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U. Glaeser

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FIGURE 37.2 Conceptual view of the routing on an XC4000 device showing the pattern of logic blocks (CLBs)<br />

embedded in “channels” of routing resources. Direct connections to the programmable switch matrix (PSM) are shown<br />

as well as the patterns for double lines connecting every second PSM. Similarly, quad lines (omitted) connect every<br />

fourth PSM. Long lines run the length of horizontal and vertical channels. This is a concept diagram only: actual<br />

devices may differ in details [1].<br />

FIGURE 37.3 Simplified block diagram showing essential features of the Xilinx XC4000 input/output block (IOB).<br />

(The XC4000 IOBs have additional capabilities [1].)<br />

CLB may be connected to any other; however, there is a penalty: the switch points are implemented with<br />

pass transistors which add to the propagation delay of any signal passing through them. Thus, the short<br />

lines through the switch matrices should not be used for critical signals connecting widely separated CLBs.<br />

The double, quad, or long lines need to be used to reduce delays. Predicting the optimal allocation for any<br />

application is obviously a hard task and many strategies may be seen in the commercially available devices.<br />

For example, Altera’s Apex 20K devices employ a hierarchical structure, grouping basic logic elements (LEs)<br />

into logic array blocks (LABs), which are in turn grouped into MegaLABs [2]. Each block has appropriate<br />

internal routing resources. Copper is also used to reduce resistance and thus propagation delay.<br />

I/O Buffers<br />

I/O buffers provide circuitry to interface with external devices. Apart from input buffers and output<br />

drivers, the main additional feature is the ability to latch both inputs and outputs. The simplified diagram<br />

of an XC4000 I/O buffer (IOB) in Fig. 37.3 shows the output driver, input buffer, registers, several<br />

© 2002 by CRC Press LLC<br />

IOB<br />

IOB<br />

IOB<br />

CLB CLB<br />

CLB CLB<br />

CLB<br />

CLB<br />

IOB<br />

Direct Connections<br />

Double lines<br />

Long lines<br />

Output<br />

Enable<br />

Out<br />

Output<br />

Clock<br />

In<br />

Input<br />

Clock<br />

PSM<br />

PSM<br />

CLB<br />

CLB<br />

IOB<br />

D<br />

Q<br />

PSM PSM<br />

PSM<br />

Q<br />

D<br />

CLB<br />

CLB<br />

IOB<br />

Slew<br />

Rate<br />

Input Buffer<br />

PSM<br />

I/O Pad<br />

CLB<br />

CLB<br />

IOB

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