描述

解决方案

Virtex and Spartan-II

On these devices, all I/O standards have an intrinsic diode between the I/O pad and ground that limits undershoot to greater than -1 V. The LVCMOS 2.5V, LVTTL, and PCI 5V standards are all 5V-tolerant (both inputs and outputs) and do not have a pull-up clamp enabled. Because configuration pins default to LVTTL, these only have pull-down clamp diodes enabled.

Virtex-E and Spartan-IIE

These devices are provided with a pull-up clamp diode between the I/O pad and VCCO that allows them to be 5V-tolerant and reduces overshoot. The clamp diode is available with all I/O standards (including all configuration pins), excluding LVDS, LVPECL, GTL, GTL+, or LVCMOS1.8/2.5.

Virtex-II Pro Only

For Virtex-II Pro devices, all JTAG pins do not have clamp diodes, and the TDO is open drain. For more information, see the Virtex-II Pro/Virtex-II Pro X 3.3V I/O Design Guidelines Application Note(XAPP659).

Virtex-II/-II Pro/-4/-5/-6 and Spartan-3

For these devices, the following pins have pull-up and pull-down clamp diodes:

• JTAG pins (except for Virtex-II Pro)
• GCK0, GCK1, GCK2, GCK3
• M0, M1, M2
• CCLK
• DONE
• PROGRAM
• TDI, TDO, TMS, TCK
• DXN, DXP (Temperature-sensing diode pins)

Note: VCCINT, VCCO, GND have a protection circuit, however,it might not be an explicit diode to Vcco and GND.

Spartan -3A/-3AN/-6

These devices have no pull-up diode clamps and use a floating n-well instead. For more information, see the Virtex-II Pro/Virtex-II Pro X 3.3V I/O Design Guidelines Application Note(XAPP659).

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