Fast Serial Link Offers Low-Cost Scalability

As processors, supporting logic, and memory chips get faster, wide parallel buses consume too much power and area, especially in low-cost systems. To help cut chip pin counts and power, many companies have developed single- or multiple-channel interfaces that can transfer data at rates from a few hundred kilobits per second to hundreds of megabits per second.

Texas Instruments has been using its VLYNQ interface as a proprietary solution on its own DSL, cable and voice broadband processors, digital media processors, and OMAP media processor chips. Cost-competitive licensing agreements will let companies integrate it into their own chips.

The VLYNQ interface is a daisy-chained, serial, full-duplex peer-to-peer communications scheme that operates at clock speeds up to 125 MHz. It can be scaled to deliver from one to eight transmit/receive (T/R) signal pairs and a common clock line for all the signal lines (17 pins plus an optional clock-request pin). The interface requires only one-fifth the silicon area of the PCI Express serial interface. Two VLYNQ blocks are required if the chip is part of a daisy chain.

Data transferred over the interface is 8B/10B encoded and packetized so there is about a 20% overhead for the encoding and additional overhead for the packetization. When clocked at 125 MHz, a single T/R pair then delivers an effective data throughput of about 73 Mbits/s (for single, 32-bit word transfers), while a dual T/R pair implementation delivers 146 Mbits/s, and a maximum eight-channel version delivers 584 Mbits/s. In-band flow-control lets the interface independently throttle the transmit and receive data streams.

If data packets contain four or 16 words, some of the overhead is eliminated. So on a single channel, data bursts of four words per packet can deliver an effective throughput of 133 Mbits/s. With 16 words per packet, the throughput goes up to 178 Mbits/s. With the maximum eight channels, an effective throughput of over 1400 Mbits/s can be achieved with 16 words per packet.

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