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Method for a low offset rise time measurement

IP.com Disclosure Number: IPCOM000033810D
Publication Date: 2004-Dec-28
Document File: 3 page(s) / 23K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a low offset rise time measurement. Benefits include improved functionality and improved performance.

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Method for a low offset rise time measurement

Disclosed is a method for a low offset rise time measurement. Benefits include improved functionality and improved performance.

Background

              Slew rate control is becoming essential to stabilize signaling on buses to achieve the best signal quality and timing performance. Conventionally, most high-speed slew rate controls are open-loop controls of the predriver rise and fall times based on a test indicator, such as the buffer strength (RCOMP) setting. However, this indicator is not very accurate because the slew rate is not represented exactly in the RCOMP measurement. To overcome this problem, chipsets are using more closed-loop slew rate controls. However, this approach is hampered by the accurate measurement of the slew rate.

 

              Conventionally, a resistor stack produces VLO and VHI reference voltages, which are fed directly to the comparators. The comparators switch when the pad swings through the respective reference voltage. However, all comparator designs have some sensitivity to the input switch level, resulting in an input offset of the comparator, a delay variation, or both. Many comparators do not have as wide a range of inputs, so different comparators have been tried with different switch ranges. However, they proved unsuccessful at balancing the comparator delay. Additionally, these errors are very test sensitive. Because typical rise and fall times for buffers must be in the 100 ps range, delay and/or variation are unacceptable (see Figure 1).

General description

              The disclosed method is a low-offset rise time measurement. It serves as a close approximation of slew rate for stabilizing signaling on buses to obtain the best signal quality and timing performance.

Advantages

              The disclosed method provides advantages, including:

•             Improved functionality due to providing a low offset rise time measurement that closely approximates the slew rate

•             Improved performance due to improving signal quality and timing performance

Detailed description

              The disclosed method provides a method for measuring rise and fall times for use as a close approximation of slew rate. The pad is connected to a buffer (typically a dummy buffer) for which the slew rate is being measured. Two identical comparators (Comp) switch when the pad swings through the 20% (VLO) and 80% (VHI) of the total typical swing. For example, assuming signaling of 0.33 VTT to VTT,...