ONFI Spec Analysis Part 2: Data Interface Evolution & ODT

Following our architecture overview in Part 1, we now dive into the physical data interfaces that enable the high-speed performance of modern NAND flash.

1. Evolution of the Interface

ONFI has evolved significantly to keep up with throughput demands:

• SDR (Single Data Rate): The classic asynchronous interface. Simple but limited in frequency.
• NV-DDR: Introduced source-synchronous clocking using the DQS strobe.
• NV-DDR2/3: Added features like differential signaling and lower voltage swings (1.2V/1.8V) to reach speeds beyond 400MT/s.

2. On-Die Termination (ODT)

As frequencies increase, signal reflections become a major bottleneck. ONFI 4.x and 5.x specifications rely heavily on ODT.

• ODT allows the NAND device to terminate the signal line internally with a specific resistance (e.g., 50Ω, 75Ω).
• This significantly improves signal integrity (SI) by reducing eye-diagram jitter.

3. Training and Calibration

High-speed modes aren’t just plug-and-play. The controller must perform:

• ZQ Calibration: Adjusts the output driver impedance.
• Read/Write Training: Aligns the DQS strobe with the data eye for reliable sampling.

In Part 3, we will look at ONFI 5.x specific enhancements for enterprise-grade performance.