After having figuring out where the cells are supposed to go, we need to figure out how to turn them into actual components on a PCB and write them out in a format that can be interpreted by a PCB design tool. As always, I spent most of the time looking for "shortcuts" and only started implementation then.

Initially I considered using the scripting capability of Eagle or Kicad, but this turned out to be quite a hazzle. Luckily I was pointed to the great Eagle file format in a discussion on µc.net. The Eagle .brd format is well structured and free from esoteric metadata - it is a pure XML format.

It turns out that it is extremely easy to handle the Eagle format using the lxml Python package.

    def loadeagle(self,filename):
        """Load eagle file """
        self.dom = et.parse(filename)
        self.n_eagle = self.dom.getroot()
        if self.n_eagle.tag != "eagle":
            raise Exception('Invalid tag name for root node - no eagle file?')

These lines read and parse an entire eagle .brd file to the element tree structure where it can be modified at will. Saving is also just a few lines.

Adding components is as simple as adding XML elements. The example below adds a transistor footprint

self.n_board = self.n_eagle.find('drawing').find('board')
n_elements = self.n_board.find('elements')
et.SubElement(n_elements, 'element', name = "Q"+cellname, library="RTL_components", package="SOT23", value="PMBT2369", x=str(x+1.65), y=str(y+1.4))

After this success it was quite clear how to proceed: The script reads a template, adds the required components and saves the board to a new file. The template consists of an empty board, design rules and a component/footprint library.Ok, remains the question, of which components to place on the PCB? The unit cell of an inverter is shown below. It consists of two 0402 resistors and a SOT323 transistor (MMBT3904 or PMBT2369). These unit cell are positioned on an equidistrant grid on the PCB, leaving some space for routing.

At this point, all unit cells are hardcoded in a Python class. Coming up with a flexible library format would add a lot of complexity at this point, especially since only very few cells have to be supported. 

In addition to placing logic cells, also pins for the I/Os are placed on the grid.

The completed placement for the counter.vhd example is shown below. The connections are shown as airwires.