Article ID: 000073720 Content Type: Troubleshooting Last Reviewed: 09/11/2012

Why are two different I(out) values used to compute the R1 and R2 values in the 5.0-V compatibility sections of applicable handbooks?

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Description To model R1 of the driving device, use the maximum short circuit current value. For example, the Using Cyclone Devices in Multiple-Voltage Systems (PDF) chapter of the Cyclone® handbook shows typical IV curves for a 5.0-V device. The instant a driver switches from driving low to driving high there is a short circuit current when both the pull-down and pull-up transistors are on. The instantaneous current is 135mA in this example, and that is used to model the resistance of the output buffer. When the buffer achieves its high state, the current is much lower. Most data sheets provide the current conditions for which the VOH is specified. The IOH current conditions is the value used to compute for R2 since that is the typical operating condition of the circuit.

Related Products

This article applies to 3 products

MAX® II CPLDs
Cyclone® II FPGA
Cyclone® FPGAs