Figure 7.10 shows the interconnect used in the Altera FLEX family of complex PLDs. Altera refers to the FLEX interconnect and MAX 9000 interconnect by the same name, FastTrack, but the two are different because the granularity of the logic cell arrays is different. The FLEX architecture is of finer grain than the MAX arrays—because of the difference in programming technology. The FLEX horizontal interconnect is much denser (at 168 channels per row) than the vertical interconnect (16 channels per column), creating an aspect ratio for the interconnect of over 10:1 (168:16). This imbalance is partly due to the aspect ratio of the die, the array, and the aspect ratio of the basic logic cell, the LAB.
As an example, the EPF8820 has 4 rows and 21 columns of LABs ( Figure 7.10 a). Ignoring, for simplicity’s sake, what happens at the edge of the die we can total the routing channels as follows:
- Horizontal channels = 4 rows ¥ 168 channels/row = 672 channels.
- Vertical channels = 21 rows ¥ 16 channels/row = 336 channels.
It appears that there is still approximately twice (672:336) as much interconnect capacity in the horizontal direction as the vertical. If we look inside the boxes A, B, and C in Figure 7.10 (b) we see that for individual lines on each bus:
- Box A connects an LE to two row channels.
- Box B connects two column channels to a row channel.
- Box C connects an LE to two column channels.
There is some dependence between boxes A and B since they contain MUXes rather than direct connections, but essentially there are twice as many connections to the column FastTrack as the row FastTrack, thus restoring the balance in interconnect capacity.