Actel Silicon Explorer II, User Manual, Page: 30 / 42

Appendix C: Termination of the VPP and Mode Pin for ACT1 Devices in a Radiation Environment
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Actel recommends that you terminate the MODE pin to a GND with a
hard jumper in parallel with a 10k W resistor. The hard jumper protects
against resistor failure. During the prototype and debug stage you can
remove the hard jumper and utilize the Silicon Explorer probing
capability. Verify the termination of the MODE pin for each Actel
device on the flight board with an ohm meter. Programming the
security/probe fuse does not eliminate the need to terminate the mode
pin.
VPP Pin
The VPP pin is the input supply pin used for device programming (the
Radiation-Hardened FPGAs’ data sheet refers to pin 22 on the RH1020
and pin 107 on the RH1280 as VCC). Actel recommends that you do
not leave VPP floating, since it may bounce around and a high voltage
might put the device in programming mode. For operating in a
radiation environment, there is a concern that unterminated leads can
charge up. In various radiation cases this effect has been observed.
Under normal operating conditions (MODE low and VPP high), the
VPP signal is used to bias transistors in the peripheral control circuitry
of the FPGA and does not directly access the user-defined logic
modules or fuses in the array core. When the device is in programming
mode (MODE pin high), the VPP signal can reach every module in the
array core. If the MODE pin is not properly terminated, there is a risk
that the device could go into programming mode and lose all
functionality. If MODE is tied low and VPP is not properly terminated,
then there is a risk of damage to the peripheral control circuitry of the
gate array.
The VPP pin is designed to receive voltages exceeding VCC. Under
normal operating conditions, the VPP signal is used to bias a high
voltage FET along with the drain of that FET. In flight, the FET would
break down at lower bias voltages. Various tests and analyses showed
that in the radiation environment, a single ion was capable of rupturing
an antifuse at 5 VDC, which corresponds to an electric field strength of
approximately 6 MV/cm. Factors for rupture include electric field
strength, oxide quality and uniformity, and any parasitic junctions that
may be biased.