

Compiling this (gcc -o simple simple.c -lftdi) and running the resulting executable should not cause any errors, and should return a successful error code if a FTDI device is attached.

This post is getting long enough for now, so I'll leave it at that. Next time will be configuring bitbang mode, where we can use the device as a configurable 8-bit IO port, blinking lights, and the wonders of Python's ctypes module...