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High Speed Signalling Utilizing a Two-Level Precompensating Driver and Dynamic Clamp Receiver

IP.com Disclosure Number: IPCOM000132439D
Original Publication Date: 2005-Dec-15
Included in the Prior Art Database: 2005-Dec-15
Document File: 3 page(s) / 367K

Publishing Venue

IBM

Abstract

Describes is a multi-gigabit single-ended source synchronous driver and receiver link. Alternate means of termination result in lower power and an increased data valid window.

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High Speed Signalling Utilizing a Two-Level Precompensating Driver and Dynamic Clamp Receiver

Disclosed is a clamp circuit is used with source terminated drivers to control overshoots and undershoots.

A clamp circuit will sense when its input voltage has exceeded a certain bias voltage and pull the input back down to that upper clamp-on voltage. Conversely, the clamp circuit will sense when its input voltage has dropped below a certain bias voltage and pull the input back up to that lower clamp-on voltage. Normally, the upper clamp-on voltage is at the voltage rail and the lower clamp-on voltage equivalent to the ground voltage. An aggressive clamp describes a situation where the clamp-on voltages are below the voltage rail and above voltage ground. Figure 1 is circuit example of an aggressive clamp. This clamped receiver combined with a voltage-mode precompensating driver explicitly means that a driver will have varying impedances and thus result in definite impedance mismatches. Previous thinking said that a precompensating driver absolutely required a terminating receiver to control effects due to impedance mismatches like reflections, overshoots, and undershoots.

A Thevenin termination for example, provides a path to both the voltage rail and ground voltage rail for excess charge (effects due to impedance mismatches) to be shunted. Unfortunately, termination is a voltage divider that takes away from a driver's ability to build or drain charge from a receiver input. The most obvious result of this effect is a slow receiver input edge-rate and therefore more of closed eye opening.

The aggressive clamp shown in figure 1 will not turn on while...