1. Technical Field of the Invention
The present invention relates to circuits and, more particularly, to sending encoded bits over interconnects.
2. Background Art
Interconnects are used to transmit signals (such as bits) over relatively long distances either inside an integrated circuit (also called a chip) and between integrated circuits.
FIG. 1 illustrates a system 10 of integrated circuits including integrated circuits IC0, IC1, IC2 . . . ICn. The integrated circuits are connected through a bus 14. Integrated circuit IC0 could be a controller for the other integrated circuits or could be the same as other integrated circuits. System 10 may be a system that is referred to as multi-drop system in which one or more of integrated circuits IC1, IC2 . . . ICn are selectively joined to bus 14. System 10, however, is not limited to a multi-drop arrangement. System 10 may be a memory system in which IC0 is a memory controller (either joined with a processor on chip or in a different chip from the processor) and in which integrated circuits IC1, IC2 . . . ICn are, for example, dynamic random access memory (DRAM) chips that are on one or more PCBs. System 10 could also involve a multi-processor system.
Address, data, and control bits on bus 14 may be on separate conductors of bus 14 or they may be, for example, time division multiplexed or packetized. For example, bus 14 may include some conductors used to carry only address bits, some conductors used only to carry only control bits, and some conductors used to carry only data bits. Or, some conductors may be used to transmit some combination of address, control, and/or data bits at different times or through a packetized or arrangement.
FIG. 2 illustrates a system 20 including integrated circuits IC0, IC1, IC2 . . . ICn. The integrated circuits are connected through busses 24-1, 24-2, . . . 24-n and an optional conductor 28. Note that IC0 may be a controller of the other integrated circuits or could be the same as other integrated circuits. System 20 is of the type that is sometimes referred to as point-to-point bus system. System 10, by contrast, is referred to as a radial bus system.
Bidirectional signaling refers to using the same conductors to transmit signals in both directions. For example, data may be transmitted either to or from an integrated circuit. If the same conductor is used in both directions, the signaling is bi-directional. The bi-directional signaling may be sequential or simultaneous. In the case of sequential bi-directional signaling, enable signals may be used to, for example, tri-state or turnoff unused drivers or receivers. In the case of simultaneous bi-directional signaling, the threshold voltage of the receiver may be changed depending on the state of the adjacent driver. For example, if the adjacent driver is transmitting a 0, the receiver threshold may be set to Vcc/4. If the remote driver is also 0, the threshold will not be met. If the remote driver is a 1, the Vcc/4 threshold will be met. If the adjacent driver is transmitting a 1, the receiver threshold may be set to 3 Vcc/4. If the remote driver transmits a 1, the threshold will be met and if it transmits a 0, the threshold will not be met.