AD8116-EB datasheet(12/26 Pages) AD

12 / 26 page

AD8116

–12–

REV. A

Using additional crosspoint devices in the design can lower the

number of outputs that have to be wire-ORed together. Figure

26 shows a block diagram of a system using ten AD8116s to

create a nonblocking 128

× 16 crosspoint that restricts the wire-

ORing at the output to only four outputs. This will prevent an

enabled output from having to drive a large number of disabled

devices. Additionally, by using the lower eight outputs from

each of the two Rank 2 AD8116s, a blocking 128

× 32 crosspoint

array can be realized.

There are, however, some drawbacks to this technique. The

offset voltages of the various cascaded devices will accumulate

and the bandwidth limitations of the devices will compound. In

addition, the extra devices will consume more current and take

up more board space. Once again, the overall system design

specifications will determine how to make the various trade-offs.

OUT

AD8116

OUT

AD8116

0–15

AD8116

OUT

AD8116

16–31

OUT 16–31

0–15

16–31

OUT 0–15

OUT

Figure 24. 32

× 32 Crosspoint Array Using Four AD8116s

OUT

AD8116

OUT

AD8116

0–15

OUT

AD8116

16–31

AD8116

32–47

OUT 0–15

OUT 16–31

OUT 32–47

OUT

Figure 25. 48

× 48 Crosspoint Array Using Nine AD8116s

Creating Larger Crosspoint Arrays

The AD8116 is a high density building block for crosspoint

arrays over 256

× 256. Various features such as output disable,

chip enable, serial data out and multiple pinouts for logic signals

are very useful for the creation of these larger arrays.

The first consideration in constructing a larger crosspoint is to

determine the minimum number of devices that are required.

The 16

× 16 architecture of the AD8116 contains 256 “points,”

which is a factor of four greater than an 8

× 8 crosspoint and a

factor of 64 greater than a 4

× 1 crosspoint. The PC board area

and power consumption savings are readily apparent when

compared to using these smaller devices.

For a nonblocking crosspoint, the number of points required is

the product of the number of inputs multiplied by the number

of outputs. Nonblocking requires that the programming of a

given input to one or more outputs does not restrict the avail-

ability of that input to be a source for any other outputs.

Thus a 32

× 32 crosspoint will require 1024 points. This number is

then divided by 256, or the number of points in one AD8116

device, to yield four in this case. This says that the minimum

number of 16

× 16 devices required for a fully programmable

× 32 crosspoint is four.

Some nonblocking crosspoint architectures will require more

than this minimum as calculated above. Also, there are blocking

architectures that can be constructed with fewer devices than this

minimum. These systems have connectivity available on a statis-

tical basis that is determined when designing the overall system.

The basic concept in constructing larger crosspoint arrays is to

connect inputs in parallel in a horizontal direction and to “wire-

OR” the outputs together in the vertical direction. The meaning

of horizontal and vertical can best be understood by looking at a

diagram. Figure 24 illustrates this concept for a 32

× 32 crosspoint

array. A 48

× 48 crosspoint is illustrated in Figure 25.

The 32

× 32 crosspoint requires each input driver drive two

inputs in parallel and each output be wire-ORed with one other

output. The 48

× 48 crosspoint requires driving three inputs in

parallel and having the outputs wire-ORed in groups of three. It

is required of the system programming that only one output of a

wired-OR node be active at a time.

It is not essential that crosspoint architectures be square. For

example, a 64

× 16 crosspoint array can be constructed with

four AD8116s by driving each input with a separate signal and

wire-ORing together the corresponding outputs of each device.

It can be seen, however, that by going to larger arrays the

number of disabled outputs an active output has to drive

starts to increase.

At some point, the number of outputs that are wire-ORed be-

comes too great to maintain system performance. This will vary

according to which system specifications are most important.

For example, a 128

× 16 crosspoint can be created with eight

AD8116s. This design will have 128 separate inputs and have

the corresponding outputs of each device wire-ORed together

in groups of eight.

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