Inductively-Coupled Inter-Chip Communication Technique Allows Chip Access without Using Debug Pins

ISSCC2007 Paper No. 20.3 covers wireless interface technology for a removable chip monitor that uses inductive coupling

The Kuroda and Ishikuro laboratories at Keio University, working with Renesas, have developed technology for inter-chip communication using inductive coupling that can be used as a debugging interface. The technology uses a probe, a flexible circuit board attached to the top surface of the LSI package, to provide high-speed wireless communications for on-chip debugging. Normally, this sort of communication is performed via wired connections to debugging pins. Renesas has provided a method for adding to standard chips the coils needed for pulse communications.

An on-chip debugger requires that pins be allocated for debugging, and using an emulator means that some I/O pins cannot be used. In response to this problem, Renesas came up with the idea that the inter-chip communication technology using inductive coupling being researched by the Kuroda laboratory at Keio University could be used as a debugging interface and suggested a joint research project. "Originally, the technology used inductive coupling to connect chips over distances of 10?m to 100?m and transfer data at 1Gbps. Results announced at ISSCC 2006 described how a speed of 1Tbps (T: tera) was achieved using 1024 bits in parallel," noted Dr. Ishikuro.

Extending the communication range was the key to implementing this new objective of communicating from outside the chip package. Three coils are used for inductive coupling, one for the clock (CLK), a transmitter (TXD), and a receiver (RXD). The three coils are formed on the microcomputer die in the aluminum-wiring layer (see Photograph 1). The wireless probe also has three coils formed on a flexible circuit board (see Figure 1).

"Although the range could be extended by making the coils larger, making the die area larger would significantly affect the cost. Also, problems with resonance would occur due to parasitic capacitance. On the other hand, trying to extend the range without making the coils larger causes the signal to become lost in noise," Dr. Ishikuro explained. Balancing these two considerations resulted in ultra-small (0.6mm) coils being used in the microcomputer, and small (1.0mm) coils in the probe. A prototype chip produced using these coil sizes achieved a communication range of 1.2mm and coupling distance of 0.5mm.

A pulse-based communication method was selected. Carrier-based communication, such as that used in RFIDs, requires a separate oscillator, modulator, and demodulator. That approach couldn't be used because it would result in a larger and more complex circuit. "Pulse communication, on the other hand, allows the circuit to be kept simple. It can easily be incorporated into the LSI production process (see Figure 2). Further, the wide spectrum used avoids complications with radio laws and has the advantage that it is easy to boost the data rate by inserting more data pulses," Dr. Ishikuro said.

Positive pulses represent a transmission value of "1" and negative pulses represent "0". Although the prototype chip has a maximum transmission rate of only 500kbps, it is theoretically possible to use pulses as short as 1ns to provide a data transfer rate of 1Gbps per channel.

A demonstration system using the prototype chip measured the waveform in the microcomputer and the waveform detected by the probe. The test results showed that the data transmissions were accurate (see Photograph 2).