Return to zero is an asynchronous 4-Phase signaling protocol
using request and acknowledge signals. It is used to transfer data electronicly from a source to a destination using a asynchronous communication channel. To initiate a data transfer the request line is asserted. One the target recieves the request it can assert the acknowledge. This inturn allows the request to disassert and once the acknowledge is released the request can innicheate another cycle.
The protocol can be used to implement bundled data or delay insensitive K of N data encoded designs. (including dual-rail / 1 of 2 and 1 of 4).
Bundled data designs can transmit the data in different stable areas of the handshake. The early protocol assumes stable data while the request signal is high. The late protocol assumes data is stable while the ackowledge signal is high. The broad protocol ensures data is stable from the first transition (R+) till the last (A-)
| Broad |
K of N
designs encode the data on the request signal. Dual-rail
is the simplest method of transporting information in a delay insensitive
manner. The request line is split into two parts: Req0 and Req1. Both request lines are acknowledged using the Ack signal. To transmit a 0 the request asserted on Req0. Alternatively to transmit a 1 a cycle is conducted using Req1.
Ack______/ \_____/ \____
All K of N return to zero
protocols use the early protocol as the data is encoded on the request signal.