connection of the sinks to the nearest mesh edge using stubs. This is done by calculating the distance between the four edges of a window from every sink using the distance formula and connecting the sink to the edge with the least distance using a wire called a stub. It is to be noted that the distance considered is the actual distance and not the manhattans distance.
Once the formation of the clock mesh and the connection of the stubs to the sinks are done, the total length of the mesh wire is calculated. This distance is calculated from a certain origin point of the mesh to the end of the stub that is the sink. The algorithmic flow chart of the structuring of the mesh is shown in the figure.
BUFFER PLACEMENT
Buffers are required three purposes. The first is the integrity of the clock signal being maintained. Next is to reduce the load capacitance seen by the clock source. Lastly, which is of great importance to us is the reduction of the skew by inducing equal delay to the sinks.
To have an almost zero skew by inducing delays in the clock path to the sinks, buffers of different sizes must be used depending on the capacitance of the sinks. To have such a layout it is necessary to form a buffer table with varying buffer sizes according to total sink capacitance in its path ( here the sum of all the skin capacitances inside a mesh window is considered) . A small sized buffer is placed at the node of the mesh window inside which the sum of the sink capacitance is less than a certain minimum value and a relatively larger sized buffer is placed where the sink capacitance is between certain values of capacitance which is stored in the buffer table. We have considered eight buffers of different sizes in accordance with certain predetermined capacitance values to form the buffer table. The placement of buffer is also done in MATLAB. The procedure is as follows.
The Centroid of a mesh window is calculated using the values of the sink capacitances inside the mesh using the formula given below:
(x_(Cap ),y_cap )=((”(cap??x(edge) ) )/(”cap),(”’cap??y(edge) ‘)/(”cap))
After the value of the Centroid is calculated the buffer is placed at the mesh node closest to the coordinates of the Centroid. The sizes of the buffers to be placed are taken from the buffer table which was formed earlier, by comparing the values of the total capacitance inside a mesh with that of the predefined values stored in the table. Hence if the sum of the capacitance is less than 25pF then buffer B1 is placed on the mesh node nearest to the centroid co-ordinates