Power-Up/-Down Behavior of ProASIC3/E Devices
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Brownout Voltage
Brownout is a condition in which the voltage supplies are lower than normal, causing the device to
malfunction as a result of insufficient power. In general, Actel does not guarantee the functionality
of the design inside ProASIC3/E devices if voltage supplies are below their minimum recommended
operating condition. Actel has performed measurements to characterize the brownout levels of
FPGA power supplies. The brownout levels of the power supplies for ProASIC3/E devices are
designed to be 0.75 V ± 0.25 V for VCC and 0.8 V ± 0.3 V for VCCI. For the purpose of
characterization, a direct path from the device input to output is monitored while voltage supplies
are lowered gradually. The brownout point is defined as the voltage level at which the output
stops following the input. Characterization tests performed on two A3PE600-PQ208 EAS devices in
typical operating conditions showed the brownout voltage levels to be within the specification.
During device power-down, the device I/Os become tristated once the first supply in the power-
down sequence drops below its brownout deactivation voltage.
PLL Behavior at Brownout Condition
When PLL power supply voltage and/or VCC levels drop below the VCC brownout levels
(0.75 V ± 0.25 V), the PLL output lock signal goes low and/or the output clock is lost. The following
sections explain PLL behavior during and after the brownout condition.
VCCPLL and VCC Tied Together
In this condition, both VCC and VCCPLL drop below the 0.75 V ± 0.25 V brownout level. During the
brownout recovery, once VCCPLL and VCC reach the activation point (0.85 ± 0.25 V) again, the PLL
output lock signal may still remain low with the PLL output clock signal toggling. If this condition
occurs, there are two ways to recover the PLL output lock signal: 1) Recycle the power supplies of
the PLL (power off and on) by using the PLL POWNDOWN signal; 2) Turn off the input reference
clock to the PLL and then turn it back on.
Only VCCPLL Is at Brownout
In this case, only VCCPLL drops below the 0.75 V ± 0.25 V brownout level and the VCC supply remains
at nominal recommended operating voltage (1.5 V ± 0.075 V). In this condition, the PLL behavior
after brownout recovery is similar to initial power-up condition, and the PLL will regain lock
automatically after VCCPLL is ramped up above the activation level (0.85 ± 0.25 V). No intervention
is necessary in this case.
Only VCC Is at Brownout
In this condition, VCC drops below the 0.75 V ± 0.25 V brownout level and VCCPLL remains at
nominal recommended operating voltage (1.5 V ± 0.075 V). During the brownout recovery, once
VCC reaches the activation point again (0.85 ± 0.25 V), the PLL output lock signal may still remain
low with the PLL output clock signal toggling. If this condition occurs, there are two ways to
recover the PLL output lock signal: 1) Recycle the power supplies of the PLL (power off and on) by
using the PLL POWNDOWN signal; 2) Turn off the input reference clock to the PLL and then turn it
back on.
It is important to note that Actel recommends using a monotonic power supply or voltage
regulator to ensure proper power-up behavior.
Internal Pull-Up and Pull-Down
ProASIC3/E device I/Os are equipped with internal weak pull-up/-down resistors that can be used by
designers. If used, these internal pull-up/-down resistors will be activated during power-up, once
both VCC and VCCI are passed their functional activation level. Similarly, during power-down, these
internal pull-up/-down resistors will turn off once the first supply voltage falls below its brownout
deactivation level.