Consumer demand for digital video, high-speed communication, and ever increasing processing speed is pushing manufacturers to adopt faster data transmission schemes. Though the data conveyed is typically digital, designers are favoring analog communication schemes that employ low-voltage differential signals, particularly for moving data within a system. The transmission medium, or “channel,” can be pairs of copper lines but is more typically pairs of circuit-board traces extending between integrated circuits. The use of differential signals allows for rejection of common-mode noise, and thus enables data transmission with exceptional speed and common-mode noise immunity.
The Telecommunications Industry Association (TIA) published a standard specifying the electrical characteristics of low-voltage differential signaling (LVDS) interface circuits that can be used to interchange binary signals. LVDS employs low-voltage differential signals to provide high-speed, low-power data communication. For a detailed description of this LVDS Standard, see “Electrical Characteristics of Low Voltage Differential Signaling (LVDS) Interface Circuits,” TIA/EIA-644 (March 1996), which is incorporated herein by reference.
Signal distortion limits the sensitivity and bandwidth of any communication system. A form of distortion commonly referred to as “intersymbol interference” (ISI) is problematic in single-ended and differential communication schemes, and is manifested in the temporal spreading and consequent overlapping of individual pulses, or “symbols.” Severe ISI prevents receivers from distinguishing symbols and consequently disrupts the integrity of received signals. To make matters more complicated, the characteristics of high-speed signals are highly destination-dependent, which is to say a received signal will appear different depending upon characteristics of the communication channel and receiver. In extreme cases, the transmitter may be so far out of adjustment for the particular communication channel and receiver that the received data is entirely unintelligible. Data transmitters must therefore be tuned to achieve and maintain optimal performance.
Receivers in high-speed communication systems sometimes include control circuitry that monitors various characteristics of incoming signals and tunes the associated transmitter accordingly. Such adjustments to the transmitter may be done once, to account for channel characteristics and process variations, or may be carried out continuously or periodically to additionally account for time-variant parameters, such as supply-voltage and temperature. Performing such adjustments requires receivers to communicate back to the associated transmitter, a process sometimes referred to as “backchannel” communication.
Providing for backchannel communication may be expensive, particularly for systems in which the high-speed communications channel being monitored and adjusted is unidirectional. The backchannel communication takes place in the direction opposite the flow of data, and so may require one or more additional signal paths and associated pins between the communicating circuits. Adding pins and signal paths is expensive and undesirable. In the alternative, bi-directional communication can support backchannel signals, but this option potentially reduces the forward communication bandwidth.
In a paper entitled “Phantom Mode Signaling in VLSI Systems,” Thaddeus Gabara describes circuits that facilitate backchannel communication in high-speed differential channels by injecting common-mode signals on the same channel but in the reverse direction as high-speed differential signals. These circuits take advantage of the ability of modern differential receivers to reject common-mode signals; in practice, however, injecting common-mode signals into a high-speed differential communication channel is, from the differential receiver's perspective, no different from injecting common-mode noise. Backchannel communication schemes relying upon common-mode signaling techniques would therefore undesirably limit forward channel transmission speed. There is therefore a need for protocols and circuits that facilitate backchannel communication over high-speed differential channels without unduly limiting the bandwidth of the differential signals.