Ampro Corporation 486E - Manual (Page 24)

Little Board™/486e Technical Manual
2–8
Powerfail Monitor
The Little Board/486e CPU has a built-in powerfail circuit that generates a clean reset signal if
power falls below 4.65V. It guarantees a minimum 140 mS reset signal, independent of how long
the power falls below the 4.65V threshold.
To enable the powerfail circuitry, short W14 with a jumper.
Non-maskable interrupt (NMI): When the supply voltage falls below (approximately) 4.7 volts,
the powerfail logic sends an NMI to the CPU. When the BIOS detects the NMI, it displays the
message “Power Fail NMI” on the screen. You have two options at this point (made by keyboard
selections). One is to mask the NMI and continue. The other is to reboot the system. This requires
operator intervention. If you want an automatic response to the NMI, you can provide an NMI
handler in your application, and patch the NMI interrupt vector address to point to your routine.
If you have configured the byte-wide socket S0 for battery backup, it will be write protected while
power is below 4.7 volts. (Its chip select is held to a logic 1.) This is to prevent writing bad data to
an SRAM in S0 when the voltage is low.
Hardware reset: If the supply voltage falls below (approximately) 4.40V, the powerfail logic
initiates a hardware reset (like pressing the RESET button). A “clean” reset during a low voltage
period prevents erratic operation or crashes. Reset is asserted for the duration of the low-voltage
period plus 100 mS after the voltage returns to above 4.40V.