Correlación entre las dimensiones de la personalidad y el rendimiento académico.

So far we have considered only push channels where the sender is the active party that initiates the communication of data, and where the receiver is the passive party. The opposite situation, where the receiver is the active party that initiates the communication of data, is also possible, and such a channel is called a pull channel. A channel that carries no data is called a nonput

channel and is used for synchronization purposes. Finally, it is also possible

to communicate data from a receiver to a sender along with the acknowledge signal. Such a channel is called a biput channel. In a 4-phase bundled-data implementation data from the receiver is bundled with the acknowledge, and in a 4-phase dual-rail protocol the passive party will acknowledge the reception of a codeword by returning a codeword rather than just an an acknowledge signal. Figure 7.1 illustrates these four channel types (nonput, push, pull, and biput) assuming a bundled-data protocol. Each channel type may, of course, use any of the handshake protocols (2-phase or 4-phase) and data encodings (bundled-data, dual-rail, mofn, etc.) introduced previously.

7.1.2

Data-validity schemes

For the bundled-data protocols it is also relevant to define the time interval in which data is valid, and figure 7.2 illustrates the different possibilities.

For a push channel the request signal carries the message “here is new data for you” and the acknowledge signal carries the information “thank you, I have absorbed the data, and you may release the data wires.” Similarly, for a pull channel the request signal carries the message “please send new data” and the acknowledge signal carries the message “here is the data that you requested.” It is the signal transitions on the request and acknowledge wires that are in- terpreted in this way. A 4-phase handshake involves two transitions on each wire and, depending on whether it is the rising or the falling transitions on the request and acknowledge signals that are interpreted in this way, several data-validity schemes emerge: early, broad, late and extended early.

Since 2-phase handshaking does not involve any redundant signal transitions there is only one data-validity scheme for each channel type (push or pull), as illustrated in figure 7.2.

It is common to all of the data-validity schemes that the data is valid some time before the event that indicates the start of the interval, and that it remains stable until some time after the event that indicates the end of the interval. Furthermore, all of the data-validity schemes express the requirements of the party that receives the data. The fact that a receiver signals “thank you, I have absorbed the data, and you may go ahead and release the data wires,” does not mean that this actually happens – the sender may prolong the data-validity interval, and the receiver may even rely on this.

A typical example of this is the extended-early data-validity schemes in fig- ure 7.2. On a push channel the data-validity interval begins some time before

Reqand ends some time after Req .

7.1.3

Discussion

The above classification of channel types and handshake protocols stems mostly from Peeters’ Ph.D. thesis [112]. The choice of channel type, hand- shake protocol and data-validity scheme obviously affects the implementation of the handshake components in terms of area, speed, and power. Just as a design may use a mix of different bundled-data and dual-rail protocols, it may also use a mix of channel types and data-validity schemes.

For example, a 4-phase bundled-data push channel using a broad or an extended-early data-validity scheme is a very convenient input to a function block that is implemented using precharged CMOS circuitry: the request signal may directly control the precharge and evaluate transistors because the broad and the extended-early data-validity schemes guarantee that the input data is stable during the evaluate phase.

n Data Ack Req n Ack Req Req Data Data Ack Nonput channel Data Ack Req

Biput channel (bundled data)

Push channel (bundled data)

Pull channel (bundled data)

Figure 7.1. The four fundamental channel types: nonput, push, biput, and pull.

Data (early) 4-phase protocol: (push channel) Ack Req Data (broad) Data (late)

Data (extended early)

Data (early) Ack Req

Data (broad) Data (late)

Data (extended early) 4-phase protocol:

(pull channel)

Ack Req

Data (pull channel) 2-phase protocols:

Data (push channel)

Another interesting option in a 4-phase bundled-data design is to use func- tion blocks that assume a broad data validity scheme on the input channel and that produce a late data validity scheme on the output channel. Under these assumptions it is possible to use a symmetric delay element that matches only half of the latency of the combinatorial circuit. The idea is that the sum of the delay of Reqand Req matches the latency of the combinatorial circuit, and

that Req indicates valid output data. In [112, p.46] this is referred to as true single phase because the return-to-zero part of the handshaking is no longer

redundant. This approach also has implications for the implementation of the components that connect to the function block.

It is beyond the scope of this text to enter into a discussion of where and when to use the different options. The interested reader is referred to [112, 77] for more details.