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AR# 1519

FPGA Configuration - What Threshold does CCLK use for 5 Volt FPGAs?


For Xilinx 5 Volt FPGA families XC2000, XC3000, XC4000(E) and XC5200, the input and output threshold level of CCLK is compatible with both 5 Volt TTL and 5 Volt CMOS threshold standards.


Xilinx FPGAs use CMOS transistors, but mimic a BJT-like structure in that outputs drop a threshold on the VOH. Therefore, when CCLK is used as an output, it swings from 0 to 4.0 volts. As an input the CCLK triggers at a threshold of 1.5 Volts. This is compatible with both TTL and CMOS threshold standards:



VILmax 0.8 1.0

VIHmin 2.0 70% (Vcc)

VOLmax 0.4 0.4

VOHmin 2.4 Vcc - 0.5V

Therefore, CCLK may be used to source or load either TTL or CMOS type external circuitry.

AR# 1519
Date Created 08/21/2007
Last Updated 05/08/2014
Status Archive
Type General Article