Programmable Clamp Diode

Both HSIO and GPIO have internal clamp diodes. Clamp diodes help reduce the voltage level at the input, and are mainly used when the voltage overshoot exceeds the maximum allowable limit. Although, the HSIO clamp is always ON; it is not a PCI clamp. PCI clamp is only on GPIO. If signaling levels of the receiver are greater than the VDDIx of the bank, the clamp diode must be OFF to support hot-swapping insertion, see Cold Sparing and Hot Swap for more information.

For GPIO, clamp diodes can be programmed to be ON or OFF by using the I/O attribute editor in Libero SoC or by using a PDC command. For HSIO, the internal clamp diode is always ON.

The following table lists programmable clamp diodes.

Table 1. Programmable Clamp Diode
I/O Standards Supported I/O Type Clamp Diode Control
LVTTL, LVCMOS33, LVCMOS25, LVCMOS18, LCMOS15, LVCMOS12, SSTL25, SSTL18I, SSTL18II, SSTL15I, SSTL15II, HSTL15I, HSTL15II GPIO OFF, ON

The following PDC command is used for programmable clamp diode settings:

set_io – -clamp_diode <value>

value can be set as listed in Table 1.

Note: The clamp diode is always on for HSUL18I, HSUL18II, SLVSE15, MIPI25, PCI, SLVS33, HCSL33, MIPIE25, LVPECL33, LVPECLE33, LVDS25, LVDS33, RSDS25, RSDS33, MINILVDS25, MINILVDS33, SUBLVDS25, SUBLVDS33, PPDS25, PPDS33, MLVDSE25, and BUSLVDSE25 I/O standards implemented in GPIO bank.