原创 FM33256 应用参考

FM33xx SPI Companion – Typical Application

DESCRIPTION

The schematic below shows a typical application of an FM33xx device. It is shown as an example of
external components that could be used, typical values, and their connections to other system devices.
In this case, a line-operated (AC powered) system is shown with microcontroller, FM33xx device, and
passive components. The micro, in this case, has a dedicated SPI interface. Your micro may not have
this port, but an SPI protocol may be written and the GPIO pins may be bit-banged to achieve the same
result.

Early Power Fail

AC line power is rectified and C1 filters the ripple. A 3.3V linear regulator supplies clean DC voltage
to the Vdd pins of the FM33xx and microcontroller. The unregulated voltage on C1 is, say, 8V with
ripple. The early power fail feature of the FM33xx can be used by connecting a voltage divider (R3,
R4) to the PFI input. Once this input goes below 1.5V, the PFO pin which is tied to the controller’s
NMI pin will drive low, signalling a Non Maskable Interrupt. The resistor values are chosen based on
how early you’d like to warn the microcontroller that the power supply is powering down. The trip
voltage VTR that causes the PFO to drive the NMI is determined by this equation:
1.5V = VTR (R3/(R3+R4))

If R3=100K and R4=300K, then VTR is about 6V.