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The control input for each register gate comes from the watcher of the next or downstream register. When the next register has received and stored a DATA wave front playing through the combinational circuit, it sends back a DATA acknowledge (DACK) that says, "I have received and stored the DATA wave front and now you can send the NULL wave front". This is conveyed by transitioning the control line value to NULL. But the watcher transitions its result to DATA when it sees a complete data set so an inverter is needed. The same is true for the reception of the NULL wave front. When the watcher sees all NULL values it transitions its result value to NULL. This means, "I have received and stored the NULL wave front and now you can send a DATA wave front". But to authorize a DATA wave front the control value must be DATA, so again the signal must be inverted.

The control line requests the sending of a DATA wave front or a NULL wave front. The register gates are feedback hysteresis gates that enforce the completeness of input for both DATA and NULL. To pass DATA values, both inputs of a gate must be DATA. Therefore a DATA wave front cannot pass the register gates until the control line is DATA, or in other words until the next register has requested the data to be passed. When the control line becomes DATA, the DATA values presented to the register gates will be passed and as long as the control line remains DATA the data will be stored by the register gates. When a complete DATA set has been passed and stored by the register gates the watcher gate will detect the complete DATA set and transition the control line to the previous register to NULL, indicating that it has received and stored a DATA wave front and the previous register can pass a NULL wave front.

Because of the hysteresis behavior of the register gates, they will not pass a NULL wave front until both inputs are NULL. So when the NULL wave front arrives at the register gates and the control line becomes NULL, the NULL values will be passed and stored as long as the control line is remains NULL. After the NULL values are passed and stored, the watcher will detect the all NULL value set and transition the control line to the previous register to DATA indicating that it has received and stored the NULL wave front and the previous register can pass a DATA wave front.

The behavior of the asynchronous register is detailed in Figures 21a, 21b, 21c and 21d.

Figure 21a

Figure 21a. Asynchronous register behavior.

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