SmartFusion2/IGLOO2 functionality is guaranteed only if VDD is above the recommended level specified in the datasheet. Brownout occurs when VDD drops below the minimum recommended operating voltage. As a result, it is not possible to ensure proper or predictable device operation. The design might continue to malfunction even after the supply is brought back to the recommended values, as parts of the device might have lost functionality during brownout. The VDD supply must be protected by a brownout detection circuit.
To recover from VDD brownout, the device must either be power-cycled, or an external brownout detection circuit must be used to reset the device for correct operation. The recommended guideline for the threshold voltage of brownout detection is mentioned in DS0128: IGLOO2 and SmartFusion2 Datasheet. The brownout detection circuit must be designed such that when the VDD falls below the recommended voltage mentioned in the datasheet, the device is held in power-down mode via the DEVRST_N pin.
Brownout detection must be implemented through a standalone circuit or included as part of power management circuitry.
SmartFusion2/IGLOO2 devices do not have a built-in brownout detection circuitry, but an external brownout detection circuitry can be implemented as shown in the following figure.
The BOD device must have an open-drain output to connect to VPP through a 10 kΩ resistor externally. During power-on, the brownout reset keeps the device powered down until the supply voltage reaches the threshold value. Thereafter, the brownout reset device monitors VDD and keeps RESET# output active as long as VDD remains below the threshold voltage. An internal timer delays the return of the output to the inactive state (high) to ensure proper system reset.
The delay time is in milliseconds and starts after VDD has risen above the threshold voltage. When the supply voltage drops below the threshold voltage, the output becomes active (low) again.