AR# 38410: Virtex-5 FPGA GTP/GTX Transceiver - What is the RX impedance value before and after power up and configuration?
Virtex-5 FPGA GTP/GTX Transceiver - What is the RX impedance value before and after power up and configuration?
The Virtex-5 FPGA GTP/GTX Transceivers are at certain impedance levels depending on before or after power up, andbefore or after configuration. This is importantfor certain protocols such as PCI-Express that relies on the impedanceto detect the receiver. This Answer Record discusses the RX impedance values for these different cases.
In protocols such as PCI-Express, the other endwould attempt to detect the impedance ofreceiver and start the link initialization process if a valid impedance is detected at the receiver end. Therefore, it is important to know the impedancethe Virtex-5 FPGA GTP/GTX receivers will show before and after power up, and configuration. The impedance values are measured for these different cases using TDR. The impedance measurement results are as follows:
The RX is at a high impedance in an unpowered state.
Whenpowered on, but beforethe device is configured, the RX is at a high impedance.
After configuration, the RX is at 50 Ohm impedance.